proposal for n tof experimental area 2...

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The n_TOF Experimental Area2 n_TOF Collaboration

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Proposal to the ISOLDE and Neutron Time-of-Flight Committee (INTC)

Proposal for n_TOF Experimental Area 2 (EAR-2)

E. Chiaveri on behalf of the n_TOF Collaboration

Spokesperson: E. Chiaveri

Technical Coordinator: E. Berthoumieux


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The n_TOF Collaboration (2009-2011)

E. Chiaveri1,2)

, S. Andriamonje1)

, J. Andrzejewski3)

, L. Audouin4)

, V. Avrigeanu5)

,

M. Barbagallo6)

, V. Bécares7)

, F. Bečvář8)

, F. Belloni2)

, E. Berthoumieux1,2)

, J. Billowes9)

,

D. Bosnar10)

, M. Brugger1)

, M. Calviani1)

, F. Calviño11)

, D. Cano-Ott7)

, C. Carrapiço12)

,

F. Cerutti1)

, M. Chin1)

, N. Colonna6)

, G. Cortés11)

, M.A. Cortés-Giraldo13)

, M. Diakaki14)

,

I. Dillmann15)

, C. Domingo-Pardo16)

, I. Duran17)

, N. Dzysiuk18)

, C. Eleftheriadis19)

,

M. Fernández-Ordóñez7)

, A. Ferrari1)

, K. Fraval2)

, S. Ganesan20)

, G. Giubrone21)

, M.B.

Gómez-Hornillos11)

, I.F. Gonçalves12)

, E. González-Romero7)

, F. Gramegna18)

,

E. Griesmayer22)

, C. Guerrero1)

, F. Gunsing2)

, M. Heil16)

, D.G. Jenkins23)

, E. Jericha22)

,

Y. Kadi1)

, F. Käppeler24)

, D. Karadimos25)

, M. Kokkoris14)

, M. Krtička8)

, J. Kroll8)

,

C. Lederer26)

, H. Leeb22)

, L.S. Leong4)

, R. Losito1)

, M. Lozano13)

, A. Manousos19)

,

J. Marganiec3)

, T. Martinez7)

, C. Massimi27)

, P.F. Mastinu18)

, M. Mastromarco6)

, M. Meaze6)

,

E. Mendoza7)

, A. Mengoni28)

, P.M. Milazzo29)

, M. Mirea5)

, W. Mondalaers30)

, C. Paradela17)

,

A. Pavlik26)

, J. Perkowski3)

, A. Plompen30)

, J. Praena13)

, J.M. Quesada13)

, T. Rauscher31)

,

R. Reifarth16)

, A. Riego11)

, F. Roman1,5)

, C. Rubbia1,32)

, R. Sarmento12)

, P. Schillebeeckx30)

,

G. Tagliente6)

, J.L. Tain21)

, D. Tarrìo17)

, L. Tassan-Got4)

, A. Tsinganis1)

, S. Valenta8)

,

G. Vannini27)

, V. Variale6)

, P. Vaz12)

, A. Ventura28)

, M.J. Vermeulen23)

, V. Vlachoudis1)

,

R. Vlastou14)

, A. Wallner26)

, T. Ware9)

, C. Weiß22)

, T.J. Wright9)

1) European Organization for Nuclear Research (CERN), Geneva, Switzerland

2) Commissariat à l’Énergie Atomique (CEA) Saclay - Irfu, Gif-sur-Yvette, France

3) Uniwersytet Łódzki, Lodz, Poland

4) Centre National de la Recherche Scientifique/IN2P3 - IPN, Orsay, France

5) Horia Hulubei National Institute of Physics and Nuclear Engineering - IFIN HH, Bucharest - Magurele, Romania

6) Istituto Nazionale di Fisica Nucleare, Bari, Italy

7) Centro de Investigaciones Energeticas Medioambientales y Technologicas (CIEMAT), Madrid, Spain

8) Charles University, Prague, Czech Republic

9) University of Manchester, Oxford Road, Manchester, UK

10) Department of Physiscs, Faculty of Science, Zagreb, Croatia

11) Universitat Politecnica de Catalunya, Barcelona, Spain

12) Instituto Tecnológico e Nuclear (ITN), Lisbon, Portugal

13) Universidad de Sevilla, Spain

14) National Technical University of Athens (NTUA), Greece

15) Physik Department E12 and Excellence Cluster Universe, Technische Universität München, Munich, Germany

16) GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany

17) Universidade de Santiago de Compostela, Spain

18) Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Italy

19) Aristotle University of Thessaloniki, Thessaloniki, Greece

20) Bhabha Atomic Research Centre (BARC), Mumbai, India

21) Instituto de Fìsica Corpuscular, CSIC-Universidad de Valencia, Spain

22) Atominstitut, Technische Universität Wien, Austria

23) University of York, Heslington, York, UK

24) Karlsruhe Institute of Technology, Campus Nord, Institut für Kernphysik, Karlsruhe, Germany

25) University of Ioannina, Greece

26) University of Vienna, Faculty of Physics, Austria

27) Dipartimento di Fisica, Università di Bologna, and Sezione INFN di Bologna, Italy

28) Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile (ENEA), Bologna, Italy

29) Istituto Nazionale di Fisica Nucleare, Trieste, Italy

30) European Commission JRC, Institute for Reference Materials and Measurements, Geel, Belgium

31) Department of Physics and Astronomy - University of Basel, Basel, Switzerland

32) Laboratori Nazionali del Gran Sasso dell’INFN, Assergi (AQ), Italy


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TABLE OF CONTENTS

1. Executive Summary ......................................................................................................... 5

2. Introduction ...................................................................................................................... 6

3. Scientific Case for EAR-2 ................................................................................................ 7

3.1 Astrophysical Research ....................................................................................................... 7 3.1.1 MACS measurement on stable isotopes to 1% accuracy .................................................. 8

3.1.2 MACS measurement on unstable isotopes ........................................................................ 9

3.1.3 Neutron induced charged particle reactions .................................................................... 11

3.2 Nuclear technology applications ....................................................................................... 11

3.3 Basic nuclear research ....................................................................................................... 14

4. Dosimetry and Radiation Damage Studies .................................................................. 16

5. Facility Performance ..................................................................................................... 18

5.1 Neutron Fluence ................................................................................................................. 19

5.2 Neutron Energy Resolution .............................................................................................. 20

5.3 Charged Particle Fluence .................................................................................................. 21

5.4 Photon Fluence ................................................................................................................... 22

5.5 Higher Signal to background ratio and Equivalent Half-Life ....................................... 23

6. Radiation Protection Analysis ....................................................................................... 26

7. Engineering Study .......................................................................................................... 29

7.1 Civil Engineering ............................................................................................................... 29

7.2 Beam line study .................................................................................................................. 31

8. Conclusion ...................................................................................................................... 32

APPENDIX I: BUDGET (EAR-2) ........................................................................................ 36

APPENDIX II: STAFF (EAR-2) ........................................................................................... 38

APPENDIX III: TENTATIVE PLANNING (EAR-2) ........................................................ 40

Appendix IV. EXPRESSIONS OF INTEREST FOR EAR-2 ............................................ 41


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1. EXECUTIVE SUMMARY

The outstanding features of the existing CERN n_TOF neutron beam (with a flight path of

185 m) are the very high instantaneous neutron flux, excellent TOF resolution, low intrinsic

backgrounds and coverage of a wide range of neutron energies, from thermal to a few GeV.

These characteristics provide a unique possibility to perform neutron-induced cross-section

and angular distribution measurements for applications in nuclear astrophysics, nuclear

reactor technology and basic nuclear physics. A wide variety of measurements have already

been performed since the facility became operational in 2001, most of them already

published [1-24] and made available to the nuclear data and nuclear physics community.

A study has been performed investigating the feasibility of new Experimental Area called

EAR-2 which, having a flight path of only 20 m from the existing spallation target (90

degrees with respect to the incoming proton beam), would fulfil the demands of the neutron

science community for a neutron time-of-flight facility with a higher neutron flux [25]. The

construction of the EAR-2 with a short flight path would offer the possibility of improving

the quality of the data essential for nuclear energy applications, nuclear astrophysics, basic

nuclear physics, dosimetry and radiation damage.

The main advantages of the planned EAR-2 with respect to existing facilities, in particular

the existing n_TOF beam line, are:

The number of neutrons at the sample position is on average increased by a factor 25.

Neutron-induced reaction measurements can be performed on very small samples (< 1

mg). This feature is of key importance for reducing the activity of unstable samples and in

cases where the available sample material is particularly rare. Limitations in sample mass

are crucial in astrophysics as well as for the field of nuclear technologies. Depending on

the particular isotope, this may enable some measurements for the first time at all.

Measurements can be performed on isotopes with very small cross-sections, which act as

neutron poisons or as bottlenecks in the reaction flow of the s-process. An optimized

signal-to-background (S/B) ratio is an essential prerequisite for such experiments.

Measurements can be performed on much shorter time scales for significantly improved

accuracy. Repeated runs with modified conditions are essential to verify corrections and

reduce systematic uncertainties.

Measurements of neutron-induced cross-sections at high energies (En >10–100 MeV),

which are very difficult in the existing EAR-1, become possible thanks to the strongly

reduced γ-flash. This concerns the important measurements of inelastic scattering cross-

sections or for reactions with charged particles in the exit channel, where the preferred Si

and Ge detectors are strongly affected by the γ-flash.

EAR-2 will contribute to a substantial improvement in experimental sensitivity and open a

new window to astrophysics, technological issues (such as transmutation or design of safety

of future nuclear energy systems) and basic nuclear physics by allowing measuring neutron-

induced reactions which are not accessible so far in existing facilities worldwide. EAR-2

should be considered complementary to EAR-1; since they would run in parallel.


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2. INTRODUCTION

The overall efficiency of the experimental programme and the range of possible

measurements could be significantly improved by constructing EAR-2, located vertically

20 m above the n_TOF spallation target (see Fig. 1). This possibility, already evoked in the

past during the first phase of n_TOF, was recently analysed in detail within the framework of

an interdepartmental working group. During the last year, this group considered in detail the

feasibility of the project and in particular the clarification and better definition of the

scientific case, expected characteristics of the neutron beam, civil engineering challenges and

radiation protection issues. The present document is a summary of the work and conclusions

reached to date.

Fig.1: Experimental Area 2 (EAR-2) schematic view

The configuration of the presently designed n_TOF EAR-2, presented in detail in

Section 5, allows neutron-induced reactions to be measured with the following advantages

compared to the existing EAR-1:

Reduction of γ-flash: Since most of the relativistic particles produced in the spallation

process and which generate the so-called „γ-flash‟ are emitted in the forward direction,

placing an experimental area at an angle of 90° with respect to the primary beam axis

strongly reduces the related background effects. The large reduction of these signals, which

for some detectors mask the signal from neutron reactions for the first few μs, opens the

possibility to extend, in these cases, the measurement of neutron-induced reactions up to

higher energies compared to those presently achievable in EAR-1.

Higher neutron flux: Being closer to the spallation target (flight path of 20 m) the

configuration provides a higher instantaneous neutron flux with respect to the present neutron

fluence in EAR-1 (flight path of 185 m). This is a clear advantage for measuring reactions on

samples with very small masses and/or very small cross-sections.


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Higher neutron rate: Another effect of the ten times shorter flight path is that the

interaction time of the neutron pulse with the sample is also ten times shorter. In this way, the

ratio between neutron-induced signals and the constant background from the activity of

radioactive samples is increased by an order of magnitude. This feature is extremely

important in studies on high activity radioactive isotopes, where the intrinsic activity

represents the dominant background component.

Small samples:. The set-up with C6D6 detectors at the present flight path appears to be

limited to samples of the order of 6 × 1017

atoms or 150 g. With the 25 times higher flux at

EAR-2 this limit can be reduced to 6 µg. Another factor of five can be gained by replacing

the C6D6 set-up with the Total Absorption Calorimeter (TAC), thus decreasing the limit to

about 1 µg or 5 × 1015

atoms. These numbers refer to samples in the important mass range A

>150, which includes major s-process branchings, but also the actinide isotopes. Assuming

ISOLDE intensities of 1011

s–1

, this means that attractive n_TOF samples for measurements at

EAR-2 could be made in a single shift at ISOLDE. This possibility is fascinating but needs of

course further investigation.

3. SCIENTIFIC CASE FOR EAR-2

The realization of the 2nd

Experimental Area with its short flight path will contribute to a

substantial improvement in experimental sensitivity and will open a new window to stellar

nucleosynthesis, nuclear technology issues (design and safety investigations for future

nuclear energy systems), and basic nuclear physics by allowing neutron-induced reactions

which are not accessible in the existing facilities to be measured. Together with

measurements of interest for nuclear technology and nuclear astrophysics, the EAR-2 will

permit studies of radiation damage on electronics devices and detectors to be performed, and

to study the response and resistance of new materials, such as moderators, in future high-flux

facilities, in particular the European Spallation Source (ESS), as well as in fusion systems,

like ITER.

3.1 Astrophysical Research

In the field of Nuclear Astrophysics, neutron capture reactions are responsible for the

origin of the heavy elements between the Fe peak and the actinides. Roughly equal

abundances of the heavy elements were produced by the slow and rapid neutron capture

processes (s- and r-process) [26,27]. Whilst the s-process is associated with comparably

quiescent stellar conditions during He and C burning, the r-process is associated with an

explosive environment. More specifically, the s-process takes place either during He shell

burning in thermally pulsing low-mass asymptotic giant branch (AGB) stars, or in the core

He and shell C burning phases of massive stars. The prevailing temperatures and neutron

densities at the s-process sites are rather moderate, corresponding to thermal energies

kT = 10–100 keV and typical neutron capture times of about a year. Owing to the slow time

scale, the s-process reaction path follows the valley of stability, starting at the pronounced

abundance maximum at Fe and Ni all the way up to the terminal point at Pb and Bi, where

further neutron captures produce short-lived α-unstable nuclei, which eventually decay back

into the Pb isotopes.

As a consequence of the slow time scale, the final s-abundances are determined by the

respective neutron capture reactions. Therefore, accurate (n,) cross-sections for all isotopes

involved in the reaction chain represent the key ingredients for any quantitative picture of


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element synthesis during the advanced He and C burning phases of stellar evolution. These

data determine the reliability of abundance predictions and are, accordingly, the basis for

testing stellar s-process models.

In a stellar environment, data are needed in the form of Maxwellian-averaged cross-

sections (MACS),

(1)

where En denotes the neutron energy, σ(En) the energy-dependent cross-section, and kT the

respective thermal energy. In mass regions between magic neutron numbers, where MACS

values are large enough for flow equilibrium to be established, there is a direct correlation

between s abundances and the respective cross-sections. Under this condition one finds that

the product MACS times the resulting s abundance, also known as σN value, is constant to

rather good approximation.

3.1.1 MACS measurement on stable isotopes to 1% statistical accuracy

The (n,) cross-sections have to be known for neutron energies between 0.3 and 300 keV

for a complete coverage of the thermal energy range 8 < kT < 90 keV determined by the

temperatures of the stellar s-process zones. For meaningful abundance predictions, cross-

sections should be available with an accuracy of 5% or better, but uncertainties as low as 1%

statistical are desired for stable isotopes and in particular for a number of key isotopes, e.g.

for the 33 s-only nuclei on the s-path and for the approximately 70 isotopes needed for the

interpretation of s-process signatures discovered in presolar grains. While the MACS

collection in the KADoNiS data base [24] contains experimental values for the 279 stable

isotopes on the s-process path, the criteria of completeness and accuracy are met only in a

minority of cases. For the accuracy aspect this is illustrated in Fig. 2, where the respective

uncertainties are plotted versus mass number. Further improvements are clearly required,

especially in the mass regions below A = 120 and above A = 180, where a large number of

cross-sections with uncertainties in excess of 10% await improvement.

The EAR2 can reduce the “statistical” accuracy of the measurements down to 1%, however

the “systematic” one will still be subject to the knowledge of the reference cross sections.

Which can give a good chance to redefine the reference cross section at EAR2 with a future

proposal.

Fig. 2: Presently quoted uncertainties for the stellar (n,γ) cross-sections required in s-process studies.


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The cross-sections of the stable s-only isotopes are required with uncertainties of ≈1%. This

goal has been reached for only half of the cases between 70

Ge and 204

Pb. At present, high-

quality measurements on s-only isotopes are still needed for 64

Zn, 76

Se, 80,82

Kr, 86,87

Sr, 96

Mo, 104

Pd, 164

Er, 180

Tam

, 192

Pt, 198

Hg, and 204

Pb. State of the art experiments were performed at

n_TOF on 186,187

Os, using C6D6 liquid scintillators combined with modern pulse height

weighting techniques [25]. The more efficient TAC [26], a large 4π detector array of 40 BaF2

crystals, which offers five times better efficiency, suffered from the γ-flash in the existing

experimental area, and was therefore limited to neutron energies below about 10 keV.

However, the higher sensitivity of the TAC is crucial in cases, where the sample material is

only available in small quantities or with low enrichment, e.g. for 180

Tam

[31].

3.1.2 MACS measurement on unstable isotopes

In contrast to the situation for stable isotopes, experimental data are almost completely

missing for the unstable branch point isotopes. Branchings occur when neutron capture and

β-decay compete whenever an isotope with a half-life comparable to the neutron capture

time is reached by the s-process flow. The relative strength of the two branches is given by

the branching ratio

(2)

which formally depends on the β-decay rate λβ = ln2/t1/2 and on the neutron capture rate λn= nn

vT σ, where nn denotes the neutron density, vT the mean thermal velocity, and σ the MACS for

the radioactive branch point nucleus. Therefore, reliable branching analyses depend critically

on accurate MACS values not only for the s-only isotopes involved, but also for the unstable

branch point nuclei. An illustrative example of this type of situation is given in Fig. 3.

Fig. 3: The nucleosynthesis mechanisms beyond Fe illustrated in the example of the Kr-Rb-Sr- region.

The neutron capture path of the s-process proceeds along the stability valley because ß-decay is

usually much faster than neutron capture. Exceptions are certain unstable isotopes – in this case 85

Kr

with t1/2 = 10.8 years – where competition between neutron capture and ß -decay leads to branchings

in the reaction flow. The explosive r- and p-processes occur in the region of unstable isotopes and

contribute to the observed abundances as indicated by dashed arrows. Isotopes shielded against α-

decays from the r-process region (full green boxes) are considered to be of s-process origin (apart

from minor p-process contributions). The small cross-sections of neutron magic nuclei act as a

bottleneck for the reaction flow and give rise to sharp s-process maxima in the observed abundance

distribution.


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Whilst the demand for accurate MACS values of stable isotopes can be satisfied with

present techniques, measurements on unstable branch point isotopes are still at the edge of

feasibility. The main difficulties are (i) to obtain suitable samples and (ii) to handle the

background induced by the radioactivity of the samples. Consequently, MACS data on

unstable isotopes are scarce and mostly limited to cases that could be measured at

kT = 25 keV by activation in a quasi-stellar spectrum. So far, TOF experiments on unstable

samples were limited to very long-lived nuclei and suffered from samples of low isotopic

purity. Future efforts will have to face the production of purer samples in sufficient quantities

and, most importantly, find ways of reducing the required amount of sample material. In this

way, the future EAR-2 at n_TOF would provide the most promising options for such studies.

First priority should be given to the important branch points 63

Ni, 79

Se, 147

Pm, 151

Sm, 152,154,155

Eu, 153

Gd, 163

Ho, 170

Tm, 171

Tm, 179

Ta, 204

Tl, and 205

Pb.

This list needs to be extended further in view of recent results in stellar evolution, which

suggest a superpulse at the beginning of the He burning phase in low metallicity stars. The

very high neutron densities in excess of 1015

cm–3

that are predicted for such superpulses

imply that the s-process path would be shifted by a few mass units into the region of

ß-unstable nuclei, thus enormously increasing the number of unstable isotopes in the s-path.

For example, the neutron capture chains in Xe and Cs would easily reach the magic nuclei 136

Xe and 137

Cs, thus bypassing the stable Ba isotopes 134 to 137, which are commonly

considered to be an integral part of the s-process scenarios.

Numerous other requests are related to (i) the interpretation of the s-process signatures of

single stars which are preserved in pre-solar grains and μm sized dust particles formed in the

s-process rich outflows from red giant stars, (ii) the neutron poison effects of the very

abundant light elements which strongly affect the s-process neutron balance despite the

extremely small cross-sections involved, and (iii) the accurate characterization of the

bottleneck regions at magic neutron numbers N = 50, 82, and 126, which are responsible for

the structure of the σN(A) curve that reflects the overall s-process efficiency. These requests

are described in more detail in Refs. [26,27] and the corresponding deficits in the present

status of recommended MACS values can be identified in the KADoNiS data base [28]

where experimental MACS values and the respective uncertainties are listed for the 279

stable isotopes on the s-process path. This collection also shows the situation for the 77

radioactive nuclei on the common s-path, which mostly rely on theoretical results with

typical uncertainties of 30–50%. The reasons for missing or uncertain experimental data in

KADoNiS are clearly related to difficulties in the underlying measurements. The highly

improved sensitivity and the much lower backgrounds reached in EAR-2 would obviously

also boost these important aspects of the s-process.

In some particular cases both the mass and the cross section may be so small that it would

be impossible to perform a differential measurement in the EAR-1 neither in EAR-2 or any

other facility in the world. For such cases there aren‟t any experimental data available and

thus any experimental information would be a breakthrough. It is in those situations that the

availability of an irradiation point at only 1.5 m accessible only from the new EAR-2 opens

the door to preforming integral cross section experiment. The sample (A,Z) would be

irradiated at a short distance form the target where the neutron fluence is very intense, and the

number of (n,g), (n,a) or (n,p) reactions would be calculated from the number of isotopes

(A+1,Z), (A-3,Z-2) and (A, Z-1) found in the sample, respectively. If these reaction products

were radioactive we would be talking about an activation measurement. In any case, the

result from such experiments would be the convolution of the cross section with the

distribution of neutrons as function of neutron energy. Although without any information on


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neutron energy, such integral cross section value would be enough to gain some information

on those cross sections which are the most difficult to measure and, if level densities were

available, could lead to semi-empirical differential cross sections. This becomes even more

interesting due to the proximity of the ISOLDE facility to n_TOF because it open the door to

measure cross section from using very small masses, even below the microgram limit.

3.1.3 Neutron induced charged particle reactions

Apart from neutron capture, (n,p) and (n,α) cross-sections are also important in

astrophysics for two reasons. (i) In the mass range below Fe, these reactions can dominate the

(n,γ) channel and determine the reaction flow and the local abundance patterns. Prominent

examples are the reactions 14

N(n,p), 17O(n,α),

26Al(n,p),

25Mg(n,α), and

33S(n,α). (ii) The

nuclear physics of the explosive p-process, which occupies the proton-rich side of the chart of

nuclides, is strongly determined by α-induced reactions. Since the reaction network of the p-

process includes about 20000 reactions predominantly among unstable nuclei, the

corresponding reaction rates have to rely on theoretical predictions. Theory, however, was

found to suffer from our poor knowledge of the α-nucleus potential at the relevant energies

below 1–2 MeV. In this situation, experimental input is extremely valuable, even for stable

isotopes between Mo and Hg. It has been found that (n,α) reactions are especially suited to

characterize the α-nucleus potential, but measurements of these very small cross-sections are

difficult because only very thin samples can be used due to the limited range of the reaction

ejectiles. Therefore, very few data exist so far, in spite of the large variety of possible

reactions that can be studied. As successful measurements are crucially dependent on the

available neutron flux, the EAR-2 would be per se the ideal place for studying this yet

unexplored territory of the p-process.

Another measurement that could be performed at EAR-2 is the 7Be(n,p) and

7Be(n,)

cross-section, of interest for the Big Bang nucleosynthesis. A sample of 7Be, which has a 53

day half-life, could be produced in microgram quantity, through the 7Li(p,n) reaction at

existing cyclotron laboratories in Europe, and by chemical processing of cooling water from

spallation sources. The accurate knowledge of these cross-sections is important to gain

information about the so-called 7Li anomaly in Big Bang nucleosynthesis.

3.2 Nuclear technology applications

The construction of EAR-2 would be a great advantage also for measurements related to

projects of nuclear waste transmutation, as well as for feasibility studies of future generation

nuclear energy systems. In fact, the much higher flux that would be available in EAR-2,

relative to the current experimental area at n_TOF and other existing facilities, could allow to

measure capture and fission cross-section of various U, Pu and Minor Actinides (MA)

isotopes with half-lives as short as a few years. Among them, some of the most important

ones are 238

Pu (87.7 yr), 241

Pu (14.1) and 244

Cm (18.1 yr). To reduce the short-term radiation

hazard, it is desirable to burn those actinides in future systems (either ADS or Gen IV fast

reactors), together with their neighbouring long-lived isotopes. 238

Pu is produced in nuclear

systems mainly by (n,2n) from 239

Pu, n capture in 237

Np and alpha decay of 242

Cm. 241

Pu is

produced by n capture in 240

Pu and 244

Cm is produced by neutron Capture in 243

Cm,

following a neutron capture in 242

Pu. So 241

Pu and 238

Pu will be present in significant

fractions in any reactor fuel loaded with Pu or Np, and both plus the 244

Cm will be

abundantly produced in transmutation fuels loaded with Pu and minor actinides. Their short

lifetime imply that they (and their decay chains) have important contributions to the activity,

heat load and neutron emission of the spent fuels from present and future reactors during the

first years after discharge from the reactor. This period of time is very important for the


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intermediate storage and, in the case of direct disposal of spent fuel, for the geological

repository. These parameters can limit the conditions, minimum acceptance time and the

capacity of the facilities foreseen for the storage and final disposal of those spent fuels and

associated high level wastes. Furthermore, the 244

Cm is the main door for the production of

heavier Cm, Bk and Cf isotopes by neutron capture. Therefore, the knowledge of their cross-

sections (capture and fission) is important for the estimation of the fuel composition of

several nuclear systems, and in this way they are key parameters for the optimization and

design of such systems, as confirmed by several sensitivity analyses. Therefore, the

knowledge of their cross-section is important for the design of such systems, as indicated by

several sensitivity analyses.

In particular, as reported in the „NEA Nuclear Data High Priority Request List‟ (NEA-

HPRL) [35], new measurements of the fission cross-section of 238

Pu are required in the range

between 9 keV and 6 MeV. Improving nuclear cross-section data for 238

Pu(n,f) is important

for reliable understanding and simulation of the behaviour of the Sodium-cooled Fast

Reactors (SFR), Lead-cooled Fast Reactors (LFR), accelerator-driven minor actinide burners

(ADMAB), and Gas-cooled Faster Reactors (GFR), in order of significance. Furthermore,

improved data for the 238

Pu(n,f) reactions are fundamental for estimating the peak power of

ADMAB and the void coefficient of SFR. According to EXFOR, due to the short half-life of

this isotope, only a few recent measurements have been performed in the energy range of

interest, and discrepancies and inconsistencies between various results need to be resolved.

The high flux that would be available in the second experimental area at n_TOF may also

allow an even more difficult measurement of interest for transmutation projects and new

generation reactors to be undertaken: the 241

Pu(n,f) reaction. The very short half-life of 14.1

years makes direct measurements on this isotope almost impossible to perform with

reasonable accuracy at current neutron facilities, even with low-resolution lead-slowing down

spectrometers (LSDS). Some results have also been obtained recently by means of surrogate

reactions, but the accuracy of such measurements is still subject to debate. The very short

half-life imposes the use of a very small amount of material, to obtain a reasonable count-rate

and extremely high instantaneous neutron flux, such as predicted at n_TOF-EAR-2.

The measurement of the 241

Am(n,f) cross-section is relatively easier and has already been

attempted in the past at n_TOF. Good accuracy data have been obtained from thermal to a

few keV of neutron energy, in particular for the most important resonances. However, the

presence of a large ɑ-decay background has resulted in low-accuracy results in the range

between 180 keV and 20 MeV, a range of interest for Fast Neutron Reactors (FNR) and

Accelerator-Driven Minor Actinides Burners (ADMAB), as indicated in the NEA-HPRL. A

recent experiment using the surrogate method was performed on this isotope but the accuracy

of the results is affected by model assumptions. A direct measurement, taking advantage of

the much higher flux of EAR-2, compared to EAR-1, would improve the situation

significantly in the fast neutron energy range.

The Nuclear Data Request List also calls for new measurements on the 242m

Am(n,f) cross-

section in the range between 500 keV and 6 MeV, for its interest especially for Fast Reactors.

The short half-life of this isotope, 141 years, has prevented measurements with the required

accuracy of a few per cent the fission cross-section. Some measurements were performed

with lead-slowing-down spectrometers (LSDS), but with scarce energy resolution and

covering mostly the low-energy region. The EAR-2 would allow new data in a wide energy

range to be collected, with reasonable resolution and with improved accuracy.


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09.02.2012

The same argument applies to another short-lived minor actinide, 244

Cm (18 yr), which has

only been studied so far by means of LSDS and surrogate reactions. The fission cross-section

of this isotope is required in the range between 65 keV and 6 MeV for the feasibility study of

Accelerator-Driven Minor Actinides Burner (ADMAB). As for 241

Pu, the very large ɑ-decay

background, combined with a relatively high probability of spontaneous fission, makes it

extremely difficult to measure the fission cross-section of 244

Cm at existing neutron facilities,

particularly in the fast neutron energy range where the cross-sections are typically low. In

contrast, the much enhanced flux of EAR-2 may allow data to be collected with a few per

cent accuracy. Apart from the fission cross-section, the neutron spectrum between thermal

energies and 10 MeV is also required for the fission reaction of 244

Cm. At present, it is very

difficult to study such a spectrum in a direct measurement. Such a measurement, again, is a

good candidate for EAR-2.

Another isotope, for which fission cross-section could be studied in n_TOF EAR-2 is 243

Cm

(29.1 yr). Since its half-life is just slightly higher than that of 244

Cm, it is also very difficult to

measure, as demonstrated by the very few results currently available. In the case of the 232

Th/233

U cycle the 232

U isotope plays a particular role, although its amount remains limited,

because its short half-life of 69 years and the emission of a very hard photon of

2.6 MeV at the end of its radioactive chain impose severe constraints on the shielding for the

preparation and handling of the fuel. It is produced by (n,2n) reactions on 233

U and by

neutron capture on 231

Pa,but it is also consumed by capture and fission reactions. The final

amount in the fuel results from this balance. Fission is an important process because 232

U is

fissile although it has an even number of neutrons. Due to the short half-life very few

measurements of the fission cross section have been achieved and in narrow energy ranges.

The high flux available at EAR-2 would allow a fission cross section measurement covering

the full energy range, from to thermal to hundred MeV, with a very limited amount of

material.

For capture measurements, the required mass is typically larger than for fission, and this

limits the number of measurements that can be performed. For transmutation projects, the

capture cross-section measurement of 245

Cm (~8500 yr half-life) would become feasible, as

would that for 231

Pa (32400 yr half-life), an important isotope involved in the Th/U fuel

cycle.

However, in the case of capture reactions, another advantage of the high flux in the

EAR-2 is that measurements of capture reactions of fissile isotopes will become feasible.

When isotopes are fissile, capture and fission reactions are competing and thus one can only

measure (n,) cross-section when both fission and capture reactions are measured

simultaneously, so that anti-coincidence techniques can be applied [38]. This implies the use

of very thin samples (~300 g/cm2) so that the fission fragments can escape the samples and

be detected. Thus the total mass that can be reached with a few such thin samples mounted in

parallel is only a few mg. With such mass, the measurements in the existing EAR-1 would

take several months and even years, and that is why the new EAR-2 is very appealing for

these kinds of measurement. Of course, the use of lower masses for these fissile isotopes

would also reduce the background from their intrinsic activity with respect to EAR-1.

Finally, the higher neutron flux also offers the possibility of using the C6D6 for this type

of measurement, since their lower efficiency with respect to the TAC would no longer be

important. In this case, the high neutron energy limit of the measurements would increase,

since C6D6 detectors are less affected by the γ-flash.


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09.02.2012

3.3 Basic nuclear research

Neutron-induced reactions measured with the time-of-flight technique form a unique tool

to investigate nuclear structure at high excitation energies by observing resolved nuclear

levels that are revealed by resonances in the reaction yields. This information can be

completed by the analysis of measured gamma-ray spectra corresponding to the decay of the

compound nucleus, which also provide information on transition probabilities below the

neutron separation energy.

Neutron resonance spectroscopy is used to obtain crucial information on level densities in

the vicinity of the neutron binding energy, i.e. at several MeV above the ground state. Level

densities are an important part in the calculation of nuclear reaction rates, having applications

in astrophysical processes and in nuclear reactor devices based on fission or fusion reactions.

A large number of level density models exist which are all calibrated by the level density

observed with neutron resonances.

As an example of the observation of nuclear levels in neutron-induced experiments, Fig. 4

shows the total cross-section as a function of neutron energy for several nuclei with

increasing mass ranging from 6Li to

241Am. The resonance structure present in the cross-

sections corresponds to nuclear levels in the compound nucleus. Note that the zero of the

neutron energy scale corresponds to the excitation energy of the neutron binding energy, i.e.

several MeV above the ground state. On the logarithmic energy scale one can observe the

decrease of the spacing between two levels, or the increase of the level density, when the

mass of the nucleus increases. The significant shell effects are illustrated by the case of 208

Pb,

a nucleus with closed neutron and proton shells, where a decrease in the level density can be

observed.

Experiments already performed at n_TOF have provided valuable information on this

topic and a specific proposal for the particular case of 88

Sr has been approved by the INTC

[35] and will be performed in 2012 in the present EAR-1.


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09.02.2012

Fig. 4: The total neutron cross-section as a function of the incident neutron energy. The resonances in

the cross-section correspond to nuclear levels in the compound nucleus at an excitation energy of

several MeV above the ground state.

Another very interesting field of research would be the study of -ray transition

probabilities, so-called Photon Strength Functions (PSF), which is based on the study of the

detector response to -rays from neutron capture reactions and its comparison with

predictions from different models for PSF. The advantage of this technique with respect to

others is that it provides information below the neutron separation energy Sn, which is not the

case in photo-absorption experiments. Indeed, the study of the region below Sn is key to

understanding hot topics such as the existence and systematics of scissor modes and low-

energy pigmy structures in the PSF. Previous experiments at n_TOF using the TAC have

already provided for the first time very valuable information on PSF of actinides beyond 238

U

[36].

The availability of a higher neutron flux at EAR-2 would imply that measurements could

be carried out in a much shorter time scale and this would open the door to dedicated

proposals devoted to the investigation of nuclear levels and PSF. This is not the case in

10-2

10-1

100

101

102

103

104

105

106

107

100

104100

104100

101

100

104

100

103

100

103

100

102

100

104

241Am

235U

208Pb

197Au

107Ag

55Mn

27Al

6Li

neutron energy (eV)

tota

l cro

ss s

ec

tio

n (

b)


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09.02.2012

EAR-1 because the number of measurements is very limited (5–8 each year), while in the

EAR-2 this number could be increased by a factor of 5 or 10, depending on the masses

available for the different samples.

Last, the study of the fission mechanism near the fission threshold, where large

asymmetries in angular distribution are found, would benefit from increase flux. In EAR-1,

these measurements are limited due to the very low fission cross section below the threshold

(~MeV).

4. DOSIMETRY AND RADIATION DAMAGE STUDIES FOR ELECTRONICS

The new experimental area at n_TOF may also be useful for the characterization and

calibration of passive and active dosimeters as well as detectors. The radiation resistance of

electronic and structural components and cables can be tested in its stray radiation field. In

order to assess quantitatively the potential of the new facility for these applications, a series

of dedicated FLUKA simulations was performed and various quantities of interest were

scored in two different positions:

Position 1: at 1.4–1.8 m above the target. This position offers very high neutron fluences

and dose rates. Samples and/or detectors of up to 16.5 cm in diameter could be placed in

this position if no neutron cross-section measurement is taking place in EAR-2. There is

also the possibility of exposing small passive dosimeters, detectors or components in

parasitic mode during normal EAR-2 cross-section measurements. A schematic layout is

shown in Fig. 1.

Position 2: the standard EAR-2 experimental area at about 19 m from the target, where

active and passive detectors and components with diameters of up to 31 cm can be

exposed when not performing neutron cross-section measurements

The main results are summarized in Table 1, where the following quantities of interest are

reported, normalized to a standard pulse of 7 × 1012

protons on target.

1 MeV (Si) equivalent neutron fluence rate. This quantity is a measure for displacement

damage in (electronic) silicon-based devices. It can be correlated to its long-term damage.

Ambient dose equivalent (H*(10)) rate. This quantity is of relevance for passive and

active dosimeters and radiation monitors. Divided by a „typical‟ quality factor it can also

give an indication of the expected absorbed dose rate. Exact values for the absorbed dose

rate are not reported since they will depend greatly on the irradiated material

composition, as is customary for an almost pure neutron spectrum with a substantial

amount of sub-MeV neutrons. The absorbed dose rate is also of interest for radiation

damage of materials such as organic insulators.

The fluence rate of „high energy‟ hadrons.

The fluence rate of thermal (En < 0.4 eV) neutrons. This quantity is important as the

charged hadrons above ~20 MeV (high-energy hadrons, HEH) or neutrons above a few

MeV are assumed to cause single event upsets (SEUs) in electronic equipment. At the

same time another part of SEUs are induced by thermal neutron capture process on light

element (such as 10

B), which might generate light recoils and -particles, which

contribute to the release of energy in the semiconductor. Levels above

107 HEH/cm

2/pulse are considered to be critical for standard electronics equipment; these

levels could be reached after ~25 pulses or 1 minute for a 2.4 s PS cycle.


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09.02.2012

Position 1MeV (Si) eq.n

n/cm2/pulse

H*(10)

Sv / pulse

High energy (> 20 MeV)

hadron fluence

h / cm2 / pulse

Thermal

neutrons

n / cm2 / pulse

Position 1 3.03 × 109

1.47 1.44 × 108

1.22 × 1010

Position 2 3.4 × 106

1.3 × 10-3

4 × 105

1.5 × 106

Table 1: Rates (per pulse) of various quantities of interest for dosimetry and radiation damage

studies. Position 1 has been assumed to be at 1.5 m from the n_TOF target for the purpose of this

table

The contribution of particles other than neutrons is minimal. At Position 1, their

contribution to the 1 MeV (Si) equivalent neutron fluence, to the high-energy hadron fluence,

and to the ambient dose equivalent is respectively 0.3%, 2.6%, and 3.1%. For Position 2, the

corresponding figures are 0.6%, 1.5%, and 1.1% with no sweeping magnet: with the

sweeping magnet there remains only 0.3% of contribution to H*(10) caused by photons.

The results presented in Table 1 demonstrate that the proposed EAR-2 will offer a unique

opportunity for the calibration of active and passive radiation detectors in an almost pure

neutron field at moderate (Position 2) and high (Position 1) dose rates. The available fluence

and dose range is very large. A short irradiation in Position 2 (10 pulses) would integrate a

few 107/cm

2 1 MeV n Si equivalent and 10 mSv, while a long irradiation (10

5 pulses) in

Position 1 will integrate 3 × 1014

/cm2 1 MeV (Si) equivalent neutrons and 1.5 × 10

5 Sv.

The neutron spectrum at Position 1 is shown in Fig. 5. Both spectra are similar to those

expected in the LHC caverns, as well as to those in the tunnel, even though in the latter case

there is also a significant component of charged hadrons. Therefore, n_TOF EAR-2 could be

successfully exploited to test and calibrate components and instrumentations used in the LHC

under very similar neutron irradiation conditions.

Fig. 5: Neutron spectrum at Position 1


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5. FACILITY PERFORMANCE

In order to quantitatively assess the performance of the facility and the improvement with

respect to the fluence achievable in the existing n_TOF Experimental Area (EAR-1), a

FLUKA Monte Carlo simulation was carried out. The geometry of the facility was set up

according to the preliminary plans discussed with the Civil Engineering (GS Department).

The implemented geometry of the proposed installation can be seen in Fig. 6.

The neutron tube extends from the spallation target up to the beam dump. The dump is

implemented with a Fe core that slows down fast neutrons, surrounded by borated

polyethylene and a thin Cd layer at the entrance that captures slow neutrons and reduces

backscattering towards the experimental area.

The bunker housing the experimental area is implemented as a square room of 40.8 m2 in

surface and 5.5 m high. According to the GS Department‟s plans, the position of the beam

tube is off-centre.

A realistic neutron collimation system is implemented. The implemented collimator has

been positioned right before the EAR-2 (to allow it to be removed if needed), starting at a

height of 15.4 m from the target‟s centre and finishing at 18.4 m. It has a 2 m long Fe section

of conical shape, followed by a 1 m long straight section of borated polyethylene, with an

inner diameter of 8 cm up to the entrance to the experimental area, similar to the collimation

used before EAR-1 during fission cross-section measurements.

Fig. 7: Cut through the lid of the target housing (top

view) and target area with the beam tube. The

position of a possible additional moderator is drawn

in blue at the bottom of the tube.

Fig. 6: Implemented geometry.


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09.02.2012

The neutron tube, having an inner diameter of 32 cm from 1.2 m up to the beam dump, has

a polygonal bottom section for the part entering in the target‟s structural container as shown

in Fig. 7. Its horizontal cross-section at this level follows the geometry of the opening present

in the target-housing lid. At the level of the target, the tube follows the curvature of the Al

target housing and is positioned at 1 mm distance to the housing. For additional moderation

of the neutron spectrum, the bottom part of the tube could be filled with a layer of

polyethylene or similar moderating material, if the radiation hardness could be proven in the

long term.

5.1 Neutron Fluence

The results of the simulations for the neutron fluence per cm2 per PS pulse of 7 × 10

12

protons for the new proposed EAR-2 are shown in Fig. 8, in comparison with the existing

EAR-1.

The strong dips in the neutron fluence spectrum of EAR-2 are due to the additional

handling and reinforcement structural material on top of the n_TOF spallation target, which is

not optimized for a vertical flight path.

It should be noted that the neutron spectrum at EAR-2 ranges from thermal to about 300

MeV, while it extends up to several GeV at the current experimental area EAR-1, due to its

forward location with respect to the incident beam on the spallation target.

The reduction of the maximum energy of neutrons (200-300MeV) is not something negative,

but rather a desired feature by design. The very fast neutrons >250 MeV opens numerous

reaction channels as well spallation reaction that in most of the cases “blinds” the detection

system, together with the gamma flash. Therefore we wanted to have a line at 90 degrees to

strongly suppress this contribution.

Fig. 8: Simulated neutron fluence per cm

2 in the existing n_TOF experimental area (EAR-1, blue line)

and in the proposed facility above the n_TOF target (EAR-2, black line). It is worth noting that, while

the neutron spectrum extends up to several GeV for the EAR-1, there is a sharp cut at ~300 MeV in

EAR-2.

101

102

103

104

105

106

107

10-12

10-10

10-8

10-6

10-4

10-2

100

dn /

dln

(E)

/ cm

2 /

7e1

2 p

pp

Neutron Energy [GeV]

Comparison of the Neutron Fluence in EAR1 and EAR2

EAR2EAR1


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Energy Interval EAR-2

n / cm2 / pulse

Statistical

uncertainty

[%]

EAR-1

n / cm2 / pulse

Statistical

uncertainty

[%]

Gain

0.02 – 10 eV 1.64e6 2.0 1.07e5 0.2 15.4

10 eV – 1 keV 1.07e6 1.4 3.98e4 0.3 26.8

1 keV – 100 keV 1.36e6 1.3 5.02e4 0.2 27.0

0.1 – 10 MeV 3.00e6 0.9 1.76e5 0.1 17.1

10 – 200 MeV 4.78e5 2.0 4.15e4 0.3 11.5

Total range

(0.02 eV - 200 MeV) 7.54e6 0.6 4.14e5 0.08 18.2

Table 2: Integrated neutron fluence for EAR-1 and EAR-2 for different energy intervals.

The integrated neutron fluence per cm2 in EAR-1 and EAR-2 for different energy intervals

is listed in Table 2, with the gain defined as the ratio between the neutron density in EAR-2

and in EAR-1 for the same collimator diameter. On average, the gain in neutron fluence of

EAR-2 with respect to EAR-1 is a factor of ~20, being 27 in the keV region of interest in

astrophysics.

The expected high dose and neutron fluence suggest the possibility of using the upgraded

facility also for material irradiation tests in the neutron beam, positioned at 1.5 m above the

target‟s centre (see Section 4). According to the simulations, a neutron fluence of

6.22e15 neutrons/cm2/week would be obtained at this position, assuming a repetition rate of

one pulse of 7 × 1012

protons every 2.4 s.

5.2 Neutron Energy Resolution

In order to investigate the quality of the possible measurements in the EAR-2, the energy

resolution in the proposed experimental area was studied. In Fig. 9 the results are given as

apparent time-of-flight, t, as a function of neutron energy. Equivalently, one can express the

resolution in effective neutron flight path ( +L0) as a function of neutron energy, with L0

being the geometrical flight path. The full width at half maximum (FWHM), Δ , of this

distribution at a given energy is considered to be representative of the neutron energy

resolution. The values for three different neutron energies and the resulting neutron energy

resolution ΔE/E = 2Δ /( +L0) are listed in Table 3 for EAR-1 and EAR-2.

Overall, the energy resolution is degraded by an order of magnitude. This does not concern

the study of heavy isotopes resonances (actinides for instance) because the corresponding

resonances are found at low neutron energies where the Doppler broadening is still dominant.

Neither will the study of (n, charged particles) reactions near and above threshold be affected

by this loss of resolution. Only the measurements of resonances in the keV region, of interest

in astrophysics, will be affected by this lower resolution. However, in these cases the quantity

of interest is the resonance integral (the MACS at 30 keV and 90 keV) and the loss of

resolution, therefore, does not have a great influence on the quality of the results.

The effect of the proton beam pulse width on the resolution has not been taken into

account in this evaluation, but it is comparable to the moderation length around 1 MeV.


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Neutron

Energy

EAR-2:

EAR-2:

L0 = 18.9 m

EAR-1:

EAR-1:

L0 = 187.5 m

Δ [cm] ΔE / E Δ [cm] ΔE / E

1 eV 4 4.3e-3 3 3.0e-4

1 keV 8 8.5e-3 5.1 5.4e-4

1 MeV 38 4.1e-2 34.1 3.6e-3

Table 3: Neutron energy resolution in EAR-2 compared to the resolution in EAR-1

Fig. 9: Resolution in the EAR-2, showing the relation between time of arrival and neutron energy.

5.3 Charged Particle Fluence

An estimate of the expected charged particle fluence in EAR-2 was made and is reported

in Fig. 10 for different contributions, as it could disturb the measurements of neutron cross-

sections. A magnet of 0.2 Tm positioned in the service gallery located 10 m above the target

centre and 8.4 m below the EAR-2 would be sufficient to deflect all charged particles from

the neutron flight path before the collimator entrance. The construction of such a magnet is

currently under study with an already existing preliminary design that fulfils all the above

requirements.


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Fig. 10: Expected charged particle fluence over particle momentum at EAR-2

5.4 Photon Fluence

The photon fluence in EAR-2, being a background component for measurements with

liquid or solid scintillators, has also been investigated in the context of this study.

In Fig. 11 the arrival time of photons (red line) and neutrons (black line) in EAR-2 (left)

and EAR-1 (right) are compared. In both facilities the prompt photons from the spallation

process can be seen as an initial γ-flash arriving together with the first high-energy neutrons.

Fig. 11: Arrival time of photons (red) and neutrons (black) at EAR-2 (left) and EAR-1 (right).

The green lines in Fig. 11 indicate the time which has been selected for each of the

facilities to separate the photons classified as contributing to the γ-flash, from the photons

classified as delayed photons. The energy spectrum of the prompt and the delayed photons in

both facilities are plotted in Fig. 12.

101

102

103

104

105

106

107

10-2

10-1

100

101

dn

/ d

ln(p

) /

cm2 /

7e1

2 p

pp

Momentum [GeV / c]

Charged Particle Fluence at 18.9 m

ElectronsProtons

PionsKaonsMuons


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09.02.2012

Fig. 12: Photon energy spectrum of prompt (left) and delayed (right) photons in both facilities.

The peak at 2.2 MeV, resulting from the neutron capture reaction of 1H in the water

surrounding the target can be seen in both spectra. The smaller absolute value of the prompt

photon fluence and the lack of very high energy photons to EAR-2 (Emax~300 MeV instead of

~10 GeV in EAR-1) are expected due to the orthogonal location of the area with respect to

the impinging proton beam on the spallation target. This will help reduce the γ-flash effects in

the different detection system and shall therefore allow higher energies to be measured in

EAR-2 than in EAR-1. The higher absolute fluence of the delayed photons is understandable

considering the significant amount of structural material at the spallation target in the

direction of the vertical flight path and the closer position of the experimental area to the

collimator and to the target.

The effect of an additional collimator closer to the spallation target (for example in the

service gallery, located 10 m away from the spallation target) in reducing this photon

distribution will be investigated.

5.5 Higher Signal to background ratio and Equivalent Half-Life

The neutron fluence, energy resolution and background are the basic parameters that

describe the performances of each facility. Typically each facility makes a great effort to

reduce the background by introducing appropriate shielding, collimation system, etc.

However, when short-lived radioactive targets are to be measured (as are the majority of most

of the recent n_TOF proposals) the background, due to the natural radioactivity, can be

decreased by decreasing the sample mass, with, as a direct consequence, a proportional

decrease on the reaction rate dNreaction/dt. The reaction rate is directly proportional to the

neutron fluence which, in the case of EAR-2, is ~25 times greater than what is available in

EAR-1 (see section 5.1).

In the case of time-of-flight measurements on radioactive samples, the background

induced by the radioactive decay of the sample is directly proportional to the time needed for

the measurement. A range of neutron energy ∆E corresponds to a window in time-of-flight

∆T, and the signal-to-noise ratio is therefore proportional to the ratio ∆E/∆T. From the

classical relation between time-of-flight and neutron energy, it follows that ∆E/∆T is

inversely proportional to the flight length L. Therefore the new short flight path will result in


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a more favourable signal to radioactive background ratio by a factor 185/19 which equals

nearly a factor 10.

The advantage of the last two points can be combined in comparisons when defining the

„equivalent half-life ‟. We would like to compare the rate of neutron-induced reactions

(3)

with the background from radioactive decays.

(4)

where dNreaction/dt is the reaction rate, dNdecay/dt the natural decay rate, t1/2 the isotope half-life

and N the number of atoms, σ(E) the reaction cross-section under investigation, Φ(E) the

neutron fluence as a function of the energy, t(E) the time-of-flight for each neutron.

From Eq. (3) and Eq. (4) we can calculate the equivalent half-life t1/2 (Eq. (5) when the

reaction rate is equivalent to the decay rate.

(5)

The equivalent half-life t1/2 value only depends on the performances of the facility and

the cross-section to be measured. This representation does not include the discrimination

efficiency of alphas versus fission fragments of the detection system to be used. Therefore,

this value gives us an indication of the measuring capabilities of the shortest lived isotopes in

each experimental area. For an accurate determination of the shortest lived isotopes, the

efficiency of the detection system should be also taken into account. It is evident that in the

EAR-2, due to the shorter flight path and the low repetition rate of the PS machine, the

measuring capabilities are increased further by a factor of 10 on top of the increase of the

neutron fluence (factor 25) resulting in a total gain of a factor of about 250 times (with

respect to what is presently achievable by performing measurements in EAR-1) on measuring

radioactive samples with half-lives as little as a few tenths of a year (Fig. 13). Figure 14

shows the neutron reaction rate assuming a cross section of 1 b (red) and 1 kb (blue) cross

section expressed in isolethargic time units, per unit of mass expressed in moles. The reaction

rate is compared with the background rate, mainly alpha background from the radioactive

decays for one mole of the isotopes. Particular attention should be paid that in this plot the

efficiency of the detector in discriminating the alpha events from the fission ones is missing.

Even in the extreme case of Cm244 the directly competing reaction (spontaneous fission) will

be below the flux induced reaction for most of the energy range. Still one has to develop

techniques able to handle very important backgrounds, because the main decays (alpha or

beta decays) of some of the samples will have a rate comparable or higher to the induced

reaction rates except in the stronger resonances of the cross sections.

Figure 15 shows the equivalent half-life in years as a function of the incoming neutron

energy, for both measuring stations, the present EAR-1 at 185 m distance and the future

EAR-2 at 19 m distance, and for two representative values of cross-section σ(E) of 1 b and

1 kb. The 1 b was chosen as a typical value of cross-section for higher energies (above

1 MeV) and the 1 kb as a typical value for the resonance region (1 eV to a few keV). Also,

the half-life of certain isotopes of the future proposals and past measurements are shown.


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Fig. 13: Neutron rate (dn/dt) for the two experimental areas EAR-1 and EAR-2.

Fig.14. Isolethargic neutron rate (dn/dlnt/mole) per unit of mass, plotted together with the expected

radioactive decay rate of various isotopes.


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Fig. 15: Equivalent half-life in years as a function of the incoming neutron energy, for both measuring

stations, EAR-1 @ 185m and EAR-2 @ 19m distance, and both for two representative values of cross-

section σ(E) of 1 b and 1 kb. Area of the half-lives of some proposed isotopes to be measured in EAR-2

(238,241

Pu) as well as the one corresponding to 245

Cm, already measured in EAR-1.

6. RADIATION PROTECTION ANALYSIS There are two radiation protection aspects associated to the operation of the EAR-2 facility. The

first one concerns the radiation levels in accessible areas induced by the neutron flux between the

target and the experimental area. The adopted mitigation measure in that case is to implement

sufficient shielding to absorb the neutrons and reduce the radiation levels to acceptable values.

The shielding analysis was performed using the Monte-Carlo code FLUKA. A detailed

description of the experimental area was implemented as a FLUKA geometry in order to

determine the thickness of the concrete walls and the dimensions of the beam dump absorbing the

neutrons which do not interact in the sample. A three dimensional view of the new experimental

area enclosure as it is implemented in FLUKA for the radiation protection analysis is shown in

Fig 16.


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Fig 16. Three dimension view of the experimental area as it is implemented in FLUKA for shielding

calculations. The lateral wall shielding could be reinforced at a later stage if the

30 cm collimator is implemented.

The source term used for the calculations takes into account the intensity and the energetic

distribution of the neutrons exiting the collimation system and entering the experimental area. For

the neutron flux intensity, it was first considered that a collimator with an 8 cm radius would be

implemented. To determine the wall thickness, it was conservatively assumed that a bulky target

scattering the neutrons toward the side walls was present in the beam path. Based on this scenario

the wall thickness was determined using a shielding design criterion of 2.5 µSv/h, which

corresponds to the limit for non-designated areas with no permanent workplace [39]. In addition,

the same exercise was repeated considering a collimator with a 30 cm radius in order to

determine the thickness of concrete which would need to be added if the facility was upgraded to

benefit from a higher number of neutrons. With those two specifications, it was decided to build

the wall between the experimental area and the rest of the building using the requirement for the

30 cm-radius collimator and the remaining three lateral walls using the requirement for the 8 cm

collimator. Those three walls could be made thicker at a later stage by adding an extra layer of

concrete blocks if the collimator is upgraded from 8 cm to 30 cm. Fig. 17 shows the dose rate

profile perpendicular to the beam direction for the 8 cm and 30 cm collimator cases. The dose

rate values show that sufficient safety margin is available to ensure that the dose rate is below the

design value of 2.5 µSv/h (shown in blue in Figure 17).


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Fig. 17. Dose rate profile perpendicular to the beam direction considering the neutrons intensity

which correspond to the 30 cm collimator (red curve) and 8 cm collimator (black curve) impinging on

a scattering target. The thick wall on the right side of the figure is in place for the two configurations

while an extra wall (in purple) is only present for the 30 cm collimator on the left.

For the beam dump design a similar approach considering the same shielding criteria was used.

However in that case, a different scenario corresponding to the use of a very thin sample

absorbing a negligible fraction of the incoming neutrons was considered. The neutron intensity

taken into account for the beam dump design corresponds to the 30 cm radius collimator and the

same design limit of 2.5 μSv/h in the surrounding accessible area (roof of the experimental area)

was used. Figure 18 shows a dose rate profile perpendicular to the beam direction and going

through the beam dump. The dose rate values can be compared to the design limit of 2.5 μSv/h

(plotted in blue) in accessible areas on the side of the beam dump shielding.

Fig.18. Dose rate profile perpendicular to the beam direction at the beam dump level considering the

neutrons intensity which correspond to the 30 cm collimator.


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09.02.2012

In addition to the shielding for the experimental area and for the beam dump, it is also foreseen

that additional shielding will be needed in the technical gallery hosting the crane used to lower

the target into the pit. Due to the implementation of the new collimation system inside the

shielding plug filling the pit, the additional shielding will be needed in order to limit the radiation

levels inside the ISR8 hall on the other side of the technical gallery. The technical gallery itself

will then become part of the primary area and will not be accessible anymore during beam

operation.

The second aspect to be considered from a radiation protection point of view is the use of

radioactive samples in the form of unsealed sources. For this reason, some technical features are

implemented in the design of the facility. Such features for example concern the performances of

the ventilation system (pressure difference between areas, use of special filters….), the fire

resistance of the walls, the possibility to easily decontaminate the area or the presence of a buffer

area in order to comply with the procedural requirements to access the experimental area. The

specifications and the implementation of those special requirements will benefit from the

experience gained from the design and operation of the existing experimental area where the

same rules apply.

Finally, as significant residual dose rates are expected close to the target, a detailed work planning

and optimization analysis will be performed following the ALARA principle for the installation

of the new collimation system.

7. ENGINEERING STUDY

7.1 Civil Engineering

A complete study of the Civil Engineering work associated to the construction of the

EAR-2 has been carried out by the GS department at CERN. The challenges were identified

and tackled without finding any glitches. In the following there is a brief description of the

characteristics of the construction and the solutions adopted.

The function of the so-called bunker (see Fig. 19 and Fig. 20) is to house the Experimental

Area 2 (EAR-2). The bunker is envisaged to be approximately 7.9 m long and 7.8 m wide.

The EAR-2 will partly be located on top of the TT2 tunnel and partly on top of the ISR

building (see Fig. 19). The bunker will be connected with the n_TOF underground facilities,

in the TT2A tunnel, via a duct of 60cm in diameter. Due to the foreseen weight of the bunker

of the Experimental Area, support pillars of roughly 12 m will have to be built with the feet

located on the concrete structure of the TT2 target foundations. This will require the opening

of a trench of roughly 8 m from the ground (at the maximum point), while the rest of the

trench would be sitting on top of the ISR building. A venting chimney on top of the TT2

tunnel will have to be partly dismantled.


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09.02.2012

Fig. 19: Overview of n_TOF spallation target area.

The walls and roof of the bunker will be made of concrete with a minimum thickness of

50 cm. This design is optimized for n_TOF measurements, for which a collimator will also be

put in place, and meets the requirements for radioprotection of the surrounding area during

physics measurements. A shield made of prefabricated blocks will be placed on the roof

surrounding the beam dump.

Additional shielding for an upgrade of the facility for testing electronics components or

material tests, which will be done without collimator, can be implemented in a second

building stage by adding a base plate around the building and by positioning concrete

shielding blocks as a wall around the facility as shown in Fig. 20.

Fig. 20: n_TOF EAR-2 surface and underground installations.


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7.2 Beam line study

An extensive feasibility engineering study has been done concerning the possible beam line

construction (Fig.21,22). As already mentioned in section 5, a possible collimator system

having an iron section of conical shape with a straight section of borated polyethylene has

been designed (Fig.21). To complete the beam line, two other elements have been considered;

a permanent magnet (0.2Tm) and a shutter acting when with the beam off an access in the

area will be requested.

The detailed design of the beam line is still on-going because the full and optimized

collimation system is still under study.

Fig. 21: Sketch of the proposed collimator design.

Fig. 22: Sketch of the proposed implementation of the vertical beam line.

Alignment

±30mm

borated

polyethylene

2x0.5m

Steel

4x0.5m

Shutter

Permanent

magnet


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09.02.2012

8. CONCLUSION

An additional measuring station, called Experimental Area 2 (EAR-2), has been proposed

for construction at the n_TOF (neutron time-of-flight) installation at CERN. The project

consists in realizing a vertical flight path of roughly 20 m above the neutron spallation target

as well as the EAR-2 bunker itself, where neutron-nucleus interaction experiments would be

performed. EAR-2 should be considered complementary to EAR-1; since they would run in

parallel.

An interdepartmental working group was created one year ago and the preliminary

outcome has been presented in this document:

The studies on the construction have been carried out by the Civil Engineering Group

(GS/SE). A feasibility study was performed and discussed along with a possible

construction plan. The conceptual design considered a realistic schedule, with the

minimum requirement of about 3FTEs/year over a period of less than 3 years. This

flexible planning offers the great opportunity to be synchronized with the period of the

LHC‟s long shutdown and the corresponding injector chain refurbishing work (see

Appendix I, II, III).

Results regarding the required shielding and beam dump design have already been

discussed and detailed radiation protection studies are ongoing. No problems have

been identified so far in the current implementation.

Detailed Monte Carlo simulations of the proposed EAR-2 beam line have been realized as

input for the radioprotection studies and to support the physics case. The presented study

shows that experimental campaigns will benefit from an increase of more than one order of

magnitude in neutron fluence combined with a strong reduction of the so called “γ-flash”

with respect to the EAR-1. The n_TOF facility with its EAR-1 is already unique in the world

in terms of the instantaneous neutron flux and low background, but the addition of the EAR-2

with its enhanced capabilities will be of utmost importance; due to the beam characteristics

the installation will open new opportunities for measurements of neutron-induced reactions

with unprecedented accuracy for various important fields of physics, among which we could

cite nuclear technology, nuclear astrophysics and stellar evolution, basic research, medical

applications, dosimetry and radiation damage. In more details, the characteristics of the EAR-

2 will enhance the capabilities of the n_TOF facility and of the related experiments allowing

– for example – to:

a) Measure samples with very low mass or very high activity;

b) Measure neutron-induced reactions up to high neutron energies (roughly 300 MeV);

c) Use very thin samples suited for (n, charged particle) reactions.;

d) Perform irradiation of various material and electronic devices for dosimetric studies,

detector development, radiation damage and other applications which could further

increase the range of possible applications of the n_TOF neutron beam.

The n_TOF Collaboration showed a strong interest (including the access of new institutes

in the Collaboration) in supporting the project, based on the new opportunities that the EAR-

2 will offer. The members of the Collaboration have prepared several “Expressions of

Interest” (EOI) related to possible experiments which could be performed in the new

proposed EAR-2 and which are impossible to be carried out in the existing EAR-1. These

EOIs are included in Appendix 2. The n_TOF Collaboration is willing to contribute, despite

their limited resources, in the construction of the n_TOF facility extension, assuming that the

civil engineering and beam line construction activities could be covered by extra

contributions from CERN‟s budget (see Appendix I).


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09.02.2012

Last, but not least, a vast campaign of collaboration has been launched with other

Institutions like IRMM (Institute for Reference Materials and Measurements, Geel, Belgium),

University of München (Germany), Oak Ridge National Laboratory (USA), and PSI

(Switzerland) for producing high purity samples, the essential ingredient for performing

measurements in both EAR-1 and in EAR-2.


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09.02.2012

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APPENDIX I: BUDGET (EAR-2)

n_TOF

Estimated Budget INDICO

Group [KCHF]

Dismantling barrack 559 GS-SE 100

CE new building n_TOF GS-SE 1,300

Ventilation EN-CV 540

Electric services EN-EL 120

Elec. general services 80

UPS 40

Access, alarms & fire detection GS-ASE 200

Access, interlock system 120

Fire detection, alarms 80

Handling equipment EN-HE 100

Crane 70

Monorail modification 30

Radioprotection, monitoring DGS-RP 100

Beam line EN-MEF 340

New target concrete tap 70

New shaft collimation, shielding 50

Dump 50

Vacuum chambers, pump, control 70

Shielded door entrance 50

Gas supply facility 20

Detector support facility (vertical) 30

Permanent Magnet TE-MSC 110

Total [KCHF] 2,910

Contingency 10%

Total [KCHF] 3,201

http://indicobeta.cern.ch/categoryDisplay.py?categId=2638


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The most important items for the budget, as shown in the pie chart above, are the civil

engineering, ventilation for Class A condition in the Area, and the beam line. The

construction of the bunker represents 45% of the total budget due to the foreseen weight of

the bunker of the Experimental Area, supporting pillars of roughly 12 m will have to be built

with the feet located on the concrete structure of the TT2 tunnel foundations.


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APPENDIX II: STAFF (EAR-2)

n_TOF Estimated Staff INDICO

Group FTE YEAR

Dismantling barrack 559 GS-SE 0.25 2012

CE new building n_TOF GS-SE 0.75 2012 Study&Purchaising Procedure

1 2013/2014 Civil Engineering work

Ventilation EN-CV 0.25 2012 Call for tender

0.50 2014 Installation

Electric services EN-EL 0.2 2012 Study

0.5 2014 Installation

Access, alarms & fire detection GS-

ASE 0.5 2014 Installation

Handling equipment EN-HE 1.0 2013/2014 Installation+actual pit modification

Radioprotection, monitoring DGS-

RP 1.0 2012/2013 Study

0.5 2014 Installation+monitoring

Beam line EN-

MEF 1.0 2012 Study

Permanent Magnet TE-

MSC 1 2013 Study&Construction

Total [FTE] 8.45

Contingency 10%

Total [FTE] 9.3

http://indicobeta.cern.ch/categoryDisplay.py?categId=2638


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The calculations of the FTEs needed for the construction and installation of the Area has been

done taking into account the availability of the staff avoiding any interference with the

activities for the refurbishment of the LHC. Most of the preparation for the different actions

will be done during 2012.


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APPENDIX III: TENTATIVE PLANNING (EAR-2)

From the point of view of the n_TOF Collaboration, the years 2013/2014 are the ideal period

for the construction of the new EAR-2. The table shows the tentative project‟s global

schedule. A more detailed analysis will be done for each item taking into account the LHC‟s

activity priorities.


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APPENDIX IV. EXPRESSIONS OF INTEREST FOR EAR-2

1. Cross sections and prompt -ray emission of fissile Pu isotopes

2. Measurement of the 25

Mg(n,α)22

Ne cross section

3. The role of 238

Pu and 244

Cm in the management of nuclear waste: simultaneous

measurements of their capture and fission cross sections

4. Measurements of (n,xn) reaction cross sections for heavy target nuclei

5. Fission cross section of the 230

Th(n,f) reaction

6. First measurement of the capture (and fission) cross sections of the fissile 245

Cm

7. Neutron capture measurement of the s-process branching point 79

Se

8. Cross section and angular distribution of fragments from neutron-induced fission of

232

U

9. Destruction of the cosmic γ-ray emitter 26

Al by neutron induced reactions

10. Measurement of 7Be(n,p)

7Li and

7Be(n,)

4He cross sections, for the cosmological Li

problem.


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TITLE:

1. Cross sections and prompt g-ray emission of fissile Pu isotopes

AUTHORS:

C. Guerrero (CERN), D. Cano-Ott (CIEMAT), E. Berthoumieux (CEA)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 239,241

Pu

MASS: 2x10mg

REACTION(S) OF

INTEREST:

(n,) (n,f)

ENERGY RANGE OF

INTEREST:

Thermal to 1-20 MeV

DETECTION SYSTEM (name and short description):

C6D6(TAC)+MGAS: Combination of C6D6 (TAC) capture detectors with MicroMegas

fission detectors

MAIN ADVANTAGE(S) OF EAR-2 with respect to EAR-1 for this particular

measurement:

Higher neutron fluence delivered in shorter time, which allows measuring mg samples:

- The combination 25 times higher fluence per pulse delivered in 10 times less time per

pulse than in EAR-1 results in a reduction of factor 250 reaction to background from

sample activity with respect to EAR-1.

- The higher fluence allow using samples of only few mg: i) the material in this

quantities is available (not the case of hundreds of mg), ii) samples can then be

produced by “thin sample” techniques like electrodepositing, evaporation, etc. (not

the case for thick samples: pressed powders, matrices, etc.)

- The higher fluence allows to use a single 1 or 2 thin layers (instead of many like in

EAR-1), thus making it possible to use C6D6 detectors (more sensitive to the

sample‟s position) instead of a 4 calorimeter (as the TAC in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 wrt. to other facilities for this particular

measurement:

The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest.

The availability of a type-A experimental area makes it possible to measure Pu isotopes,

which is not the case of instance in other TOF facility.

The high instantaneous fluence makes is possible to measure low mass samples. These

sample would have to be of small mass because 239Pu is a strategic material and because

241Pu (14y) has a very high activity.

MOTIVATION (Nuclear technology, astrophysics, basic physics):

Nuclear Technology: The capture cross section of fissile isotopes (particularly difficult to

measure due to the fission background) is of utmost importance in the operation of current

Nuclear Systems and the design of future devices. The simultaneous measurement of capture

and fission improves the accuracy, reducing systematic errors associated to absolute cross-

section normalization. The measurement would provide as well information on prompt g-ray

emission following capture and fission reaction, which are the dominant sources of heat in a

nuclear reactor and need to be measured with high accuracy according to the NEA.

BEAM TIME (assume neutron fluence of EAR-1 x25, and En<300 MeV):

3x1018

protons per isotope (equivalent measuring time in EAR-1 = 5 years)

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES (3

pages maximum):

The goal of these measurements would be to measure the capture and fission cross sections,


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as well as alpha ratios, of the fissile isotopes of plutonium 239

Pu and 241

Pu. The main

objective would be in the thermal and unresolved resonance regions. Both isotopes play a

key role in the operation of existing reactors, but their (n,) and (n,f) cross sections lack the

required accuracy for burn-up calculations of present PWR and calculations for all types of

fast reactors. The four cross sections are indeed included among those of in the NEA High

Priority Request List [1].

The measurements are very difficult because of the very high activity of the samples to be

measured, and also because independent measurement of the two reaction channels may not

help to solve the existing problems of the evaluations. For that reason, a simultaneous (n,)

and (n,f) measurement is highly desirable. The particularities of the 2nd

experimental area at

n_TOF (the highest intense neutron flux covering the full energy range of interest: thermal to

20 MeV) [2] make such a measurement feasible.

The setup would consist in a MGAS fission detector loaded with a 2 thin samples of

350µg/cm2 of PuO2 each placed in between 2 C6D6 -ray detectors. The samples would be

placed back-to-back in 2 MGAS detectors in order to cancel out the effects of the non-

uniform angular distribution of fission fragments. Both have been individually tested at

n_TOF in the EAR-1 and their combination would be straightforward. The only open

question would be their response to the -flash in EAR-2 and the associated high energy limit

that could be reached.

The expected counting rates for 2x10 mg samples and a total of 3x1018

protons per

measurements are shown in the Figure, where it is observed that a minimum of 3% statistical

uncertainty would be reached in the complete energy range.

Fig. 1. Expected counting rates for a total of 3x10

18 protons allocated to each isotope.

The simultaneous measurement of the two reactions channels with such accuracy and

covering in a single shot the thermal to MeV regions would be the first of a kind, improve

significantly the actual evaluations and opening the door to a new range of measurements of

fissile isotopes such as 233,235

U, 244

Am or 245

Cm.

References

[1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/

[2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study,

CERN-INTC-2011-032; INTC-O-013 (2011)

http://www.oecd-nea.org/dbdata/hprl/


of 69

09.02.2012

TITLE:

2. Measurement of the 25

Mg(n,ɑ)22

Ne cross section

AUTHORS: C. Massimi (University of Bologna and INFN), M. Barbagallo (INFN), Jozef Jan Andrzejewski

(Uniwersytet Lódzki, Lodz, Poland), N. Colonna (INFN), F. Käppeler (KIT), Paul Koehler (ORELA),

C. Lederer (University of Vienna), P.F. Mastinu (INFN), G. Tagliente (INFN), P. Milazzo (INFN), G.

Vannini (University of Bologna and INFN), Christina Weiss (Atominstitut, Technische Universität

Wien, Austria)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 25

Mg

MASS: few mg

REACTION(S) OF

INTEREST:

(n,ɑ)

ENERGY RANGE OF

INTEREST:

Thermal to 1 MeV


SiMon and/or MGAS. Combination of an ionization chamber and a modified silicon detector

(SiMon). Other solutions are available, but must be reconsidered according to the final EAR-

2 setup.


measurement:

Higher neutron fluence delivered in shorter time, which allows measuring very small reaction

cross section:

- The 25 times higher fluence per pulse, relative to EAR-1 results in a higher count-

rate, allowing one to measure the cross section with reasonable uncertainty.

- This measurement has never been performed because the fluence at existing neutron

time-of-flight facilities is much too low.


Nuclear Astrophysics: The 22

Ne(ɑ,n)25

Mg reaction is the main neutron source of the s-

process, which is responsible for the production of elements heavier than iron. Despite its

importance no data is available in the energy range of interest: 1<Eɑ<500 keV. Therefore

reaction rates at stellar temperature for this important reaction are rather uncertain. A reliable

estimation of the reaction rate can be deduced from the inverse reaction 25

Mg(n,ɑ)22

Ne.

Unfortunately this reaction cross section is very small and up to date it was not possible to

measure it since available neutron fluxes do not permit the measurement.


The measurement can be performed in parallel with other measurements, since there is

practically no effect on the neutron beam. The protons available in a whole measurement

campaign would be the ideal case: 2x1019

protons (impossible to measure in EAR-1)

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES:

The measurement is very difficult because the Q-value of the reaction is 480 keV and the

(n,ɑ) reaction cross section is expected to be small.

Ultra thin layers of material are required to let the low-energy ɑ particles escape from the

sample. The range of 480-keV ɑ particles in magnesium is few micrometers. The

corresponding areal density of a 1-µm thick 22

Mg sample is 4.2×10-6

atoms/barn.

In literature there are only two measurements [1,2] of the 25

Mg(n,ɑ)22

Ne cross section, using

monoenergetic neutrons with energies of 5, 7, 13 and14 MeV produced at a Van de Graaff

accelerator. The cross section in this energy region ranges is comparably high, from 1.4 to 95


of 69

09.02.2012

mb.

The expected counting rate CR (counts per second) for a thin sample is proportional to the

reaction cross section σ (barn), the areal density of the Mg sample n, the neutron fluence

ϕ (neutrons/s), the overall detection efficiency Ɛ, and the escape probablilty of the ɑ particles

from the sample Pɑ:

PnCR . (1)

Assuming the following values: n = 4.2×10-6

atoms/barn, σ = 1 mb which correspond to the

worst case, Ɛ = 50% (using a thin magnesium layer and a 2 ɑ particle detector), Pɑ=80%,

the resulting counting rate, as a function of the neutron fluence is:

CR 4.2 105 103 0.4 2 109 . (2)

Equation 2 shows that a higher neutron fluence is crucial for obtaining a reasonable counting

rate.

For instance in EAR-1, ϕ EAR-1=7.1×104 neutrons are delivered per proton burst in the 10-100

keV energy range.

Therefore, the count rate in EAR-1 is limited to ≈0.00014 counts per pulse in the energy

region of greatest interest. On the other hand, in EAR-2 the fluence is 25 times larger

(ϕ EAR-2=25 ϕ EAR-1), providing a rate of about 1 counts per 300 pulses.

Unless other interfereing reaction channels are open, e.g. the (n,p) channel, particle-

identification is not required. As a consequence the setup can be very simple. Suited detectors

for the reaction products are, for instance, Si detectors, MicroMegas, compensated ionization

chambers, and diamond detectors. At the moment we propose two use two versions, which

are already in use in EAR-1:

A) a modified version of the silicon detector SiMon. It would consist of two magnesium

samples viewed by 8 Si detectors outside of the beam;

B) a micromegas detector loaded with two magnesium samples.

In both cases, very thin samples of highly enriched 25

Mg are placed back-to-back in order to

compensate angular distribution effects of reaction products.

Option A) has the advantage of a very low background, since the only material in the beam

are the Mg samples on ultrathin substrate layers. Drawbacks are the low geometric efficiency

of about 10% and A remaining sensitivity to angular distribution effects. The advantage of

option B) is the higher efficiency close to 90%, but the background induced by neutron

reactions in the detector windows and backing materials can be important.

Fig.2. Experimental cross section of the 25

Mg(n,ɑ)22

Ne reaction.

It is worth nothing that this measurement could be considered as a pioneering step for a series

of (n,ɑ) measurements of great importance for Nuclear Astrophysics, which could not be

performed in the past.


of 69

09.02.2012

References

[1] B. Lavielle, H. Sauvageon, and P. Bertin, Phys. Rev. C 42, 305–308 (1990)

[2] M. Brendle, R. Enge and G. Steidle, EPJ A 285, 293-304 (1978)

[3] www.cern.ch/ntof

http://www.cern.ch/ntof


of 69

09.02.2012

TITLE:

3. The role of 238

Pu and 244

Cm in the management of nuclear waste: simultaneous

measurements of their capture and fission cross sections

AUTHORS: E. Mendoza (CIEMAT), D. Cano-Ott (CIEMAT), E. González (CIEMAT), C. Guerrero (CERN)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 238

Pu, 244

Cm

MASS: ~0.1mg each

REACTION(S) OF

INTEREST:

(n,) (n,f)

ENERGY RANGE OF

INTEREST:

Thermal to 1-20 MeV


C6D6 or a new Total Absorption Calorimeter+MGAS: Combination of C6D6 (or a new Total

Absorption Calorimeter) capture detectors with MicroMegas fission detectors


measurement:





- The higher fluence allows using samples of less than 1 mg: i) the material in these




- The higher neutron fluence allows using a 1 or 2 thin samples (instead of the several

necessary at EAR-1). The samples can be placed close to the C6D6 detectors (more

sensitive to the sample‟s position) or at the centre of a 4 calorimeter (as the TAC

used in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 for this particular measurement:


The availability of a type-A experimental area offers the required safety conditions for

measuring short lived Pu and Cm isotopes.

The high instantaneous neutron fluence makes it possible to measure high activity samples

like the 238

Pu and 244

Cm isotopes.


Nuclear Technology: The capture cross section of 238

Pu and 244

Cm are highly relevant for

the nuclear waste managment: both of them constitute an important source of heat in the

irradiated fuel due to their high specific activities (i.e. short half lives). Furthermore, the

neutron capture in 244

Cm opens the path to the build up of higher mass and longer lived Cm

isotopes. The simultaneous measurement of their capture and fission cross sections improves

the accuracy, reducing systematic uncertainties associated to the normalisation to absolute

cross-sections.


3x1018



pages maximum):

The goal of these measurements would be to measure the capture and fission cross sections,

as well as alpha ratios, of the isotopes 238

Pu (88 y) and 244

Cm (18.1 y). The energy range of

range of interest comprehends both the resolved and unresolved resonance regions. Both


of 69

09.02.2012

isotopes have high specific activities due to their short half lives and thus play a key role in

the management of irradiated fuels, the separation of minor actinides and the preparation of

fresh nuclear fuels for the transmutation of minor actinides: 238

Pu is an intense alpha emitter

and 244

Cm undergoes spontaneous fission and is one of the main neutron emitters in the

irradiated nuclear fuel. Furthermore, the neutron capture in 244

Cm builds up heavier and

longer lived Cm isotopes.

Both fission cross sections are indeed included among those of in the NEA High Priority

Request List [1].

The capture cross sections of both isotopes have been poorly measured. For the case of 238

Pu,

there is only one capture measurement [3], excluding the thermal point, between 17.8 eV and

200 keV, and one transmission measurement [4], between 0.0082 eV and 6.5 keV.

For the case of 244

Cm, there are two capture measurements available [5,6], between 20eV and

10keV and between 1 and 950keV, respectively. No capture measurements below 20 keV,

excluding the thermal point, have been performed. Two transmission measurements have

been performed for the 244

Cm isotopes [7,8], between 4.27 and 780 eV and between 1keV

and 3.5 MeV. Differences in the evaluated cross sections are presented in Figure 1.

Fig.1. Existing differences in the libraries in the evaluated capture cross sections of

238Pu and

244Cm.

The measurements are very difficult because of the very high activity of the samples to be

measured, and also because independent measurement of the two reaction channels may not

help to solve the existing problems of the evaluations. For that reason, a simultaneous (n,)

and (n,f) measurement is highly desirable. The particularities of the 2nd

experimental area at

n_TOF (the highest intense neutron flux covering the full energy range of interest: thermal to

20 MeV) [2] make such a measurement feasible.


~1.7µg/cm2 of

238PuO2 or

244CmO2 each placed in between 2 C6D6 -ray detectors (or a new

Total Absorption Calorimeter if available). The samples would be placed back-to-back in 2

MGAS detectors in order to cancel out the effects of the non-uniform angular distribution of

fission fragments. All detector types have been individually tested at n_TOF in the EAR-1

and a test experiment with the combined use of the TAC + MGAS for measuring the 235

U(n,γ) cross section has been performed as well. The only open question would be their

response to the -flash in EAR-2 and the associated high energy limit that could be reached.

The expected counting rates for 2x0.05 mg samples and a total of 3x1018

protons per

measurements are shown in Figure 2.


of 69

09.02.2012

Fig. 2. Expected counting rates for a total of 3x10

18 protons allocated to each isotope. 30bins/decade

has been used in both figures.

References [1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/ [2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN-

INTC-2011-032; INTC-O-013 (2011) [3] M.G.Silbert, J.R.Berreth, Neutron Capture Cross Section of Plutonium-238; Determination of

Resonance Parameters, Nucl.Sci.Eng. 52, 187 (1973). [4] T.E.Young et al., Neutron Total and Absorption Cross Sections of Pu238, .Sci.Eng. 30, 355

(1967). [5] M.S. Moore, G.A. Keyworth, Analysis of the Fission and Capture Cross Sections of the Curium

Isotopes, Phys. Rev. C, v3 (1971) 1656. [6] H.Fernandez Gianotti, Fast neutron cross-sections for curium-244, EXFOR # V0006.007.

[7] R.E.Cote et al., Total Neutron Cross Sections of Cm244, Physical Review Vol.134, p.B1281

(1964).

[8] H.Fernandez Gianotti, Fast neutron cross-sections for curium-244, EXFOR # V0006.002.



of 69

09.02.2012

TITLE:

4. Measurements of (n,xn) reaction cross sections for heavy target nuclei

AUTHORS: R.Vlastou (NTUA), M.Kokkoris (NTUA), V. Avrigeanu (IFIN-HH), E. Berthoumieux (CEA),

Strasbourg group

SAMPLE(S) OF

INTEREST:

ISOTOPE: 197

Au, 181

Ta,

etc

MASS: 2-3g

REACTION(S) OF

INTEREST:

(n,2n), (n,3n), (n,4n), (n,5n)

ENERGY RANGE OF

INTEREST:

10 to ~60 MeV


2 Ge detectors specially modified by the Strasbourg group +MGAS: Combination of 2 HPGe

detectors at 120o and 90

o with respect to the beam direction and MicroMegas fission

detectors for the determination of the neutron flux via the 235-238

U(n,f) cross section. The

cross section for the exit channels of each reaction will be determined by the prominent

gamma rays emitted from the residual nuclei [1].


measurement:

The higher neutron fluence of EAR-2 delivered in shorter time, allows measuring low

(n,xn) cross sections (~100-200 mb at high energies).

The lower gamma-flash in EAR-2 would allow HPGe detectors to be operative.


Nuclear Technology: The generation, maintenance and validation of nuclear data libraries

relevant to ITER, IFMIF and DEMO nuclear engineering design demand, among others,

improvement of nuclear models and update of databases. Moreover, the neutron energy range

concerned in this respect is extended to 50 MeV. On the other hand, the cross sections for

nuclear reactions induced by fast neutrons below 20 MeV are generally considered to be

reasonably well known in spite of many fast neutron reactions for which the several data are

either conflicting or incomplete even around 14 MeV. It is the reason why having recent sets

of accurate measured cross sections still below 20 MeV and especially between 20 and ~60

MeV is highly desirable.

Basic physics: Accurate and consistent data of (n,xn) reactions over a wide energy range (10-

50MeV) are very important to test and improve nuclear models and investigate reaction

mechanisms.


3x1018



pages maximum):

The objective of the present proposal is two-fold. Since only several data with large

uncertainties are known above 20-30 MeV (e.g., the case of the target nucleus 197

Au in Figure

1), it is important to estimate these cross sections theoretically. Therefore the predictive

power of the nuclear model calculations is a major challenge. On the other hand, the accuracy

of these predictions should be checked on the basis of some reference experimental reaction

cross sections. Data with error bars larger than even 50% or clearly distinct in spite of their


of 69

09.02.2012

already 15-20% errors, as shown in Figure 1, can not be used in this respect. Further n_TOF

EAR-2 measurements may bring the data status at larger energies close to the enhanced

accuracy presently achieved around 10 MeV ([2] and Refs. therein).

Fig. 1: Comparison of experimental [2,3], ENDF/B-VII.1 evaluated [4], and calculated (n,2n),

(n,3n), and (n,4n) reaction cross sections by using various model assumptions, for incident energies

up to 40 MeV. [5].

Beyond the nuclear technology motivation of these measurements there is also a basic

interest for the related studies. Actually the model calculations of the fast-neutron reactions

cross sections data below 20 MeV are most sensitive to the parameters related to nuclei in the

early stages of the reaction, i.e. within the pre-equilibrium emission (PE) processes which

then become dominating at higher energies. Thus, there is a good opportunity through these

studies to look for the understanding of the model constraints which are responsible for the

calculated cross section variations, concerning particularly (a) the incident energies below 20

MeV, where the statistical model (SM) calculations are most sensitive to the parameters

related to residual nuclei and emitted particles which are populating them, and (b) the

energies above 20-30 MeV, where the PE processes become dominating so that the measured

data analysis may better validate the corresponding model assumptions of the PE model

(several being shown in Figure 1 and discussed in Ref. [5]). Certain conclusions on the

suitability of the corresponding models could be obtained obviously only on the basis of an

increased accuracy of the measured data.

10 15 20 25 30 35 400.0

0.5

1.0

1.5

2.0

2.5

197Au(n,2n)

196AuTewes+ (1960)

Prestwood+ (1961)

Bayhurst+ (1975)

Paulsen+ (1975)

Veeser+ (1977)

Frehaut+ (1980)

Goutiere+ (1981)

Csikai (1982)

Daroczy+ (1985)

Ikeda+ (1988)

Lu Hanlin+ (1989)

Soewarsono+ (1992)

Uwamino+ (1992)

Filatenkov+ (1999)

Filatenkov+ (2003)

Tsinganis+ (2011)

ENDF/B-VII.1 No PE

gp(): FGM

gp()=a+b

BSFG

10 15 20 25 30 35 400.00

0.05

0.10

0.15

0.20

0.25 197Au(n,2n)

196mAu

Tewes+ (1960)

Prestwood+ (1961)

Goutiere+ (1981)

Ghorai+ (1984)

Ikeda+ (1988)

Soewarsono+(1992)

Filatenkov+ (1999)

Filatenkov+ (2003)

Tsinganis+ (2011)

Philis+ (1977): eval.

No PE

gp(): FGM

gp()=a+b

BSFG

[12-, 9.6h ]

15 20 25 30 35 40

0.5

1.0

1.5

2.0

(

b)

E (MeV)

197Au(n,3n)

195Au

Bayhurst+ (1975)

Veeser+ (1977)

Lu Hanlin+ (1989)

Iwasaki+ (1993)

Philis+ (1977): eval.

ENDF/B-VII.1

No PE

gp(): FGM

gp()=a+b

BSFG

25 30 35 40

0.5

1.0

1.5

2.0

197Au(n,4n)

195Au

Bayhurst+ (1975)

Philis+ (1977): eval

Uwamino+ (1992)

Soewarsono+(1992)

Svoboda+ (2011)

ENDF/B-VII.1

No PE

gp(): FGM

gp()=a+b

BSFG


of 69

09.02.2012

Fig. 2. Comparison of measured [2], evaluated (MENDL-2) and calculated cross sections

for the 181

Ta(p,xn) reactions [6].

The experimental procedure for these measurements will be tried for the first time at the

n_TOF facility. The cross section for the formation of the residual nucleus for each exit

channel of the reaction under study, will be determined by the prominent gamma rays emitted

from the residual nucleus, after applying the appropriate corrections with the help of

statistical model calculations [1]. The gamma rays will be measured by using a HPGe

detector placed at 125o where the second order Legendre polynomial P2 vanishes and the

spectra can be assumed to represent angle integrated yields. A second HPGe detector at 90o

will help for the identification of certain Doppler-shifted delayed gamma-rays. The neutron

flux will be determined via the 235-238

U(n,f) cross section measurements with the fission

Micromegas detector already used in n_TOF, while the neutron time-of-flight will be

determined by the gamma-flash as START signal and the gamma-ray as STOP signal.

Overall, the final goal of this proposal would be to bring the neutron induced reactions

studies above 20 MeV, at the experimental accuracy and theoretical understanding of

charged-particle induced reactions (e.g., the (p,xn) reactions on 181

Ta shown in Figure 2 [6],

while there are no similar neutron-induced reaction data). They are altogether quite useful for

nuclear technology achievements and the nuclear model validation as well, due to their large

cross sections and decreased number of questionable parameters which may affect the

calculated cross sections. The present proposal, if adopted for EAR-2, opens a vast field of

research on a variety of medium and heavy isotopes, not just 197

Au and 181

Ta.

References [1] C.T. Papadopoulos, R. Vlastou, E.N. Gazis, P.A. Assimakopoulos, C.A. Kalfas, S. Kossionides

and A.C. Xenoulis, Phys. Rev. C 34, 196 (1986). [2] A. Tsinganis, M. Diakaki, M. Kokkoris, A. Lagoyannis, E. Mara, C. T. Papadopoulos, and R.

Vlastou, Phys. Rev. C 83, 024609 (2011). [3] Experimental Nuclear Reaction Data (EXFOR), http://www-nds.iaea.or.at/exfor. [4] M. B. Chadwick et al., Nucl. Data Sheets 107, 2931 (2006). [5] V. Avrigeanu, M. Avrigeanu, and F.L. Roman, in: Third International Workshop on Compound

Nuclear Reactions and Related Topics, Sept. 19-23, 2011, Prague, EPJ Web of Conferences (in

press); http://www-ucjf.troja.mff.cuni.cz/cnr11/presentations_dir/avrigeanu_v.pdf. [6] M. Avrigeanu, R.A. Forrest, F.L. Roman, and V. Avrigeanu, in: Advances in Nuclear Analysis and

Simulation (PHYSOR 2006), Vancouver, Sept. 10-14, 2006, ANS, La Grange Park, 2007, p. D102.

10 20 30 40 50 60 700.0

0.5

1.0

1.5

2n

1n

No PE

Shubin+ (1994, MEDNL-2)

TALYS-0.64 [(p,3n)179m

W]

STAPRE-H.6 [(p,3n)179m

W]4n

3n

Thomas+ (1967): (p,n)

Thomas+ (1967): (p,2n)

Birattari+ (1971): (p,3n) ?

Birattari+ (1971): (p,4n)

Zaytseva+ (1994)

Michel+ (2002)

181Ta(p,xn)

(

b)

En (MeV)

http://www-nds.iaea.or.at/exfor

http://www-ucjf.troja.mff.cuni.cz/cnr11/presentations_dir/avrigeanu_v.pdf


of 69

09.02.2012

TITLE:

5. Fission cross section of the 230

Th(n,f) reaction

AUTHORS:

R.Vlastou (NTUA), M.Kokkoris (NTUA), N.Colonna (INFN-Bari), M. Calviani (CERN)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 230

Th

MASS: 10mg

REACTION(S) OF

INTEREST:

(n,f)

ENERGY RANGE OF

INTEREST:

0.01-50 MeV


MGAS: MicroMegas fission detectors currently used for Pu fission measurements in EAR-1.


measurement:



pulse than in EAR-1 results in an increase by a factor of 250 of the reaction to

background signal from sample activity with respect to EAR-1.

- The higher fluence allows using samples of only few mg: i) the material in these

quantities is available (not in the case of hundreds of mg), ii) samples can then be



- The higher fluence allows to measure low reaction cross sections at a high accuracy

impossible to be achieved in EAR-1.


measurement:


The availability of a type-A experimental area makes it possible to measure Pu isotopes,

which is not the case of instance in other TOF facility.

The high instantaneous fluence makes is possible to measure low mass samples. These

sample would have to be of small mass because 239Pu is a strategic material and because

241Pu (14y) has a very high activity.


Nuclear technology: The 230

Th(n,f) cross section is important for the Th-U fuel cycle which

has several advantages with respect to radioactive waste management and nonproliferation,

as compared with the conventional U-Pu fuel cycle. There is lack of data in the low energy

region and the existing data of the 230

Th(n,f) reaction cross section above the fission

threshold, cover only the energy range up to 25 MeV and with many discrepancies and low

accuracy.

Basic physics: The peak in the excitation function of the cross section measured by James

et.al. [1] in the vicinity of 720 keV, has been interpreted in terms of a vibrational mode

resonance state in the secondary minimum of a double-humped fission barrier, with K=1/2

[2]. Better experimental data in the energy region close to the fission barrier may enable us to

reveal finer structures of the fission mode and extract all possible spectroscopic information

on the states associated with the second well of the fission potential.


3x1018

protons (equivalent measuring time in EAR-1 = 5 years)


of 69

09.02.2012


pages maximum):

The goal of these measurements would be the accurate and consistent fission cross sections

of the natural but very rare isotope 230

Th, which is very interesting for the investigation of the

fission process and very important for the Th-U fuel cycle. To achieve improved design

calculations for thorium-based reactors, the determination of neutron-induced reaction cross

sections on isotopes of thorium is required. Little data exist on the 230

Th(n,f ) cross section in

the energy range relevant for fast reactor systems with poor agreement between the

measurements, as can be seen in Fig. 1. The most recent measurements are based on an

indirect determination of the 230

Th(n,f) cross section via the surrogate 232

Th(3He,α) reaction,

over an equivalent neutron energy range of 220 keV to 25 MeV [3]. Due to the lack of data

both in the low and high energy region and to the discrepancies among the existing data,

especially in the energy region around and below the fission threshold, this reaction is

included among those of High Priority Request List in NEA [4].

The measurements are difficult due to the fact that 230

Th is a very rare isotope and can be

produced in very small quantities, while the fission cross section is relatively low (less than

1b), thus requiring a very high neutron flux in order to achieve accurate measurements. The

particularities of the 2nd

experimental area at n_TOF, with the high-intensity neutron flux

covering the full energy range of interest from 0.01 to 50 MeV)[5], make such a

measurement feasible.

The setup would consist of a MGAS fission detector loaded with a thin sample of 10mg of

Thorium oxide of high isotopic composition (>99%), spread over a circular area of 8 cm

diameter on an Al backing. The cross section will be deduced with respect to the 255

U(n,f)

cross section. The Thorium and Uranium samples would be placed in 2 MGAS detectors,

which have already been successfully used at n_TOF in the EAR-1.

Fig. 1. Fission cross section data for 230

Th available in literature.

References

[1] G.D.James et. al. Nucl.Phys. A189(1972)225.

[2] G.Yuen et al. Nucl.Phys. A171(1971)614.

[3] B. L. Goldblum et al. Phys.Rev.C80(2009) 044610

[4] NEA High Priority Request Listhttp://www.oecd-nea.org/dbdata/hprl/

[5] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study,

CERN-INTC-2011-032; INTC-O-013 (2011)



of 69

09.02.2012

TITLE:

6. First measurement of the capture (and fission) cross sections of the fissile 245

Cm

AUTHORS: E. Mendoza (CIEMAT), D. Cano-Ott (CIEMAT), E. González (CIEMAT), C. Guerrero (CERN)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 245

Cm

MASS: 0.1mg

REACTION(S) OF

INTEREST:

(n,) (n,f)

ENERGY RANGE OF

INTEREST:

Thermal to 1-20 MeV


C6D6 (or a new Total Absorption Calorimeter)+MGAS: Combination of C6D6 (or a new Total

Absorption Calorimeter) capture detectors with MicroMegas fission detectors.


measurement:





- The higher fluence allows using samples of less than 1 mg: i) the material in these




- The higher neutron fluence allows using a 1 or 2 thin samples (instead of the several

necessary at EAR-1). The samples can be placed close to the C6D6 detectors (more

sensitive to the sample‟s position) or at the centre of a 4 calorimeter (as the TAC

used in EAR-1).

MAIN ADVANTAGE(S) OF EAR-2 for this particular measurement:


The availability of a type-A experimental area offers the required safety conditions for

measuring radioactive samples such as the Cm isotopes.

The high instantaneous fluence makes it possible to measure enriched samples only available

in small amounts, as it is the case for the Cm isotopes.


Nuclear Technology: The capture cross section of 245

Cm is particularly difficult to measure

due to the five times larger fission background. Accurate cross section data for 245

Cm are of

utmost importance for the transmutation of the long lived nuclear waste. The simultaneous

measurement of capture and fission will improve the accuracy, reducing systematic

uncertainties associated to the normalisation to absolute cross-sections.


3x1018



pages maximum):

The goal of this measurement is to measure the capture and fission cross sections, as well as

alpha ratios, of the fissile isotope 245

Cm (and heavier Cm isotopes when available). The


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09.02.2012

energy range of range of interest comprehends both the resolved and unresolved resonance

regions. The (n,) and (n,f) cross sections of 245

Cm lack the required accuracy for the

calculations for evaluating the performance of different transmutation strategies. The 245

Cm

fission cross section is indeed included among those of in the NEA High Priority Request

List [1], and it has been measured several times, including a measurement at n_TOF in the

past [3]. However, there is no capture or transmission data available in EXFOR. Some

capture measurements have been performed in the past [4], but without the required

accuracy: the measurement was performed using a nuclear explosion as a neutron source,

with several Cm isotopes together, and the capture cross section of 245

Cm was only

“obtained” between 20 and 60 eV. Other measurements estimate some resonance integrals

[5]. The very poor information of neutron capture in 245

Cm leads to big differences between

evaluations, as it is presented in the left panel of Figure 1.

The (n,γ) measurement is very difficult because of the large fission γ-ray background due to

the unfavourable fission to capture cross section ratio, and also because independent

measurement of the two reaction channels may not help to solve the existing problems of the

evaluations. For that reason, a simultaneous (n,) and (n,f) measurement is highly desirable.

The particularities of the 2nd

experimental area at n_TOF (the highest intense neutron flux

covering the full energy range of interest: thermal to 20 MeV) [2] make such a measurement

feasible.


~1.7µg/cm2 of

245CmO2 each placed in between 2 C6D6 -ray detectors or inside the TAC.

The samples would be placed back-to-back in 2 MGAS detectors in order to cancel out the

effects of the non-uniform angular distribution of fission fragments. All detector types have

been individually tested at n_TOF in the EAR-1 and a test experiment with the combined use

of the TAC + MGAS for measuring the 235

U(n,γ) cross section has been performed as well.

The only open question would be their response to the -flash in EAR-2 and the associated

high energy limit that could be reached.

The expected counting rates for 2x0.05 mg samples and a total of 3x1018

protons per

measurements are shown in Figure 1.

Fig.1. Different evaluated capture cross sections on the left and expected counting rates for

a total of 3x1018

protons allocated on the right.

References [1] NEA High Priority Request List http://www.oecd-nea.org/dbdata/hprl/ [2] E. Chiaveri et al., n_TOF Experimental Area 2 (EAR-2) preliminary feasibility study, CERN-

INTC-2011-032; INTC-O-013 (2011) [3] M.Calviani et al., “The neutron-induced fission cross-section of 245Cm: new results from

n_TOF”, to be submitted to Pys. Rev. C.



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09.02.2012

[4] M.S. Moore, G.A. Keyworth, Analysis of the Fission and Capture Cross Sections of the Curium

Isotopes, Phys. Rev. C, v3 (1971) 1656. [5] V.D.Gavrilov,V.A.Goncharov, Thermal Cross Section and Resonance Integrals of Radiation

Capture of Neutrons for Cm-244-248 Isotopes and for Cf-250, Journ.: Atomnaya Energiya Vol.44,

Issue.3, p.246.


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09.02.2012

TITLE:

7. Neutron capture measurement of the s-process branching point 79

Se

AUTHORS:

C. Domingo-Pardo (IFIC), C. Guerrero (CERN), R.Reifarth (Univ.Frankfurt), J.L. Tain

(IFIC).

SAMPLE(S) OF

INTEREST:

ISOTOPE: 79

Se

MASS:2.7g, approx.

1/10000 content in 79Se,

the rest mostly 78

Se

REACTION(S) OF

INTEREST:

(n,)

ENERGY RANGE OF

INTEREST:

Thermal to 1 MeV, of particular

interest for astrophysics 1keV to

100 keV.


BaF2-TAC


measurement:

Higher neutron fluence delivered in shorter time, which allows measuring (n,g) CS with the

sub-mg content of 79

Se in the sample (mostly 78

Se), in a reasonable beam-time of aobut 30-40

days. Furthermore, the goal of the present measurement is to extract later the Maxwellian

averaged neutron capture cross section in the relevant neutron energy range for astrophysics,

i.e. between 1keV and 100 keV. In this respect, the high TOF resolution of EAR-1 is less

relevant than the high neutron fluence that will be available at EAR-2. Finally, the one order

of magnitude lower neutron flux at EAR-1 would imply a prohibitively low capture-to-

background ratio and too long beam-time request.


measurement:


With respect to other high-resolution TOF facilities, the advantage of n_TOF-EAR-2 is the

same, but of even higher magnitude, than when comparing to n_TOF-EAR-1 (see section

above). Both the high instantaneous flux, as well as the availability of type-A experimental

area are of relevance for measuring a sample, which has an intrinsic activity of more than 10

kBq.


Astrophysics: Selenium-79 is a branching point in the slow neutron capture process (s-

process) with relevant implications in nucleosynthesis and in AGB stellar models. Indeed, the

products of the s-process nucleosynthesis after 79

Se are the s-only 80,82

Kr, whose solar system

abundance are accurately known from chemical analysis of pre-solar grains. This

information, in conjunction with the experimental CS of 79

Se, will allow one to extract

reliable conditions for the neutron density, as well as the role of the main and weak s-process

contributions to the nucleosynthesis in the A=80 mass region.


One month.


pages maximum):

Introduction:


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09.02.2012

Neutron capture reaction cross sections are a crucial element for stellar models, which

describe the production of elements heavier than iron. The slow neutron capture process (s-

process) is responsible for about half of the solar system isotopic abundances of elements

heavier than iron. This mechanism operates in AGB stars, where two alternate burning shells,

one of H and an inner of He, surround an inert degenerate CO-core. S-process elements are

produced by neutron captures on seed nuclei in the He-rich intershell, between the two

burning shells. The He intershell is periodically swept by convective instabilities induced by

He-burning runaway, where 12

C is synthesized. The main neutron source for AGB stars of

mass M<4M⊙ and T=108 K is the

13C(ɑ,n)

16O reaction (main s-process component) with a

neutron density of Nn<107 cm

-3[1]. In the case of more massive stars, the s-process takes

place during pre-supernova evolution, i.e. during convective core He-burning and convective

C-shell burning. In this case also the neutron source 22

Ne(ɑ,n)25

Mg is activated at

temperatures of 3.5·108 K and neutron densities larger than 10

10 cm

-3 can be reached. Such

high neutron density strongly favours the production of neutron rich nuclides for s-process

branching. Therefore, a different s-process pattern is expected depending on whether the first

or the second neutron source reaction is more active. The knowledge of the neutron capture

cross section sin such branching nuclei together with observed element abundances allow to

estimate the neutron density at the s-process site. Since such s-process branching nuclei are

unstable, direct neutron capture measurements are generally very difficult at conventional

neutron facilities.

The reaction to be considered in this expression of interest for n_TOF EAR-2 is 79

Se(n,).

The knowledge of the neutron capture cross section of 79

Se provides a crucial test for the

understanding of s-process nucleosynthesis in massive stars[2]. The unstable isotope 79

Se is

an s-process branching point, and it is located in a region where two scenarios may

contribute, the one from massive stars (weak s-process component) and AGB stars main s-

process component). Furthermore, the 79

Se(n,) reaction is particularly relevant, because it

leads to the production of the s-only 80

Kr and 82

Kr isotopes, which are shielded from the rapid

neutron capture process by their stable (or almost stable) isobars 80

Se and 82

Se (t1/2 = 1020

a).

The solar abundances of 80

Kr and 82

Kr isotopes are well characterized from chemical analysis

of pre-solar grains [3], thus allowing to extract reliable conclusions about the stellar

conditions by comparing predicted abundances (based on the stellar models and the 79

Se(n,)

versus the measured abundances (in the pre-solar grains).

Furthermore, since 80

Se is stable, the activation method is not applicable in this case. Thus,

the only possibility to measure the 79

Se cross section is to use a neutron source with a very

intense flux and detect prompt gamma-rays emitted in the capture events.

Experimental approach:

The n_TOF EAR-2 facility at CERN represents, at present, the unique place to accomplish

the challenging measurement proposed here.

A sample of 79

Se can be produced with sufficient mass by several methods. At present, the

most promising technique seems to be the irradiation of an enriched 78

Se sample, of about 2.6

g (2 cm diameter, 2 mm thickness) at the ILL facility of Grenoble or at the HIRF reactor of

Oak Ridge. After one week of thermal neutron irradiation in these facilities, one would obtain

about 1018

and 1019

atoms of 79

Se, respectively. A high enrichment of 78

Se in the primary

sample is convenient in order to isolate reliably the CS contributions from the several Se

isotopes, which have similar Q-values. Another possibilities for the production of a 79

Se

sample rely on the chemical separation and purification of either fission residues, or from

decommissioned spallation targets (e.g. from ISOLDE or from PSI).

The measurement of the capture events will be carried out with the Total Absorption

Calorimeter (TAC). This way, a high detection sensitivity for the channel of interest will be

achieved. Other techniques, like the PHWT with C6D6 are excluded owing to the low


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09.02.2012

efficiency of the gamma-ray detectors, which would lead to an excessively long measuring

time and too low signal-to-background ratio.

References

[1] F. Käppeler, et al., Rep. Prog. Phys. 52, 945 (1989).

[2] G. Walter, H. Beer, F. Käppeler, et al., Astron. Astrophys. 167, 186 (1986).

[3] R.S. Lewis, S. Amari, and E. Anders, Geochim. Cosmochim. Acta 58, 471 (1994).

[4] Z.Y. Bao, H. Beer, F. Käppeler, et al., Atomic Data Nucl. Data Tables 76, 70 (2000).


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09.02.2012

TITLE:

8. Cross section and angular distribution of fragments from neutron-induced fission of 232

U

AUTHORS: L. Tassan-Got (IPNO), L. Audouin (IPNO), L. S. Leong (IPNO), C. Paradela (USC), I. Duran (USC)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 232U

MASS: 4x60mug

REACTION(S) OF

INTEREST: (n,f)

ENERGY RANGE OF

INTEREST: Thermal to 100 MeV


PPAC assembly : 10 PPAC detectors enclosed in a single reaction chamber


measurement:

Higher neutron fluence delivered in shorter time, which allows measuring fission of mug

samples:




- The higher fluence allow using samples of only few tens of mug : the material in this

quantities is available and possible to handle (not the case of mg). Samples should be

produced by the usual electro-deposition method


measurement:

The n_TOF EAR-2 neutron fluence covers in a single shot the full energy range of interest

The availability of a type-A experimental area makes it possible to measure very active U

isotopes.

The high instantaneous fluence makes is possible to measure high activity samples (like the

mentioned 232U).


Nuclear Technology: 232

U is an especially dangerous byproduct of the Th/U3 fuel cycle because

its decay chain includes the emission of a 2,6 MeV gamma ray. 232

U is mainly formed by (n,2n)

reaction on 233U. Its concentration in the spent fuel will be the main factor in the definition of

gamma shielding required to handle this fuel. Hence, the measurement of its fission cross section

is a key data in order to quantify its presence in spent fuel. Unfortunately, the very high activity

of this nuclei has hindered neutron-induced reactions measurements: only 6 measurements have

ever been performed, the most recent in 1986, and several energy ranges (< 6 eV, 5-50 keV, > 10

MeV) have never been measured at all.

BEAM TIME (assume neutron fluence of EAR-1 x25):

4x1018

protons


pages maximum):

The goal of this measurement would be to measure the fission cross section, as well as the

angular distribution of fission fragments, of 232

U. We will take advantage of the full energy range

of the n_TOF EAR-2 facility, from thermal region to several MeV: 232

U, although being an even-

even nuclei, behaves as a fissile (thresholdless) isotope. As all even-even nuclei, it is expected to


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09.02.2012

exhibit strong fission anisotropies.

Existing data are extremely scarce: only 6 data sets exist. They cover only parts of the energy

range of interest and, where overlap exists they are significantly discrepant with one another.

The preparation of the experiment, its decommissioning and the measurement itself are all

exceptionally challenging because of the very high activity of the samples of interest : the 69 yr

half life corresponds to an activity of 840 MBq/mg. Samples of 60 mg would have an activity of

50 MBq each, which appears as a manageable value for PPACs detectors. Obviously such ultra

low mass samples may only be measured at the 2nd

experimental area at n_TOF.

A total of 1,5x1018

protons are necessary for a clean measurement of fission cross sections for a

15 mg target: hence a factor of 60 in the sample mass and a factor 25 of gain in the fluence

results in a requirement of 4x1018

protons. This number of protons should be obtained in the

shortest possible time, as the harmfulness of the targets will increase with time due to the build-

up of the 232

U decay chain.

The setup would consist in a 10 PPAC fission detector, with 4 232

reference target of 235

U, each placed in between 2 PPACs. In empty target slots, inert material

would be in order to limit the stress of each detector to the reactions taking place in one single

target. Targets and detectors would be inclined by 45 degrees with respect to the beam direction

in order to measure all angles with respect to the neutron beam, a configuration which has been

successfully used at n_TOF in the EAR-1.


of 69

09.02.2012

TITLE:

9. Destruction of the cosmic γ-ray emitter 26

Al by neutron induced reactions

AUTHORS:

P.J. Woods (University of Edinburgh), C. Lederer (University of Frankfurt) F. Käppeler

(Karlsruhe)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 26

Al

MASS: ~μg

REACTION(S) OF

INTEREST:

(n,p)(n,ɑ)

ENERGY RANGE OF

INTEREST:

Thermal to 1 MeV


Large solid angle Double-Sided Silicon Strip Detector ΔE-E telescope system


measurement:

~25 higher neutron flux – low statistics experiment to due small sample of radioactive target

material.


Nuclear Astrophysics: The first observation of γ-rays from the radioisotope

26Al (t1/2 ~ 1 Myear) showed

nucleosynthesis is ongoing in our galaxy. Detailed satellite telescope observations indicate

that galactic 26

Al is predominantly produced in different burning stages of massive Wolf-

Rayet stars [1]. Recent reaction network calculations have shown the rates of the neutron

induced destruction reactions 26

Al(n,p) and 26Al(n,α) are the major source of uncertainty in

predicting the amount of 26

Al material ejected into the interstellar medium by Wolf-Rayet

stars. The limited experimental data sets on the 26

Al(n,p) and 26Al(n,α) reactions exhibit

major discrepancies, and do not cover all the relevant stellar burning energy range.

Consequently stellar modelers have identified new measurements on these reactions as the

highest priority for predicting 26

Al abundances. We propose to measure the 26

Al(n,p) and 26Al(n,α) cross-sections at the n_TOF facility from thermal energies up to 1 MeV covering

the entire energy range of astrophysical interest.


Reduction of statistical (and systematic errors) are critical for this experiment, a good

measurement could be made with ~3 months running, possibly in parasitic mode although

this would compromise Silicon detector geometry/solid angle for experimental set-up.

SUMMARY/FIGURES/PREVIOUS DATA OR MEASUREMENTS/REFERENCES:

Trautvetter and Kaeppeler reported the first measurement of the 26

Al(n,p1)26

Mg reaction [2]

which was followed up by a more comprehensive study [3], in which measurements of both

the 26

Al(n,p0)26

Mg and 26

Al(n,p1)26

Mg reactions were reported. A later study of the 26

Al(n,p1)26

Mg reaction, and a first study of the 26Al(n,α0)

23Na reaction, both performed at the

Los Alamos Neutron Science Center (LANSCE), were reported by Koehler et al. [4]. The

cross-section data from ref. [4] are plotted in Figures 1 and 2 for these reactions as a function

of neutron energy. The 26

Al(n,p1)23

Na reaction rate was found to be in disagreement with that

of [3] in the one region of burning temperature overlap between the two sets of measurements

(see Figure 16 from ref. [5]).


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09.02.2012

Fig. 1. Cross-section data for the

26Al(p,n1)

26Mg reaction [4].

Fig.2. Cross-section data for the

26Al(n,α)

23Na reaction [4].

Illiadis et al. [5] highlighted this as the key issue that needs to be resolved for calculating

abundances of 26

Al produced in the convective carbon shell and explosive neon carbon shell

burning phases of Wolf-Rayet stars. Koehler et al. suggested discrepancies between the

measurements could be due to angular distribution effects, since the experiments covered

different angular ranges, or more likely differences in normalization procedure [4]. Ref [3]

used the 197

Au(n,γ)198

Au reaction for normalization, which Koehler et al. comment is

sensitive to any small excess in thermal neutrons in the beam [4], whereas Koehler et al. used

the 26

Al(n,p1)26

Mg cross-section at thermal neutron energies, and the 10B(n,α)

7Li reaction, for

normalization purposes [4]. Most recently, De Smet et al. reported measurements of the


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09.02.2012

26Al(n,α0+1)

23Na reactions using the GELINA neutron time-of-flight facility at the Institute for

Reference Materials (IRMM), Geel [6], with the 10B(n,α)

7Li reaction used for the neutron

flux determination. The GELINA measurements overlapped the lower neutron regime

covered by the LANSCE experimental study (maximum energy 10 keV [4]) and also

obtained data at higher energies, identifying a number of new resonances for the 26Al(n,α)

23Na reaction. A resonance at 6 keV was identified in both studies (the only one

identified in [4]) but there was disagreement in the total width Г. of the resonance. De Smet

suggested this may be due to the relatively small solid angle covered by the silicon detection

system used in the LANCSE measurements, rendering the measurements sensitive to

anisotropy effects - the 6 keV resonance was assigned to a p-wave [6]. The GELINA

measurements had a much larger statistical accuracy mainly due to the use of much larger

sample of 26

Al atoms [7]. Illiadis et al. noted the experimental discrepancies between these

low energy measurements of the 26Al(n,α)

23Na reaction rate (see Figure 15 from [5]), and the

absence of measurements of the 26Al(n,α)

23Na reaction rate in the higher energy astrophysical

burning regime above T~0.3-3 GK , and called for new measurements [5]. This regime

covers the burning temperatures for convective shell C/Ne burning, and explosive Ne/C

burning in Wolf-Rayet stars, and requires measurements up to 1MeV in neutron energy. In

particular, there may be hitherto unobserved strong resonances at high energies that can

significantly influence the Maxwellian Averaged Cross-Section (MACS) in the high

temperature burning regime.

References

[1] R. Diehl et al., Nature (London), 298, 445 (2006).

[2] H.P. Trautvetter and F Kaeppeler, Z. Phys. A318, 121 (1984).

[3] H.P. Trautvetter, H.W. Becker, U. Heinemann, L. Buchmann, C. Rolfs, F. Kaeppeler, M.

Baumann, H. Freiseleben, H.-J. Lutke-Stezkamp, P. Geltenbort and F. Gonnenwein, Z. Phys.

A323, 1 (1996).

[4] P.E. Koehler, R.W. Kavanagh, R.B. Vogelaar, Yu. M. Gledenov and Yu. P. Popov,Phys.

Rev. C56, 1138 (1997).

[5] C. Iliadis, A. Champagne, A. Chieffi and M. Limongi, Ast. Phys. J. Supp. 193, 16 (2011).

[6] L. de Smet, C. Wagemanns, J. Wagemanns, J.Heyse, J. van Gils, Phys. Rev. C76, 045804

(2007).

[7] C. Ingelbrecht et al., Nucl. Inst. Meth. A480, 114 (2002).


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09.02.2012

TITLE:

10. Measurement of 7Be(n,p)

7Li and

7Be(n,)

4He cross sections, for the cosmological Li

problem

AUTHORS: N. Colonna (INFN), M. Calviani (CERN), F. Kaeppeler (KIT), C. Massimi (INFN), D. Schumann

(PSI), C. Guerrero (CERN), E. Berthoumieux (CEA), M. Barbagallo (INFN), A. Ferrari (CERN),

E. Chiaveri (CEA)

SAMPLE(S) OF

INTEREST:

ISOTOPE: 7Be

MASS: < 10 µg

REACTION(S) OF

INTEREST:

(n,p) (n,)

ENERGY RANGE OF

INTEREST: Thermal to 1-20 MeV


To be defined, possibly MicroMegas, Si-based E-E telescope, diamond detector


measurement:

Higher neutron fluence delivered in shorter time, which allows measuring sub-mg samples:




- The higher fluence allow using samples of less than 1 mg: i) the material in this

quantities is available from PSI, ii) samples can then be produced by “thin sample”

techniques like electrodepositing, evaporation, etc. (not the case for thick samples:

pressed powders, matrices, etc.)

- The higher fluence minimizes the background related to the natural radioactivity of

the sample, an aspect particularly important in this measurement, considering the very

short half-life of 7Be (53 d), which results in an extremely high specific activity.


measurement:


The availability of a “type-A” experimental area makes it possible to measure the cross

section for short-lived 7Be isotope, which is not the case of several other facilities.

The high instantaneous fluence makes it possible to measure high activity samples (like the

mentioned 7Be isotopes), and low cross sections as the one expected for the (n,ɑ) reaction.


Nuclear Astrophysics: The (n,p) and (n,) reaction on 7Be are of interest for Big Bang

Nucleosynthesis. In particular, they are responsible for the destruction of 7Be, from which

7Li

is produced. An accurate measurement of their cross section could provide important

information on the so-called “Cosmologic Li problem”. Very few data exist on this reaction,

in particular on the (n,) one, which reflects in completely different evaluated cross sections.

According to some recent calculations of BBN, a factor of 100 in the (n,) cross section may

ease the problem on the primordial Li abundance.


5x1018

protons per isotope (equivalent measuring time in EAR-1 = several years)


pages maximum):


of 69

09.02.2012

One of the important unresolved problems of Astrophysics is the so-called “Cosmological

Lithium problem”. It refers to the large discrepancy, of more than a factor of 3, between the

abundance of primordial 7Li predicted by the standard theory of Big Bang Nucleosynthesis

(BBN), and the value inferred from observations of galactic halo dwarf stars, the so-called

“Spite plateau halo stars” (for a review on the subject, see Ref. [1,2]) . Several mechanisms

have been put forward to explain this difference: new physics beyond the Standard Model,

errors in the inferred primordial 7Li abundance from the Spite plateau stars and, finally,

systematic uncertainties in the nuclear physics inputs of the BBN calculations. Recently,

several measurements of charged-particle induced reactions have mostly ruled out a possible

solution of the cosmological Li problem based on conventional nuclear physics [3,4].

However, new and accurate measurements of neutron-induced reactions are necessary in

order to completely clarify this issue. In this letter of intent, we propose to perform a new

measurement of (n,p) and (n,) reactions on 7Be, of relevance for the cosmological

7Li

problem, as well as for basic nuclear physics. Given the intrinsic difficulty of the

measurements, related to the relatively short half-life of 7Be (53.29 d), the second

experimental area (EAR-2) at n_TOF could be one of the few facilities in the near future

where accurate results on these reactions could be obtained in the energy range of interest for

Big Bang Nucleosynthesis.

In standard BBN, primordial 7Li is mostly produced by electron capture of

7Be relatively

late after the Big Bang, when the Universe has cooled down sufficiently for nuclei and

electrons to combine into atoms. Therefore, the abundance of 7Li is essentially determined by

the production and destruction of 7Be in the early stages of BBN. The main reaction leading

to the production of 7Be is the

3He(α,γ)

7Be, while several reactions are responsible for its

destruction. Among them, the 7Be(n,p)

7Li followed by

7Li(p,α)

4He is one of the most

important. In 1988 Koehler et al. [5], measured the cross-section of the 7Be(n,p) reaction

from thermal to 13.5 keV at the 7 m station of the LANSCE neutron facility, Los Alamos.

The results were used to estimate the astrophysical reaction rate as a function of the

temperature. The authors found reaction rates 60 to 80% lower than previously thought, thus

excluding a significant impact of this reaction on the 7Li problem. It should be considered,

however, that for the reaction rate at high temperatures, the authors had to rely on some

assumptions, given the limited energy range covered by their measurement. Although it is

highly unlikely that a solution to the 7Li problem could be related to a large error in the

7Be(n,p) cross section, a more precise estimate of the reaction rates at temperatures between

0.3 and 1 GK (i.e. 25-80 keV) would be desirable to improve BBN calculations.

Together with the (n,p) reaction, the (n,) reaction could in principle contribute to the

destruction of 7Be during Big Bang Nucleosynthesis. However, the contribution of this

channel is neglected in BBN calculations at present, due to its much lower cross-section

compared to the (n,p) reaction. The figure below shows the comparison between the

evaluated cross sections, in ENDF/B-VII and RUSFOND-2010, for the two reactions. In the

energy range of interest, the (n,) cross section is between two and four orders of magnitude

lower than the (n,p) channel. It should be considered, however, that there are essentially no

direct measurements of this reaction in the energy range of interest for BBN. In EXFOR, a

single (n,) measurement at thermal energy performed at the ISPRA reactor is reported. For

this reason, an uncertainty of a factor of 10 is typically assigned to this reaction in BBN

calculations.

In view of the lack of data on the 7Be(n,)

4He cross section, it can not be excluded that

the astrophysical reaction rate for this reaction might be significantly underestimated (note

the opposite trend in the cross section between ENDF/B-VII.0 and RUSFOND-2010

evaluations). Recent BBN calculations [6] have estimated that a cross section a factor of 100

higher than currently thought may possibly account for a large reduction in the primordial 7Li


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09.02.2012

abundance, thus at least partially solving the 7Li problem. Although an error of two orders of

magnitude in the (n,) cross section is not very likely, it cannot be completely excluded. It is

therefore mandatory to measure the 7Be(n,) cross section in the keV energy range.

Fig.1: Comparison of the evaluated

7Be(n,p)

7Li and

7Be(n,)

4He cross sections in

ENDF/B-VII.0 (left panel) and RUSFOND-2010 (right panel) from 1 eV to 20 MeV

neutron energy.

The second experimental area at n_TOF would offer the unique opportunity to perform

the very difficult measurements of the important 7Be(n,p) and

7Be(n,) reactions, thanks to

the very high instantaneous neutron flux, and the possibility to cover a wide neutron energy

range, from thermal to over 1 MeV. Such a measurement would be the first to be performed

in this region, with the results of interest for other fields as well.

One of the main difficulties in the measurement is related to the small mass typically

available for a 7Be sample. In the measurement of Koehler et al., a sample of 90 ng was used,

which had to be produced not long before the measurement, due to the relatively short half-

life of the isotope. This problem could be easily solved at n_TOF, thanks to the recent

agreement between the n_TOF Collaboration and the RadWasteAnalytics group at PSI. The

long-time experience of this group, and the availability of a large amount of 7Be from the

cooling water of the SINQ spallation neutron source at PSI, guarantees for the production of

a sample of mass as high as 8 µg (100 GBq), just when needed for the measurement. The

main remaining difficulty for this measurement is related to the detector, which should be

able to clearly identify protons and alpha particles, with high background rejection capability.

For this aspect, the n_TOF Collaboration is currently investigating various possibilities, in

particular the use of high performance gas detectors (MicroMegas), Si-based E-E

telescopes, or devices based on diamond detectors. It should be considered that the energy of

the reaction products is very different, being 1.64 MeV for protons and several MeV for alpha

particles (the Q-value of the 7Be(n,) reaction is 19 MeV). We are therefore confident that

this very difficult measurement can be performed at n_TOF EAR-2, thanks of the

combination between high-flux, availability of a sufficient quantity of 7Be, and state of the art


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detection and acquisition systems.

References

[1] D. Tytler et al., Physica Scripta 85, 12 (2000)

[2] R.H. Cyburt et al., Phys. Rev. D 69, 123519 (2004)

[3] C. Angulo et al., Astrophys. J. 630, L105 (2005)

[4] O.S. Kirsebom and B. Davids, Phys. Rev. C 84, 058801 (2011)

[5] P. Koehler et al., Phys. Rev. C 37, 917 (1988)

[6] F.L. Villante, Private communication

proposal for n tof experimental area 2...

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