Department of Physics

Faculty of Nuclear Sciences and Physical Engineering

Czech Technical University in Prague

Neutron Production in Spallation Reactions of

Relativistic Light Ions on Thick, Heavy Targets

Report for the PhD State Exam, 2006

Antonın Krasa

Branch: Nuclear EngineeringSpecialization: Experimental Nuclear Physics

Supervisor: RNDr. Vladimır Wagner, CSc.Affiliation: Department of Nuclear Spectroscopy

Nuclear Physics InstituteAcademy of Sciences of the Czech Republic

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Abstract

The ADS idea and principle of spallation neutron source is described, main transmutationconcepts and current investigation trends in the world are mentioned with focus on exper-iments being performed in JINR Dubna. Measurements of the neutron field produced inp+Pb (at 0.7 – 2.0 GeV proton energy) and d+Pb (at 2.52 GeV deuteron energy) reactionsby the means of the Activation Analysis Method are compared with Monte-Carlo calcula-tions performed with the MCNPX 2.4.0 code. Simulated quantities are spatial distributionsand energy spectra of neutrons and protons, and yields of radioactive isotopes produced inactivation samples.

Key words: spallation reactions, neutron production, neutron transport, activation analy-sis method, Energy plus Transmutation setup, MCNPX code

Acknowledgments

My special thanks go to my excellent supervisor RNDr. Vladimır Wagner, CSc., for hispatience and permanent support of my work; the director of the Department of NuclearSpectroscopy at NPI Rez RNDr. Andrej Kugler, CSc., for valuable suggestions and newvisual angles on the topic; my colleague Mitja Majerle, graduated physicist, for his huge workon simulations and the mastery he reached in that; my colleagues Mgr. Filip Krızek, OndrejSvoboda, ing. Karel Katovsky for inspiring comments and hot discussions; Mgr. VladimırHenzl, PhD., and Mgr. Daniela Henzlova, PhD., for their brilliant ideas. I am gratefulto all of them and to all colleagues from the E+T collaboration for careful cooperationduring experiments, sharing their geniuses and attainments. My deep thanks belong also toJaroslav Frana, CSc., for the possibility of using the γ-spectrometer for long-lived isotopesmeasurements and his DEIMOS code for spectra analysis; the LHE JINR for the possibility ofusing the Synchrophasotron and Nuclotron accelerators and the LNP JINR for the possibilityof using the Phasotron accelerator.

This work was carried out under support of the Grant Agency of the Czech Republic(grant No. 202/03/H043) and under support of the Grant Agency of the Academy of Sciencesof the Czech Republic (grant No. K2067107).

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Contents

Abstract 2

Acknowledgments 2

Introduction 4

1. Accelerator Driven Systems (ADS) 51. Transmutation 52. Spallation 53. Alternative neutron production 84. Transmutation concepts 105. Advantages and disadvantages of ADS 116. Main experiments concerning ADS 11

1. Gamma-2 and Gamma-MD 112. Energy plus Transmutation (E+T) 123. Cross-sections measurements 144. Subcritical Assembly at Dubna (SAD) 14

2. Codes for modelling of high-energy nuclear reactions 16

3. Experimental instruments 181. Accelerators 18

1. Synchrophasotron 192. Nuclotron 203. Phasotron 22

2. Setups 231. Bare target, target surrounded by moderator 232. Energy plus Transmutation 24

3. Beam monitoring 284. Neutron Activation Analysis Method 32

4. Data analysis 35

5. Examples of results 371. Experimental results 372. Calculations 37

6. Conclusions and perspectives 40

References 41

4

Introduction

Increasing interest is being focused on the transmutation of long-lived fission products andminor actinides from nuclear waste, of plutonium from nuclear weapons, or of thorium (asan energy source). Different concepts of transmutation involve also the Accelerator DrivenSystems (ADS) [1] based on a subcritical nuclear reactor driven by an external spallationneutron source (section 1).

This study is a part of a complex research of ADS carried out by a collaboration of theNuclear Physics Institute of the Academy of Sciences of the Czech Republic (NPI AS CR)in Rez and the Joint Institute for Nuclear Research (JINR) in Dubna near Moscow, RussianFederation. The experiments were performed using the Synchrophasotron/Nuclotron accel-erator complex at the Veksler and Baldin Laboratory of High Energies and the Phasotronaccelerator at the Dzhelepov Laboratory of Nuclear Problems at JINR Dubna (section 3).Relativistic protons/deuterons interacting with a massive, cylindrical, lead target inducespallation reactions. The spatial and energetic distributions of the produced neutron fieldare measured by the activation of Al, Au, Bi, Co, In, Ta, and Y samples. The activities ofthe samples are measured by the HPGe detectors at the Dzhelepov Laboratory of NuclearProblems at JINR Dubna (of short-lived radioisotopes) and at the Department of NuclearSpectroscopy at NPI Rez (of long-lived radioisotopes). The yields of the radioactive nucleiproduced in these samples are calculated from their resulting γ-spectra (section 4).

The aim of the performed experiments is to check the accuracy of the model descriptionsand of the cross-section libraries used in the corresponding Monte-Carlo simulations (section2) of spallation reactions, and of the propagation of the produced high-energy neutronsthrough a thick target (section 5). Investigations of this energy domain were not of highinterest in the past because of its minor importance for classical light-water nuclear reactors.But reliable predictions of the relevant physical processes strongly dependent on the accuracyof available nuclear data.

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1. ACCELERATOR DRIVEN SYSTEMS (ADS)

1. Transmutation

Generally, nuclear transmutation1 is an arbitrary process in an atomic nucleus in whichits structure changes. In the case of the change in the number of neutrons, the nucleus’physical properties are changed (e.g., half-life, activity, radiation energy), in the case of thechange in the number of protons, the nucleus obtains also different chemical properties (e.g.,reaction rate, chemical coupling). Apparently, appropriately induced transmutations couldbe used to decrease the half-life of long-lived radioisotopes included in the high level waste,which is produced during nuclear fuel burnup in nuclear reactors. Radioactive nuclei areproduced during the operation of nuclear reactors in two ways:

• fission products are generated by the fission of 235U in classical reactors, and byfission of 239Pu in fast reactors,

• higher actinides or transuranic elements, representing 1.1% of the spent fuel, aregenerated by the neutron capture within the nucleus of 235U, 238U, or 239Pu and theirresultant β-decays.

Almost all nuclides produced these ways are unstable, with some of them having a half-lifeup to tens of millions of years, see Table I. Fission products are mostly above the line ofβ-stability. To transmute them to stable nuclides, they should capture one or more neutronsand then disintegrate by β-decay. To transmute higher actinides to stable nuclides, theyshould undergo neutron capture and consecutive fission which is a process that producesenergy and makes the method attractive from the point of view of energetics.

Table I: Fission products and higher actinides with half-lives greater than 1000 years (dataextracted from [4]).

fission product 79Se 93Zr 99Tc 107Pd 126Sn 129I 135Cshalf-life [years] 1× 106 2× 106 2× 105 7× 106 1× 105 2× 107 2× 106

higher actinide 234U 235U 236U 238U 237Np 243Am 245Cmhalf-life [years] 2× 105 7× 108 2× 107 4× 109 2× 106 7× 103 9× 103

higher actinide 246Cm 247Cm 248Cm 239Pu 240Pu 242Puhalf-life [years] 5× 103 2× 107 3× 105 2× 104 7× 103 4× 105

2. Spallation

As follows from the last paragraph, we use the concept of “transmutation” in the meaningof a conversion of radioisotopes with long half-lives to short-lived or stable ones. The trans-

1 Historical note: In 1919, E. Rutherford [2], as the first, demonstrated transmutation on example of14N + α →17O. In 1932, J. D. Cockroft and E. T. S. Walton demonstrated the first accelerator-driventransmutation, bombarded lithium target with 125-500 keV protons (from linear accelerator) and “trans-muted” lithium nucleus into two alpha particles: p + 7Li → α + α (they won the Nobel prize for this19 years later).

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High-energy fission Fission products

Evaporation

Spallation product

Intranuclear cascade

Incident particle

Heavy nucleus

n µ

n π

n

n γ

γ

Figure 1: A scheme of spallation (according to [8]).

mutation can be effectively done by the means of the placement into an intensive neutronfield.2 Even large neutron flux densities in a classical nuclear reactor (typically φ ∼ 1014

neutrons·cm−2·s−1) are not enough efficient for transmutation purposes. Required neutronflux density for ADS should be at least two orders higher [5]. The spallation reactions on athick target can be used as an intensive source of neutrons. The spallation reaction3 isthe process in which a light projectile (proton, neutron, or light nucleus) with energy froma about 100 MeV to a few GeV, interacts with a heavy nucleus (e.g., lead). Spallation hasfew stages (Fig.1):

• Fast direct stage (∼ 10−22 s) is called Intra-Nuclear Cascade (INC) [10], see Fig.2.De Broglie wavelength of 1 GeV particle is 1.24 fm and it interacts with individualneutrons and protons in the target nucleus, sharing its energy with them by elasticcollisions and starting up a cascade of nucleon-nucleon collisions. The size and form ofthe INC depends linearly on the number of target nucleons and on the incident particleenergy. At low energies (∼ 100 MeV), all interactions occur just between nucleonsand the process is called nucleon cascade. Gradually, with growing incident particleenergy, the threshold energies for particle production in nucleon-nucleon collisions areexceeded. Initially, pions are coming up (at energies of about hundreds of MeV), athigher energies (∼ 2 − 10 GeV) heavier hadrons are emitted. Produced hadrons canalso participate in the intra-nuclear cascade and interact between each other, what iscalled hadron cascade. Particles that obtained energy high enough to escape from the

2 The use of charged particles is examined as well, but the use of neutrons appears more practicable.3 Historical note: Spallation reactions have been discovered by B. B. Cunningham at Berkeley [6] in 1947,

where also the theoretical description was given by R. Serber soon after [7].

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Figure 2: A scheme of intra-nuclear cascade generated by a proton in a heavy nucleus withthe impact parameter b. The solid circles represent the positions of collisions, the open

circles represent the positions forbidden by the Pauli exclusion principle. The short arrowsindicate “captured” nucleons contributing to the excitation of the nucleus (taken from [9]).

nucleus are emitted mainly in the direction of the incident particle. The rest of theenergy is equally distributed among nucleons in the nucleus which is left in a highlyexcited state.

• The intra-nuclear cascade is not sharply separated from the equilibrium decay. In apre-compound stage (< 10−18 s), the pre-equilibrium emission can happen. In thecourse of this stage, fast particles or fragments may be emitted after each interactionbetween the incident or other cascade particle and a nucleon inside the nucleus. Theenergies of pre-equilibrium particles are greater than energies of particles emittedduring the equilibrium decay.

• Finally, the equilibrium stage comes up4. Energy is equally distributed throughoutthe nucleus that is in a highly excited state and loses its energy by the evaporationof neutrons5 or light charged fragments (e.g., deuterons, α-particles) with energies upto tens of MeV. The particles are emitted isotropically. When the nucleus does nothave enough energy to emit neutrons, it deexcites by γ− and β-transitions.

• In the case of a thick target, high-energy particles escaping from the nucleus in thecourse of spallation can induce further spallation reactions and generate inter-nuclearcascade. It relates mainly to neutrons because they do not lose their energy byionization losses, thus, among all emitted particles, they penetrate deepest into thetarget material.

4 A few other processes, called collectively fragmentation [11], are possible as well: emission of a heavyfragment, fission into two fragments similar in proton number, multi-fragmentation (production of manyfragments of relatively small charges), or break up into individual charged particles.

5 Fission products from high-energy fission (see previous footnote) also evaporate neutrons.

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• For some target materials, low-energy spallation neutrons (i.e., low-energy cascadeplus evaporation neutrons) can enlarge neutron production by (n,xn)-reactions.

Globally, the incident proton induces the production of a large amount of neutrons 6 withwide energy spectra. These neutrons can be used for transmutation of relevant nuclei.

3. Alternative neutron production

Spallation reactions induced by relativistic protons on thick targets are not the only pos-sibility to produce intensive neutron fluxes for ADS purposes. Other possibilities with areresumed in Table II.

Electron induced neutron production is one of investigated possibilities, mostly in the∼ 10 MeV range [14], recently also in the GeV range [15]. The principle consists is thegeneration of the cascade shower of electrons, positrons, and photons by the reactions ofelectrons with a thick target. Predominantly, bremsstrahlung photons (emitted by decel-erated electrons) interacting with a target in the giant resonance region produce neutrons.The direct interaction of electrons with nuclei is negligible (α = 1

137-times smaller) in com-

parison with photon reactions. Although, the cost to produce one electron is smaller thanone proton, the final amount of neutrons produced per incident electron is much smallerthan in the case of protons and the benefits of the use of protons prevail.

The fusion of deuterium and tritium catalyzed by a negative muon [16] is also a method forproducing neutrons. Muon captured on the orbit of deuterium or tritium cancels Coulombfield of this atom and enhances the probability of t-d fusion. Muon becomes available foranother cycle after fusion, until it decays or is captured by a heavy nucleus.

Table II: Neutron sources in point of ADS (according to [33]).

Nuclear reaction Beam energy Beam current [particle/s] Neutron yield [n/inc.part.](e,γ) & (γ,n) 60 MeV 1015 0.04

d(t,n)α 0.3 MeV 1019 10−5 – 10−4

Fission 0.025 eV 2.4Spallation 1 GeV 1015 30

The use of spallation reactions itself has several options (e.g., beam particle and energy,target and blanket material). For example, the deuteron-induced spallation reactions wereexplored mainly theoretically (using the LAHET+MCNP code systems [17] or the standardLiege INC model supplemented by the Dresner evaporation-fission model [18]). A set ofexperiments with 3.65 AGeV light nuclei beams have been carried out in JINR Dubna [19]in 1980’s. They have concluded that the number of the produced neutrons per one incidentproton as well as the power consumption for neutron generation are preferable for lightnuclei to protons in this energy range, see Table III. On the other hand, they cause highercontamination of the accelerator.

The key aspect for the choice of the target material is the number of produced neutronsthat is linearly dependent on its atomic number and density. The number of the produced

6 around 55 neutrons per one 1.6 GeV proton striking a large Pb target [5], around 70 neutrons per one 1.5GeV proton striking a large Th target [12].

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Table III: Relative consumption of energy for the production of one neutron in spallationreactions of relativistic protons and light ions on thick, lead target (taken from [19]).

Projectile 1H 2H 4He 12CEnergy [GeV] 3.65 8.1 3.65

Relative energy consumptionfor production 1 0.92 0.89 0.72 0.81of one neutronEnergy losses

per one ionization 8 4 4 6 11[%]

Decrease of energy consumptionin comparison with - 7±6 12±6 27±6 17±6

3.65 GeV protons [%]Decrease of energy consumption

after deduction difference - 3±6 8±6 25±6 20±6of ionization losses [%]

neutrons can be increased by the using of a fissile material. The number of the producedneutrons per one incident proton as a function of the target material and the beam energywere investigated for both thin and thick targets at the COSY accelerator at FZ-Julich [20].They concluded that the neutron production depend rather weakly on both target materialand beam energy in the case of thin targets and weakly on target material but strongly onbeam energy for thick targets.

Important parameters of the target material are also thermal conductivity, caloric recep-tivity, melting point, boiling point. Moreover, the target should have such a size7 that atonce it incepts main part of the high-energy cascade, and let spallation neutrons escape.

Range of protons in W

0

20

40

60

80

100

0 500 1000 1500 2000 2500E p [MeV]

Ran

ge [c

m]

Table of Isotopes (Bethe-Bloch formula)Monte-Carlo simulation (MCNPX code)COSY experiment (A.Letourneau)

Range of protons in Pb

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500E p [MeV]

Ran

ge [c

m]

Table of Isotopes (Bethe-Bloch formula)Monte-Carlo simulation (MCNPX code)COSY experiment (A.Letourneau)

Figure 3: Range of protons in tungsten and lead.

7 typically cylinder with 10 cm diameter, tens of cm long (e.g., range of 1 GeV protons in thick tungstentarget is about 30 cm, in lead target about 55 cm, see Fig.3).

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4. Transmutation concepts

Initially, the ADS principle based on a subcritical nuclear reactor driven by an externalspallation neutron source was designed to produce fissile material and has already beensuggested in the late 1940’s. Nowadays, this idea is also studied for transmutation of long-lived waste and for electrical power generation.

Thanks to the Lawrence’s invention of a cyclotron in 1929, the production of largeamounts of neutrons by high-power accelerators became possible. After participating theManhattan Project, E. O. Lawrence brought in the idea of the accelerator as a neutronsource with the intention to produce fissionable material. His MTA project (Materials Test-ing Accelerator) began at the Lawrence Livermore National Laboratory in California in 1950[21]. He proposed to irradiate various thick targets (U, Be, Li) by protons and deuteronsusing a cyclotron, to measure the cross-sections, neutron yields, and the feasibility of con-verting the fertile (depleted uranium or thorium) to fissile material (239Pu, 233U). That wasthe first motivation, because USA was dependent on foreign uranium sources. The MTAproject was closed after rich domestic uranium ores were found in the Colorado plateau.

During next decades, investigations important for the estimation of the efficiency ofvarious modes of transmutation were performed. For example, neutron yields and spectrain lead and uranium targets irradiated by relativistic protons and neutron cross-sections fora number of isotopes have been measured in JINR Dubna [19, 22].

First quite conceptual and complex study of the radioactive waste transmutation hasstarted at the end of 1980’s in JAERI (Japan Atomic Energy Research Institute). A long-term program for research and development on nuclide partitioning and transmutation tech-nology was called OMEGA [23] (Option Making Extra Gains from Actinides and FissionProducts). The program initiated the global interest in transmutation topic that startedfrom the beginning of 1990’s.

C. Bowman from LANL (Los Alamos National Laboratory) created detailed concept ofthe Accelerator Transmutation of Waste (ATW) [5] using thermal neutrons. He suggestedthe use of a linear accelerator with high-intensive proton current (∼ 250 mA) of 1.6 GeVenergy.

C. Rubbia from CERN (Conseil Europeen pour la Recherche Nucleaire = EuropeanCouncil for Nuclear Research) proposed a basic concept of the Energy Amplifier [12], alsocalled Accelerator Driven Energy Production (ADEP). As the name implies, it does not payinterest to the disposal of radioactive waste directly. This idea is based on the use of 232Th8 as a fuel for the production of fissile 233U:

n +232 Thγ−→ 233Th

β−, 22m−→ 233Paβ−, 27d−→ 233U.

ADEP reckons with a use of a 1 GeV cyclotron with smaller beam current than in LANL(12.5 mA) for transmutation by fast neutrons. In the case of using the Energy Amplifier forwaste transmutation, the fast neutron flux produced in EA could fission all higher actinides,while the thermal neutron flux in a classical nuclear reactor do not fission many of them.

Another idea of using “the ash” as the fuel is called the Accelerator Based Conversion ofplutonium (ABC). It is a proposal of burning 239Pu from struck nuclear warhead [25].

8 about three times more abundant element in the earth’s crust than uranium. Particularly, India, thanksto its large reserves of thorium, plans to base their nuclear power program on thorium [24].

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5. Advantages and disadvantages of ADS

Besides the elimination of the nuclear waste and energy generation, another advantage ofADS is its safety. The fissile material will have a composition such that a chain reactioncould not run there and neutrons will be generated from an external source (i.e., from spal-lation reactions in the target). Therefore, a nuclear accident caused by an uncontrolledchain reaction will be foreclosed. In any case, transmutations will not be the final solutionof nuclear waste problem. We will not be able to transmute completely all long-lived ra-dioactive waste, so permanent storage in deep, underground, geologically-stable repositorieswill be necessary anyway. Nevertheless, the ADS could significantly decrease the volume ofradioactive waste.

The main project problem of the accelerator driven facility is its size and high technologicrequirements to run it. It is not possible to build a small, functional facility that couldverify our assumptions. There exists a number of possibilities of how a real acceleratordriven facility could look. However, properties of the eventual project will considerablyinfluence its efficiency. If the facility were poorly designed, it would lead to a significantloss. Therefore, we must be able to describe the transmutation and related processes witha very high level of confidence. We should be able to describe as perfectly as possiblethe course of the spallation reactions between protons and target nuclei, the spatial andenergetic distributions of the produced neutron field, the transport of neutrons (throughvarious materials) which follows the spallation, and the probabilities of individual isotopetransmutations.

6. Main experiments concerning ADS

Therefore, lot of projects all around the world have been established to carry out experimentsfor nuclear data acquisition, for complement of the cross-section libraries, for testing theaccuracy of models describing spallation and transmutation reactions. The aim of suchinvestigations is to design the optimal parameters of accelerator, beam, target, and blanket.

In European scale, several projects to verify the fundamental physics principle of ADSwere established, e.g., n TOF [36], FEAT (First Energy Amplifier Test) [30], and TARC(Transmutation by Adiabatic Resonance Crossing) [31] in CERN; SATURNE [27] in France;TRADE (TRiga Accelerator Driven Experiment) [28] and TRASCO (TRAsmutazioneSCOrie) [35] in Italy; MYRRHA (Multi-purpose hYbrid Research Reactor for High-techApplications) [29] in Belgium; YALINA [34] in Belorussia; HINDAS project (High andIntermediate energy Nuclear Data for Accelerator-driven Systems) [26] using six facilitiesthroughout the Europe.

One of such places, where the investigation of ADS has been intensively carried out isalso JINR Dubna. Currently, there are several directions of this research there, see next fourparagraphs.

1. Gamma-2 and Gamma-MD

Gamma-2 project, under the leadership of V. M. Golovatyuk at LHE JINR, is instrumentalto study spallation neutron production by GeV protons on a thick target and influence ofmoderator on the produced neutron field [37]. The setup consists of a cylindrical target

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(Pb, U) of the diameter d = 8 cm and the length l = 20 cm surrounded by moderator,see Figure 4. Until now, several experiments have been carried out with the paraffinemoderator of the thickness of 6 cm (front part of the target was without any shielding).New setup called Gamma-MD with the graphite moderator of a cubic size (∼ 1 m3) willbe used in next experiment. The low-energy neutron spatial distribution is being measuredby the activation sensors of 139La (by (n,γ)-reaction) along the beam axis on the top of themoderator. Transmutation of higher actinides and fission products in moderated neutronfield is also being studied [38]. Thanks to the simple setup geometry, the experimentalresults from Gamma-2 experiments are useful for testing the accuracy of high-energy codes(see section 2).

Figure 4: Photos of the old Gamma-2 setup with La-sensors on the top of the moderator(left) and the new Gamma-MD setup (right).

2. Energy plus Transmutation (E+T)

E+T is a wide international collaboration (scientists from Armenia, Australia, Belarus,Czech Republic, Germany, Greeke, India, Mongolia, Poland, Russia, Serbia and Montene-gro, Ukraine) under the leadership of M. I. Krivopustov at LHE JINR. It uses the setup(Figure 5) of the same name, which consists of a 43 kg cylindrical Pb target with a 206.4 kgdeep-subcritical natural uranium blanket surrounded by a polyethylene shielding (the wholeassembly mass is 950 kg), for details see paragraph 3 2 2. The complex investigation withinthe frame of E+T project pursues the following aspects:

• the transmutation of fission products (129I) and higher actinides (237Np, 238Pu, 239Pu,241Am) by spallation neutrons [39–42, 76–78],

• the spatial and energetic distributions of spallation neutron by the activation analysismethod using Al, Au, Bi, Co, Cu, Dy, Fe, In, La, Lu, Mn, Nb, Ni, Ta, Ti, and Ysensors (neutron capture for thermal, epithermal, and resonance component, thresholdreactions for fast-energy component of neutron spectra) [79, 80, 85, 88], solid statenuclear track detectors [43–45], and nuclear emulsion techniques [46],

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1060

polyethylene ρ = 0.8 g/cm3

300

beam

280480

220

460

380

300 400

220

460

164

216

1060

300

1110

756

38steel+wood textolite

72

1 mm of Cd

polyethylene wood cover

Figure 5: Side and front sketch of the E+T setup (dimensions are in millimeters) and thephoto of the U/Pb-assembly in front of the polyethylene shielding.

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• tests of the accuracy of the computer codes for calculation of neutron spectra andtransmutation yields [82, 87].

3. Cross-sections measurements

The Phasotron accelerator is being used to study the cross-sections of the reactions ofprotons (Ep = 600 MeV) on thin targets (Figure 6) of fission products (129I), naturaluranium [49], and higher actinides (237Np, 241Am [50], 239Pu). Many produced isotopes(with wide spectrum of half-lives, from minutes until years) were observed, the comparisonswith computer codes were done [47] to check the tested models. In comparison with theinverse-kinematics method [48], the experimental method used in these experiments hasthe possibility to measure the yields of the meta-stable states of residual nuclei, of usingradioactive targets, and of better accuracy of the cross-sections. On the other hand, thismethod has lower sensitivity of registration and it is impossible to measure the yields of verylong-lived, stable, and very short-lived nuclei. The plan is to continue in the cross-sectionsmeasurements and to carry out experiments with 232Th, 238Pu, and 235U.

2

PROTON BEAM

Neutrons

48

36

25

14

37

26

15

7

15 mm

0.0488 mm

Aluminium foil

Uranium foil

First experiment5 min. irradiation

Second experiment27 min. irradiation

30 cm

By proton induced neutrons irradiation (32 min.)

PROTON BEAM

Neutrons

48

36

25

14

37

26

15

48

36

25

14

37

26

15

7

15 mm

0.0488 mm

Aluminium foil

Uranium foil

First experiment5 min. irradiation

Second experiment27 min. irradiation

30 cm

By proton induced neutrons irradiation (32 min.) Figure 1: Set-up of the experiment

II. EXPERIMENTAL METHODS – IRRADIATION, GAMMA MEASUREMENT, AND DATA HANDLING The experiment was carried out in the external beam of the JINR LNP Phasotron accelerator with a total beam current of 2.20 µA (2.02 µA respectively); the beam current on the targets was 0.8 µA. Proton irradiation was made in two steps – 5 min short irradiation with a total proton flux of 1.5·1015 for the detection of short-lived isotopes and 27 min long irradiation with a total proton flux of 8.09·1015 for the measurement of intermediate- and long-lived isotopes. There was also a third uranium-target sample (No.7 in Figure 1), which was placed 30 cm perpendicular to the beam on the plane of the targets No. 1-6 and which was irradiated by background neutrons produced in these targets. Targets made from natural uranium (consisting of three isotopes: 234U - abundance 0.0054 % and T1/2 = 2.455 (6) ·105 y, 235U - 0.7204 %; 7.038 (5) ·108 y and 238U - 99.2742 %; 4.468 (3) ·109 y [9]) metal foils were exposed to the proton beam with an energy of 660 MeV. The diameter of the irradiated target samples was 15 mm; the thickness was 0.0477 mm and their weights ca. 165 mg (Table 3). The experimental set-up is shown in Figure 1. The sample sets number one natU(1) (No. 1-3) and number two natU(2) (No. 4-6) were irradiated by the proton beam. A two-coordinate proportional chamber controlled the profile and the position of the beam during irradiation of the targets. The size of the beam in horizontal (x) and vertical (y) direction could be described by Gaussians with the FWHM(x) = 19.2 mm and FWHM(y) = 16.2 mm. Aluminum foils were used in order to monitor the beam. For monitoring purposes, the reaction 27Al(p,3pn)24Na was used. Good agreement gives also the reaction 27Al(p,10p11n)7Be, while 27Al(p,3p3n)22Na gives ca. 30% smaller values for the proton current (Table 1). For the current calculations, the following reaction cross-sections were used: σ(24Na) = 10.8 (7) mb, σ(22Na) = 15.0 mb, σ(7Be) = 5.0 mb [10]. The third uranium-target sample natU(3) was placed 30 cm perpendicular to the beam on the plane formed by the targets natU(1) and natU(2), and was irradiated by background neutrons produced by Resn)p(p,U A

Znat

(1,2) yx reactions. Final results for 11 neutron-induced reaction yields are shown in Table 6. In Table 5a, the final results for the 42 proton-induced reaction yields of long-lived residual nuclei are shown, while Table 5b displays upper limits for the yields of isotopes observed in the spectra but without final results determined. The final yields of intermediate- and short-lived isotopes produced in the proton-induced reactions will be presented at the ND2004 conference. Neutron results are presented as B-factor – number of residual nuclei ResA

Z per one incident proton and per one gram of the target.

Figure 6: Measurements of the cross-sections of natU(p,xpyn)AZRes on direct proton beam

from the Phasotron, Al-folis were used for beam monitoring (taken from [49]).

4. Subcritical Assembly at Dubna (SAD)

SAD [51, 52] is a project, under the leadership of V. N. Shvetsov at the Frank Laboratoryof Neutron Physics in JINR. Its aim is to construct the facility consisted of a replaceablespallation target (Pb, W) with a subcritical MOX blanket (UO2 + PuO2), using also thePhasotron accelerator. The motivation is to study neutron production in such a setup,power release, fission rates of higher actinides, and transmutation rates of fission products.Final design is not given yet, preparation of equipments and theoretical calculations of thesetup parameters are in progress, current draft is given on Figure 7. Basic tasks which willbe solved during project implementation are the investigations of:

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• the spallation product yields;

• the core neutron physics characteristics (spectral and angular flux distributions, powerrelease, prompt neutrons life-time, effective fraction of the delayed neutrons;

• the fission rates of minor actinides, uranium and plutonium isotopes;

• the transmutation rates for long-lived fission products;

• shielding efficiency;

• the accuracy of the computer codes and the nuclear data bases used for calculationsof the ADS characteristics.

Top cover

Exp. Chan.

Lead Target

Lower support frame

Target cooling

AC cooling

Lower reflector

Figure 7: Cross section of the SAD active core (left) and general view of the SAD core(taken from [51]).

16

2. CODES FOR MODELLING OF HIGH-ENERGY NUCLEAR REACTIONS

As mentioned above, there is a strong need of simulation codes for ADS assembly projec-tion. There are several simulation codes and combinations of these codes (LAHET+MCNP[53, 54], MCNPX [55], FLUKA [57], HETC [58], INCL (Liege INC) [59], SHIELD [60],CASCADE [61], CEM (Cascade-Evaporation Model) [62], GEM (Generalized EvaporationModel) [63], LAQGSM (Los Alamos Quark-Gluon String Model) [64], GEANT4 [65]) de-scribing spallation reactions, interactions of secondary particles, and the following neutrontransport through the target material. They are based on the mathematical Monte-Carlomethod9, and they use various physical models of spallation reactions and cross-sectionlibraries of neutron-induced reactions with nuclei.

Formerly, we used a combination of LAHET plus MCNP codes. LAHET (Los AlamosHigh Energy Transport Code) [53] can model spallation reactions and transport of nucleons,pions, muons, antinucleons with energy E ≥ 20 MeV. LAHET generates cross-sectionsfor individual processes. MCNP (Monte Carlo N-Particle Transport Code) [54] is able tomodel the transport of neutrons (and photons and electrons) in an energy range 10−11 MeV≤ E ≤ 20 MeV. It uses libraries of evaluated data (such as ENDF/B-VI) as a source of thecross-sections.

Currently, we use MCNPX (MCNP eXtended) code [55] and we are members of theMCNPX β-tester team. MCNPX improves and links the advantages of both LAHET andMCNP. MCNPX supports 34 particle types, the ability to calculate interaction probabilitiesdirectly with physics models for energies where tabular data are not available, and exploitsnew libraries of evaluated cross-sections up to 150 MeV [56]. The MCNPX simulations usemodels of individual stages of the spallation reaction (see also paragraph 1 2):

• INC models (Bertini [67], Isabel [68], CEM [62]) describe interactions between imping-ing particle and target nucleons during intra-nuclear cascade as a sequence of binarycollisions (valid for the incident particle energy in region about 20-2000 MeV, wherethe incident particle wavelength is smaller than a mean length between nucleons of thetarget nucleus, thereto, a mean free path of the incident particle in target nucleus isgreater than the inter-nucleon lengths) separated in space and time. The nuclear den-sity distribution is approximated by a step-function distribution, where the densitiesin regions with constant density are fitted to the folded Saxon-Woods shape.

• Pre-equilibrium models (e.g., MPM (Multistage Preequilibrium Exciton Model) [69])describe the process of energy equalization as a sequence of two-particle interactions,whereas the nucleus is, in each phase (i.e., after each interaction), defined as the num-ber of particles and wholes. This description of a nucleus is called the Exciton Model(an exciton is either a nucleon excited above the Fermi level or a vacancy under theFermi level). The nucleus comes near to the equilibrium particle-whole configurationwith each interaction of incident or cascade particle with other nucleons. When the

9 Monte-Carlo method [66] is a numerical technique used for simulating the behavior of various systems(from economics to nuclear physics) more complex than we otherwise can. In contrast to deterministicalgorithms, it is a stochastic method. It is based on an executing of many random experiments (with amodel of a system). The essential point is to have a high-quality generator of pseudo-random numbers (itis not necessary to use really random numbers). The result is a probability of some effect.

17

equilibrium state is reached, the pre-equilibrium model is replaced by evaporationmodel.

• Evaporation models (e.g., Dresner evaporation model [70]) describe the equilibriumdecay of an equilibrium nucleus with the excitation energy reached at the end ofthe pre-equilibrium stage. The probability of the nucleus decay into a certain channeldepends on level densities in a final channel and on the probability of a passage throughthe energy barrier.

• As a competitive process to equilibrium decay, high-energy fission can happen.MCNPX includes two models of residual nuclei fission: ORNL (Oak Ridge NationalLaboratory) model [71] for actinides with Z ≥ 91 and RAL (Rutherford Appleton Lab-oratory) model [72] which covers fission of actinides and subactinides with Z ≥ 71.High-energy parts above the range of INC physics usability are taken from FLUKA[57].

For details how we use the MCNPX code, see paragraph 5 2.

18

3. EXPERIMENTAL INSTRUMENTS

Our research group [73] at the Department of Nuclear Spectroscopy in NPI AS CR hasan eight-year experience [74–88] with experiments on the topic of ADS performed in JINRDubna, partly in cooperation with the international group “Energy plus Transmutation”.Our work is mainly focused on the measurement of neutron production in reactions of lightions in an AGeV range on a thick, metal target (by the means of the activation analysismethod, see section 3 4) and on the transmutations of long-lived radioactive samples (refinedfrom burned-up nuclear fuel) induced by this secondary neutron field.

In this section, the description of used experimental apparatus, i.e., types and parametersof accelerators, setups, irradiation data, and description of the measurement methodologyis given.

1. Accelerators

Principal research instruments in JINR Dubna include a Synchrophasotron/Nuclotron accel-erator complex (Figure 8), a Phasotron, three isochronous cyclotrons (120, 145, 650 MeV),and a pulse reactor (1500 MW pulse). Nowadays, the Nuclotron and the Phasotron are usedfor investigations in the transmutation field.

Figure 8: Accelerator complex of the Laboratory of High Energies, JINR Dubna (takenfrom [89]).

19

Table IV: Parameters of the Synchrophasotron, Nuclotron, and Phasotron accelerators inJINR Dubna (according to [90–92, 94]).

Name Synchrophasotron Nuclotron PhasotronType of proton superconducting space-varying field

accelerator synchrotron synchrotron phasotronDate of first operation 1957 1992 1984 (originally 1949)Tmax (protons) [MeV] 9000 12 800 659± 6Tmax (Z

A = 12) [AGeV] 4 6 -

Extraction time [s] 0.5 10 0.004Intensity (protons per cycle) 4 · 1012 3 · 1010 (design 1013) 1011

Maximal proton current 320 nA 1.6 nA 3.2µAVacuum [Pa] 10−4 10−8

Power consumption [MW] 8 1.5Magnetic field [T] 1.1 2.2Circumference [m] 207.3 251.5 5

Operational temperature [K] ambient 4.5 ambientCold mass [t] 36 000 80

1. Synchrophasotron

Figure 9: Magnet of the Synchrophasotron (left) and operating console of theSynchrophasotron (right) (taken from [89]).

The Synchrophasotron10 (Figure 9) was built under the leadership of V. I. Veksler at theVeksler and Baldin Laboratory of High Energies (LHE) during years 1953-1957. The firstcomputer was put into operation in 1975 to measure and control beam slow extractionparameters. It had the record parameters for that time, it accelerated protons up to E = 9GeV and light ions (including 28Si) for Z

A= 1

2up to E = 4 AGeV. Its main strikes were the

discovery of Σ− hyperon in 1953, acceleration of deuterons to relativistic energies 4.5 AGeVin 1971 (as the first accelerator in the world). Because the Synchrophasotron used a weakfocusing, focusing chamber had size about 0.5 m and magnets weighed about 36 000 tons

10 proton synchrotron; it has fixed-orbit radius, magnetic field increasing with time and variable frequencyof accelerating voltage

20

(it got into the Guinness World Records Book for the heaviest magnet). But it is definitelyout of service from 2002 and it was fully replaced by the Nuclotron.

Figure 10: Nuclotron ring (left) and operating console of the Nuclotron (right) (takenfrom [89]).

Figure 11: Nuclotron superconducting magnets. Dipole magnet (left) is anchored in thevacuum shell of the cryostat by 8 parts of stainless steel (m = 500 kg, l = 1462 mm, B = 2

T). Quadrupole magnet (right), m = 200 kg, l = 450 mm, grad B = 33.4 T/m (takenfrom [89]).

2. Nuclotron

Nuclotron (Figure 10) is the superconducting (SC), strong focusing (Figure 11) syn-chrotron. It was built under the leadership of A. M. Baldin during years 1987-1992 andit has worked since March 1993. The Nuclotron ring (with circumference of 251.5 m) isinstalled in the tunnel around the synchrophasotron, whereas the Nuclotron median planeis 3.7 m below the synchrophasotron one (Figure 8). The total cold mass of its magnet is

21

80 tons. In total, it has 96 SC dipoles and 64 SC quadrupoles. It accelerates protons up toE = 12.8 GeV and nuclei (including 238U) for Z

A= 1

2up to E = 6 AGeV. Basic research

proceedings at Nuclotron regards investigation in the fields of a pre-asymptotic manifesta-tion of quark and gluon degrees of freedom in nuclei, the study of the spin structure of thelightest nuclei, the search for hypernuclei, the study of polarization phenomena using polar-ized deuteron beams. There is also a number of projects being implemented in the frameof an applied research - radiobiology and space biomedicine, the impact of nuclear beamson the microelectronic components, the use of a carbon beam in cancer therapy, and trans-mutation of radioactive waste associated with the problems of the electro-nuclear energygeneration method [93]. Lately, Nuclotron has been suffered from adversities as a leakageof helium (December 2004) and an outage of electricity (owing to the storm in June 2005)and after-pollution of vacuum tube. Presently (March 2006), the Nuclotron is temporarilyout of order because of injector repair (estimated until summer 2006).

Nuclear spectroscopy Phasotron

Figure 12: Schematic view of the Phasotron and its beam lines (taken from [52]).

22

Figure 13: Phasotron (left) and operating console of the Phasotron (right) (taken from[94]).

Figure 14: Photos of the deflecting magnet OM-1 (left) and of the doublet of quadrupolelenses ML-3 (taken from [52]).

3. Phasotron

The first accelerator in Dubna and the largest accelerator in the world at that time wassynchrocyclotron11 with proton energy 560 MeV. It was built at the Dzhelepov Laboratoryof Nuclear Problems (LNP) during years 1947-9. The first operation of this machine wastimed to be on Stalin’s 70th birthday. The synchrocyclotron was reconstructed into a space-varying field Phasotron (Figure 13, 14) during 1979-84. The conversion allowed an increasein proton energy to 660 MeV and an about 20-times increase in the intensity of the ejectedproton beam to∼ 1011 per cycle. While the frequency of proton acceleration cycles of 250 Hz,it matches electrical current of ∼ µA (compare with 1000-times smaller value for the presentstate of the Nuclotron, see Table IV, and also compare the irradiation time and the integral

11 a cyclotron with varying frequency of the driving radio frequency (RF) electric field (to compensate forthe relativistic mass gain of the accelerated particles)

23

proton flux in performed experiments, see Table V). The ejected proton beam has a micro-structure, particle bunches of 10 ns duration follow one by one with an interval about 70 ns.The Phasotron has 10 beam channels (Figure 12) employed for basic research experimentsfor a study of rare particle decays, µ-catalysis investigations, nuclear spectroscopy at theYASNAPP-212 complex, and for the hadron therapy (in collaboration with the OncologicalScientific Centre of the Russian Academy of Medical Sciences). Presently (March 2006),the Phasotron is temporarily out of order because of the part damage during the fire inApril 2005 (repair for medical purposes estimated until spring 2006, for physical purposesyet unknown).

2. Setups

One can see an obvious development of increasing system complexity have been used byour group for neutron production measurement. Personally, the author participated 0.7,1.0, 1.26, 2.0 AGeV experiments using the “Energy plus Transmutation” setup and the 660MeV experiment at the Phasotron.

1. Bare target, target surrounded by moderator

Our group began experiments with the simplest setup of a bare, cylindrical target fromlead (see Figure 15) or tungsten, optionally surrounded by moderator. Five experimentswere carried out using proton beams from the Synchrophasotron. The main tasks of theseexperiments were to study the neutron field produced in the spallation target irradiatedby relativistic protons, to study the transmutation of radioactive iodine by these neutronsand the influence of moderators on the produced neutron field. Later, one experiment wascarried out using very intensive proton beam from the Phasotron, see Table V.

Table V: Experiments with a bare target and a target surrounded by moderator.

Proton energy [GeV] 1.5 1.5 0.885 1.3 2.5 0.66Date [dd-mm-yy] 24-06-98 25-06-98 05-11-99 21-06-00 14-12-03

Irradiation time [h:m] 8:35 2:03 3:17 2:02 0:10Proton flux [1013] 8.9 3.46 2.77 4.07 158Target material Pb W Pb Pb Pb

diameter [cm] 9.6 2.0 9.6 9.6 9.6length [cm] 50 60 50 48.6 45.2

Moderator material polyethylene - polyethylene - polystyrenedimensions [cm3] 106× 106× 111 - 106× 106× 111 - 52× 16.5× 16.7

Accelerator Synchrophasotron PhasotronPublished [75] [79] [78] [84]

Range [cm] (from [95]) 93 53 47 79 172 30

12 the abbreviation means (in Russian): nuclear spectroscopy on proton beam

24

Figure 15: Photos of a Pb target (d = 9.6 cm, l = 48.6 cm) with holders for beammonitors, placed in a wooden grid.

2. Energy plus Transmutation

Using the new Nuclotron accelerator, five experiments (Table VI) were carried out with amore complex setup called “Energy plus transmutation” (Figure 5). It is divided into foursections of 114 mm in length separated by 8 mm empty intervals, totally 480 mm (Figure16). Each section is composed of a cylindrical lead target (of 84 mm diameter) and ofa natural uranium blanket with a hexagonal cross-section (side length of 130 mm). Eachof the blanket sections contains 30 uranium rods of 36 mm diameter and 102 mm length.The lead target and the uranium rods are sealed in an aluminium cover of 2.0 mm and1.27 mm thickness, respectively. The target and the blanket are fixed by iron holders of4 mm thickness, front and back ends of every section are closed by aluminium plates of6 mm thickness (Figure 17)).

This U/Pb-assembly is fixed on a wooden-metal rack (362×505×72 mm3, see Figure 18)and surrounded by a biological shielding consisted of a cubic container (106×106×111 cm3)with walls from wood (thickness of 1 cm; the ribs of the box are made from wood barswith cross-section 5x5 cm2) filled with granulated polyethylene (CH2)n (ρ = 0.802 g.cm3)with an admixture of boron. The inner walls of the container are coated with a 1 mm thickCd layer (used for absorption of thermal neutrons). The front and the back ends of thesetup are without any shielding. The polyethylene moderates high-energy neutrons outgoingfrom the setup and then partly scatters them back. Herewith, the moderator creates ahomogeneous field of neutrons with energies 1 eV < E < 0.1 MeV inside the containerand, thus, disallows the study of spatial distribution of low-energy neutrons produced in thetarget. The influence of the shielding on high-energy neutron component (En > 1 MeV) isnegligible (see Figure 19).

E+T installation was irradiated with proton beams Ep = 0.7, 1.0, 1.5, 2.0 GeV anddeuteron beam Ed = 1.26AGeV (Table VI), the courses of irradiations are shown in Fig-ures 20, 21. The accuracy of the beam energy is estimated at the level of 0.5%. Thetotal numbers of beam particles were determined by activation of beam monitors, see nextparagraph.

25

DT I A AR M G E E T T E R

224

102 114 8 6

480TARGET LENGTH

Pb

U

84

Al

U

2

Figure 16: Side view of the U/Pb assembly (dimensions are in millimeters).

Figure 17: Photo of an unjointed target-blanket assembly. The aluminium plate coveringits front part is forwarded, detectors are fixed on a yellow, plastic foil pushed right.

26 1

62

46

31 31

Figure 1. Front view

anti-corrosive steel 305

4

72300

362

(a)

wood

Figure 2. Top view (from above)

362

24.5 68 26.5 13 24.5

505

(b)

The wooden-metal rack on which the Pb/U-blanket is located

505

62

10

300 31 31

362

anti-corrosive steel

wood

305

(c)

guide-rails for fixing of four sections of

the uranium blanket

Figure 18: Front (a), top (b), and general (c) views of the wooden-metal rack on which thefour-section U/Pb-assembly is located (dimensions are in millimeters).

27

Energy [MeV]

Neu

tron

flux

[M

eV-1

pro

ton-1

cm-2

]

without polyethylene and Cdwith polyethylene without Cdwhole setup

10-910-8

10-7

10-7

10-5

10-6

10-3

10-5

10-1

10-4

101

10-3

103

10-2

10-1

0.90

0.95

1.00

1.05

1.10

Energy [MeV]

Neu

tron

flux

rat

io

10-2 10-1 1 101 102 103 104

Figure 19: Influence of the polyethylene container and the Cd-layers on neutron spectra atX = 11.8 cm, R = 3 cm (left). Ratio of neutron fluxes (from high-energy region of the left

figure) between the whole setup (Pb+U+Cd+(CH2)n) and the setup only with Pb+U(right). MCNPX simulation of p+Pb at 1.5 GeV experiment.

0.7 GeV

0.0

1.0

2.0

3.0

4.0

0 10000 20000 30000 40000time [seconds]

inte

nsity

[1010

pro

tons

]

1 GeV

0.0

1.0

2.0

3.0

4.0

0 10000 20000 30000time [seconds]

inte

nsity

[1010

pro

tons

]

1.5 GeV

0.0

1.0

2.0

3.0

4.0

0 20000 40000 60000time [seconds]

inte

nsity

[1010

pro

tons

]

2 GeV

0.0

1.0

2.0

3.0

4.0

0 10000 20000 30000time [seconds]

inte

nsity

[1010

pro

tons

]

Figure 20: Proton beams intensities with E+T experiments.

28

Figure 21: Deuteron beam intensity with E+T experiment.

Table VI: Experiments with the E+T setup. ∗ the value of the beam integral for thedeuteron irradiation is preliminary, given by current integrator (probably, the value from

activation monitors will be considerably smaller).

Beam energy [AGeV] 0.7 1.0 1.5 2.0 1.25Beam particle p p p p d

Date [dd-mm-yy] 27-06-04 30-11-03 11-12-01 27-06-03 30-11-05Irradiation time [h:m] 8:51 6:03 12:03 7:43 8:00

Proton flux [1013] 1.47 3.40 1.14 1.25 4.7∗

Published [85] [41, 88] not yetRange [cm] (from [95]) 33 55 93 132

3. Beam monitoring

The total number of beam particles hit the target was determined by the current integratorsuffers from a significant systematic error that can reach tens of per cent (at least thedetermination of a relative change of the beam intensity during such a short time period asour irradiations is accurate enough, with error about 1%). Therefore, the total beam fluxand the beam profile were measured independently by three groups of E+T collaboration:

29

• Mr. W. Westmeier used a big circular 27Al monitor (with thickness of 0.03 mm) cutinto an inner circle with diameter of 21 mm, and three concentric rings with externaldiameters of 80, 120, and 160 mm (Figure 24). This Al-monitor was placed 100 cmin front of the target front in order to avoid interactions of backscattered neutronswith the monitor foils.

• Mr. I. Zhuk used solid state nuclear track detectors (SSNTD). This method is based onthe measurement of distributions of natural lead induced fission rates. The two sets oflead samples were placed just on the front of the target in different directions (Figure22). The same sets were placed also between first and second sections to check theparallelism between the beam and the target axes, and to measure the defocusing ofthe beam while passing through the target.

• Our group used two sets of activation monitors from 27Al and natCu (69.17% 63Cu,30.83% 65Cu), one set for the measurement of the beam flux and one set for themeasurement of the beam diameter and shape. Because we analyzed only these beammonitors, this technique is explained in more detail below.

Figure 22: Typical placement of the solid state nuclear track detectors used for beammonitoring (taken from [43]).

Beam flux monitors were composed of 8×8 cm2 27Al and natCu sensors with a thick-ness of 50µm and 25µm, respectively. In the case of Al, the yields of 27Al(p,3pn)24Na,

30

Figure 23: The traces of one bunch on Polaroid foils placed in front of the target (left) andbehind it (right). Photo from d+Pb at 1.26 AGeV experiment

Table VII: Parameters of irradiations (FWHM = 2.35σ for the Gaussian curve).

Proton energy [GeV] 0.7 1.0 1.5 2.0Beam integral [1013] 1.47(5) 3.40(15) 1.14(6) 1.25(6)

Beam integral on Pb target [1013] 1.04(8) 3.24(15) 1.10(5) 1.07(10)FWHM vertical [cm] 5.9(2) 4.1(3) 3.7(5) 5.4(3)

FWHM horizontal [cm] 5.9(2) 2.5(3) 2.4(5) 3.8(3)Position vertical [cm] -0.4(9) 0.2(2) 0.1(2) 0.3(2)

Position horizontal [cm] 0.2(2) 0.0(2) 0.3(2) -1.4(2)

27Al(p,3p3n)22Na, and 27Al(p,10p10n)7Be reactions were measured. They have well knowncross-sections with weak energetic dependence [97, 98]. The beam flux monitors were placed1 m in front of the target, what is sufficient to screen neutrons evaporated from the targetin the direction against the beam, which could cause competitive reaction 27Al(n,α)24Na[96]. In the case of Cu, the yields of many (p,X)-reactions were measured and the followingisotopes were identified: 24Na, 59Fe, 52Fe, 58Co, 57Co, 56Co, 55Co, 54Mn, 52Mn, 51Cr, 48Cr,48Sc, 47Sc, 46Sc, 44mSc, 57Ni, 48V, 43K, 42K. The same can be observed in the case of thedeuteron beam.

31

120160

8021

84

60

Figure 24: The position of group of nine Al and Cu beam monitors closely in front of thetarget (left) and the position of concentric Al-monitors 1 m in front of the target (right).

The beam geometry was also studied with the use of high-energy proton reactions on27Al and natCu. First, before the beginning of the proper irradiation, one bunch of beamparticles was sent to Polaroid foils (placed at the positions of the front and the end of thetarget) in order to check the shape, location, and direction of the beam, see Figure 23.A group of nine Al and Cu foils (size 2×2 cm2 with approximately 50 µm thickness) wasplaced closely in front of the target (Figure 24). The yields in different foils were comparedusing the assumption that the central foil is fully covered. The beam profile was fitted withthe 3D-Gaussian distribution (this approximation is good for the central part of the beam,but not for its tails).

The results obtained independently from the solid state nuclear track detectors and fromthe activation monitors can be seen in Table VII. The errors include statistical errors andinaccuracies of determination of the corresponding cross-sections of (p,X)-reactions, whichwere acquired by interpolation using EXFOR/CSISRS data base values [99].

We found out from simulations [86] that the accuracy of experimental data is not muchinfluenced by the uncertainties of the beam profile width, but it strongly depends on theuncertainties in the beam position. The uncertainty of the beam position is around 3 mm,what means uncertainties in the neutron field up to 10%.

32

4. Neutron Activation Analysis Method

The spatial distribution of the produced neutron field was measured by the ActivationAnalysis Method [100] that is based on the nuclear activation of the stable isotopes presentedin the sensors into unstable nuclides with appropriate half-lives. Detecting of the emittedradiation provides quantification of the amount of the newly formed radioactive nuclei. Therate of nuclear reactions

R = φnσ (1)

is given by the flux of the incident particles φ, by the activation cross-section σ (the proba-bility of the reaction between a projectile and a target nucleus), and by the number of thetarget nuclei

n =m

MNAθ, (2)

where m is a mass of the sample, M is an atomic weight of element, Avogadro’s numberNA = 6.022× 1023 mol−1, θ is isotopic abundance.

Figure 25: Samples of radio-chemical sensors used for activation analysis method, from leftto right: Al, Au, Bi, Co, In, Ta, and the calibration point-like source of 60Co.

Used activation sensors were packed into sandwiches compound of 27Al, 197Au, 209Bi,59Co, natIn, 181Ta, 89Y samples (Figure 25) and placed in the gaps between sections of theU/Pb-assembly (Figure 26). Aluminium, gold, and tantal samples had the 20×20 mm2

square shape (of ≈ 0.5 g, 0.3 g, and 0.8 weight, resp.), bismuth samples had the 25×25 mm2

square shape (of ≈ 6 g weight), indium samples had the 15×15 mm2 square shape (of ≈ 0.3g weight), cobalt and yttrium samples had the circular shape with the diameter of 10 mm(of ≈ 3 g and 1 g weight, resp.). A typical13 set of the activation sensors is shown in Figure

13 One can find exceptions, e.g., in cases of 0.7 and 1.0 GeV experiments, the last sandwich of the radial setwas placed not at radial distance R = 13.5 cm, but at R = 10.7 cm.

33

26. The first set of sandwiches is placed at a radial distance R = 3 cm from the target axisat five longitudinal distances X = 0.0, 11.8, 24.0, 36.2, 48.4 cm from the target front. Thesecond set is placed at a longitudinal distance X = 11.8 cm from the target front at fourradial distances R = 3.0, 6.0, 8.5, 13.5 cm from the target axis.

Figure 26: Scheme of a typical placement of activation sensors (dimensions are inmillimeters).

In the process of irradiation, the stable isotopes of sensor materials were transmuted by(n,γ), (n,α), (n,xnyp)-reactions into radioactive ones. Examples of studied neutron-inducedreactions, both with a threshold in neutron energy and without it, are shown in Table VIIIand Figure 27.

The activity of the irradiated sensors was measured by HPGe γ-spectrometers EG&GOrtec and Canberra companies. The coaxial GMX-20190-P detector had relative effi-ciency 28.3% and energy resolution (FWHM of 60Co at 1.33 MeV) 1.90 keV, the coaxialGR1819-7600SL detector had relative efficiency 18% and energy resolution 1.90 keV. Eachsensor was measured few times in order to identify isotopes with different half-lives.

34

Table VIII: Examples of threshold and non-threshold reactions studied by the ActivationAnalysis Method. The threshold energies were calculated (with help of [101] based on [102]tables) as the difference between outgoing and incoming particles masses. ∗ in the case of

27Al(n,α)24Na reaction, the nuclear coulombic barrier is necessary to be taken intoaccount, therefore, this Ethresh was estimated from ENDF/B-VI library [103].

reaction threshold energy half-life reaction threshold energy half-life[MeV] of product [MeV] of product

27Al(n,α)24Na 5.5∗ 14.959 h 197Au(n,γ)198Au - 2.69517 d209Bi(n,4n)206Bi 22.6 6.243 d 197Au(n,2n)196Au 8.1 6.183 d209Bi(n,5n)205Bi 29.6 15.31 d 197Au(n,3n)195Au 14.8 186.10 d209Bi(n,6n)204Bi 38.1 11.22 h 197Au(n,4n)194Au 23.2 1.584 d209Bi(n,7n)203Bi 45.2 11.76 h 197Au(n,5n)193Au 30.2 17.65 h209Bi(n,8n)202Bi 54.0 1.72 h 197Au(n,6n)192Au 38.9 4.94 h209Bi(n,9n)201Bi 61.4 1.8 h 197Au(n,7n)191Au 45.7 3.18 h209Bi(n,10n)200Bi 70.8 36.4 m 59Co(n,γ)60Co - 5.271 y209Bi(n,11n)199Bi 78.4 27 m 59Co(n,2n)58Co 10.6 70.82 d209Bi(n,12n)198Bi 87.9 10.3 m 59Co(n,3n)57Co 19.4 271.79 d59Co(n,4n)56Co 30.9 77.27 d 59Co(n,5n)55Co 41.2 17.53 h

Fig. 6. Examples of cross sections of reactions studied by the activation analysis method(single items in legend are sequenced in line with increasing threshold energy)

target axis) and on the right side of the polystyrene box (9.1 cm to the right fromthe target axis). The foils were located side by side all along the target in orderto determine the spatial distribution of the neutron ˇeld, see Fig. 7.

Fig. 7. A scheme of placement of activation foils at the ®Pb¯ set-up

In the second experiment the foils were placed at a radial distance of 3 cmfrom the target axis at ˇve longitudinal distances of 0.0, 11.8, 24.0, 36.2, 48.4 cm

6

Figure 27: Examples of cross-sections [99] of reactions studied by the activation analysismethod (single items in legend are sequenced in line with increasing threshold energy).

35

4. DATA ANALYSIS

The measured γ-spectra of irradiated sensors, covering a region approximately from 50 upto 3500 keV, were processed by the DEIMOS32 code [104] that provides a Gaussian fit ofγ-peaks taking into account the background fitted with a parabola. The acquired areas werecorrected for coincidence summing effects, for a peak efficiency of the HPGe detector, forbeam fluctuations and interruptions during irradiation, and for decay during irradiation,cooling, and measurement. The yields (i.e., number of activated nuclei per one gram ofactivated material and per one incident particle) of each produced isotope were determinedaccording to the relation

Nyield =Sγ(E)

Iγ(E)εp(E)COI

treal

tlive

B

mI(p)

exp(λtcool)

[1− exp(−λtreal)]

λtirr[1− exp(−λtirr)]

, (3)

where Sγ(E) is fitted area of peak of γ-transition with energy E, Iγ(E) is intensity of thisγ-transition per decay, εp(E) is detector efficiency, COI is coincidence correction factor (seenext paragraph), treal is real time of measurement, tlive is live time of measurement, B isthe correction for beam instabilities during the irradiation, m is mass of a sensor, I(p)is total beam flux, tcool is time from the end of the irradiation until the beginning of themeasurement, tirr is time of irradiation, λ = ln 2

T1/2is decay constant. The relation between

experimental yield Nyield(E) and neutron flux Φ can be derived with use of Fredholm’sequation as follows

Nyield =1

ArmuI(p)

∫ ∞

0Φ(En)σ(En)dEn, (4)

where Ar is specific atomic mass of a chemical element from which the sensor is made, mu

is unified atomic mass unit (mu = 1, 66× 10−27 kg). Integration of product of neutron flux

0.01

0.1

1

10 100 1000 10000Energy [keV]

Eu154 Eu133 Ba57 Co60 Co88 Y137 Cs Th

p

R p / t

152

154

133

57

60

88

137

228

Figure 28: The curve of peak efficiency εp(E) and the curve of the photofractionR = εp(E)/εt(E) (Ortec detector, d = 1.3 cm).

36

Table IX: Energies and absolute intensities of calibration standards (taken from [107]);calculated COI factors for used geometries at both detectors.

isotope E Iγ COI COI isotope E Iγ COI COI[keV] [%] (Ortec) (Canberra) [keV] [%] (Ortec) (Canberra)

60Co 1173.2 99.97 0.92 0.93 152Eu 1212.9 1.42 0.90 0.9260Co 1332.5 99.98 0.91 0.93 152Eu 1299.1 1.63 0.90 0.9257Co 122.1 85.60 1.00 1.00 152Eu 1408.0 21.01 0.99 1.0057Co 136.5 10.68 1.00 1.00 154Eu 123.1 40.79 0.90 0.92133Ba 81.0 34.06 0.93 0.94 154Eu 247.9 6.95 0.88 0.90133Ba 276.4 7.16 0.99 0.99 154Eu 591.8 4.99 0.88 0.90133Ba 302.9 18.33 1.00 1.00 154Eu 723.3 20.22 0.90 0.92133Ba 356.0 62.05 1.00 1.00 154Eu 756.8 4.57 0.88 0.90133Ba 383.8 8.94 1.00 1.00 154Eu 873.2 12.27 0.90 0.9288Y 898.0 93.70 0.92 0.94 154Eu 996.3 10.6 0.92 0.9488Y 1836.1 99.20 0.91 0.93 154Eu 1004.8 18.01 0.98 0.98

137Cs 661.7 85.1 1.00 1.00 154Eu 1274.4 35.19 0.99 0.99152Eu 121.8 28.37 0.90 0.93 154Eu 1596.5 1.80 1.27 1.20152Eu 244.7 7.53 0.91 0.93 228Th 238.6 43.5 1.00 1.00152Eu 295.9 0.447 0.88 0.91 228Th 241.1 4.10 1.00 1.00152Eu 344.3 26.57 0.93 0.94 228Th 277.4 2.30 0.84 0.87152Eu 367.8 0.816 0.90 0.92 228Th 300.1 3.28 1.00 1.00152Eu 411.1 2.24 0.85 0.86 228Th 549.8 0.114 1.00 1.00152Eu 778.9 12.94 0.91 0.92 228Th 583.2 30.6 0.89 0.91152Eu 867.4 4.25 0.90 0.92 228Th 727.3 6.58 0.97 0.98152Eu 964.1 14.61 0.96 0.97 228Th 860.6 4.50 0.80 0.84152Eu 1085.8 10.21 1.00 1.00 228Th 1620.7 1.49 1.01 1.01152Eu 1089.7 1.73 0.92 0.93 228Th 2614.5 35.86 0.89 0.91152Eu 1112.1 13.64 1.00 1.00

Φ = Φ(En) [neutron.(MeV.proton.cm2)−1] and cross-section σ = σ(En) of correspondingreaction is made over neutron energy En.

Calibration of detector efficiency was performed with the use of standard calibrationpoint-like sources (dimensions in order of tenths of mm, see Fig.25) 133Ba (with relativeerror of reference activity 2.0%), 57Co (1.8%), 60Co (1.7%), 137Cs (2.0%), 152Eu (2.0%),154Eu (2.0%), 88Y (1.7%), 228Th (1.5%) (and daughters products) with several gamma linesranging from 80 keV up to 2700 keV, see Table IX. All necessary corrections on possiblecoincidence summing effects were done and they are included in COI correction factor [105]in equation (3), see also Table IX. COI depends on the full energy peak efficiency εp(E)(i.e., the detector ability to detect the total energy of the γ-ray), and the total efficiencyεt(E) (i.e., the detector ability to detect such part of the γ-ray energy which is bigger thandetector differentiation ability). For example, for used geometry at Ortec detector, the peakefficiency curve εp(E) (fitted for different ranges with a polynomial function of third order,and with a linear function; of course in logarithmic scale), and the curve of the photofractionR = εp(E)/εt(E) [106] (fitted with polynomial function of second order in logarithmic scale)are shown in Fig.28.

37

5. EXAMPLES OF RESULTS

1. Experimental results

The yields of radioactive isotopes produced in Al and Au sensors in units of [g−1 proton−1]are presented on example of the 1.5 GeV experiment in Figure 29. The trends are identicalfor all three experiments. Dependencies of yields on the position along the target are givenon the left side, dependencies of yields on the radial distance from the target axis are given onthe right side of the figures. The delineated errors are only of statistical character (hardlyvisible on this scale). Systematic experimental errors, such as uncertainties of detectorefficiency or beam integral determination contribute about next 5%.

The longitudinal distributions of yields of all isotopes produced in threshold reactionshave maximum around 10 cm from the target forehead. However, the neutron field aroundthe setup is a complicated mixture of spallation, fission, moderated, and back-scatteredneutrons, the isotopes generated in threshold reactions are produced mainly by neutronsisotropically evaporated in spallation reactions. Thus, one could expect the maximum shouldbe in the centre of the target. But because of ionization losses and out-scattering of protons,the proton beam energy and intensity decrease along the target. Therefore, the maximumintensity of the fast neutron field is shifted from the centre to the target’s front.

The radial distributions of yields of all isotopes produced in threshold reactions decrease,as the intensity of spallation part of neutron spectra falls down with growing distance fromthe spallation target.

The production maximum of isotopes 198Au and 60Co produced in (n,γ)-reaction is evi-dently not so sharp in comparison with those produced in threshold reactions (in longitudinalas well as in radial distributions). The reason can be the different course of cross-sections ofneutron capture contrary to cross-sections of threshold reactions. The polyethylene shieldingpartly moderated high-energy neutrons outgoing from the setup and could scattered low-energy neutrons back. Herewith, a homogeneous field of low-energy neutrons was createdand this field gives constant contribution to the production of 198Au and 60Co in the wholesetup, what explains a very flat shape of its distributions.

2. Calculations

Monte-Carlo simulations of the neutron production in the setup and of the activation re-actions in the sensors were performed by MCNPX 2.4.0 code [55]. The intranuclear cas-cade of spallation reactions was simulated using the Bertini INC model, the MultistagePre-equilibrium Exciton Model was used for the pre-equilibrium emission of particles (onlynucleons and charged pions were taken into account). Simulations were done using SSW(Surface Source Write) card, neutron spectra Φn(E) and proton spectra Φp(E) were countedwith HTAPE3X program, see Figure 30. The calculated yields of produced nuclei weredetermined (using F4 tally with the FM multiplier card) by the convolution of the simu-lated spectra with the corresponding cross-sections σn(E), σp(E) (inbuilt in MCNPX fromENDF/B-VI library [103]):

Nyield(r, z) =1

ArmuI(p)sim

∫ ∞

0[Φn(E, r, z)σn(E) + Φp(E, r, z)σp(E)]dE, (5)

38

0.1

1

10

100

1000

-10 0 10 20 30 40 50 60Position along the target X [cm]

Yie

ld [1

0-6]

198Au 196Au 194Au 193Au192Au 191Au 24Na

0.1

1

10

100

1000

0 2 4 6 8 10 12 14 16Radial distance from the target axis R [cm]

Yie

ld [

10-6

]

198Au 196Au 194Au 193Au192Au 191Au 24Na

0.1

1

10

-10 0 10 20 30 40 50 60Position along the target X [cm]

Yie

ld [1

0-6]

206Bi 205Bi 204Bi203Bi 202Bi 201Bi

0.1

1

10

0 2 4 6 8 10 12 14 16Radial distance from the target axis R [cm]

Yie

ld [

10-6

]

206Bi 205Bi 204Bi203Bi 202Bi 201Bi

0.01

0.1

1

10

100

1000

-10 0 10 20 30 40 50 60Position along the target X [cm]

Yie

ld [1

0-6]

60Co 58Co 57Co 56Co 55Co

0.1

1

10

100

1000

0 2 4 6 8 10 12 14 16Radial distance from the target axis R [cm]

Yie

ld [

10-6

]

60Co 58Co 57Co 56Co

Figure 29: Longitudinal (left) and radial (right) distributions of yields of nuclei producedin Al, Au, Bi, and Co sensors (p+Pb at 1.5 GeV). The lines linking experimental points

are delineated to guide readers’ eyes.

where Ar is specific atomic mass of a chemical element from which the sensor is made, mu isunified atomic mass unit, and I(p)sim is the number of simulated incident protons (3× 106

for all cases). Apparently, the energetic spectrum is harder at the end of the target whencompared to its beginning, see Figure 30.

The simulations describe the shape of longitudinal distribution of yields very well, seethe example of 196Au in Figure 31. A quantitative agreement between experimental and

39

0 10 20 30 40 50

0.01

1

100

1e-06

0.0001

0.01

1

Neutrons per incident proton

Distance along the target [cm] Neutron energy [MeV]

Neutrons per incident proton

0 10 20 30 40 50

100 200

300 400

500 600

1e-06

0.0001

0.01

Neutrons per incident proton

Distance along the target [cm] Neutron energy [MeV]

Neutrons per incident proton

Figure 30: Longitudinal neutron spectrum at R = 3 cm in logarithmic scale (left) and afocus on high-energy component in linear scale (right). MCNPX simulation of p+Pb at 1.0

GeV experiment.

0.0

0.5

1.0

1.5

2.0

2.5

-10 0 10 20 30 40 50Position along the target X [cm]

exp/

sim

Au196 1.0 GeV Au196 1.5 GeV Au196 0.7 GeV0.0

0.5

1.0

1.5

2.0

2.5

0 5 10 15Radial distance from the target axis R [cm]

exp/

sim

Au196 1.0 GeV Au196 1.5 GeV Au196 0.7 GeV

Figure 31: Comparison of experimental and simulated yields of 196Au in longitudinal (left)and radial (right) directions.

simulated values is worse, the differences reach about 50 %. The trends of experimentaldata and simulations in the case of radial distributions are in perfect agreement for the 0.7and 1.0 GeV experiments. Discrepancies in absolute values do not exceed 40 %. Absolutelydifferent situation is for the experiment with the 1.5 GeV beam energy, the discrepancybetween the experimental and simulated values increases quickly with growing perpendiculardistance from the target axis up to more than two times. The preliminary results of 2.0 GeVexperiment show the same tendency. Such great difference cannot be caused by experimentaldata uncertainties. Even other INC models (as Isabel or CEM) included in MCNPX are notable to describe better the neutron production in the used setup. We assume, the reasoncould be in the cross-section libraries or in the INC models (maybe they do not pass forhigher beam energies, where the limit for usability indicates to be between 1.0 and 1.5 GeV).We hope to obtain exact identification of the sources of inaccuracies using detailed analysisof wide sets of experimental data.

40

6. CONCLUSIONS AND PERSPECTIVES

The high energy neutron production in spallation reactions of AGeV protons and deuteronsin a thick, lead target with the uranium blanket surrounded by the polyethylene moderatorwas studied. The shape and the intensity of the produced neutron field were measured bythe Activation Analysis Method.

Due to the hard part of the neutron spectrum in the U/Pb-assembly, isotopes producedin (n,xn)-reactions (the emission of up to x = 9 neutrons) with high threshold energy (up to∼ 60 MeV) were observed. The maximum intensity of the fast neutron field (En > 1 MeV)produced in the spallation target is located in the region around 10 cm from the targetforehead. The energetic spectrum becomes harder at the end of the target.

There is a good agreement between theoretical description of neutron production andexperimental data obtained for proton beam energies E ≤ 1 GeV, but an underestimationappears for E ≥ 1.5 GeV. We are going to identify all sources of uncertainties coming fromsimulation codes, using systematic analysis of complex sets of experimental data.

Usual frequency of E+T experiments is two times per year. Goals for near future are tocarry out experiments with

• higher beam energies - up to 3.0 GeV,

• heavier projectiles,

• graphite as a moderator (GAMMA-MD installation),

• extensions of Pb-target and U-blanket,

• Pb+Bi eutectic as a target,

• E+T setup without any moderator,

• helium-jet transport systems of fission-products, which are able to quickly transfershort-lived isotopes, with help of a helium flux, from their source to detector. Formerly,this experiment was planned for Dubna Phasotron [85], but because of the accidentlast year, the irradiation will be performed at Nuclotron.

Promises for longer-range future could be our participation in SAD [52] or IREN (IntenseREsonance Neutron source) [109] projects being constructed in JINR Dubna.

41

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