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Irradiations at the High-Energy Neutron Facility at iThemba LABS

A. Buffler, G. Reitz, S. Röttger, F. D. Smit, F. Wissmann (Eds.)

European Radiation Dosimetry Group e. V.

EURADOS Report 2016-02

Neuherberg, June 2016

ISSN 2226-8057

ISBN 978-3-943701-13-5

EURADOS Report 2016-02

Neuherberg, June 2016

Irradiations at the High-Energy Neutron Facility

at iThemba LABS

A. Buffler1, G. Reitz2, S. Röttger3, F. D. Smit4,

F. Wissmann3 (Eds.)

1 University of Cape Town, Department of Physics

Rondebosch 7700, South Africa

2 German Aerospace Center, Radiation Biology,

Linder Hoehe, 51147 Koeln, Germany

3 PTB Physikalisch-Technische Bundesanstalt

Bundesallee 100, 38116 Braunschweig, Germany

4 iThemba Laboratory for Accelerator Based Sciences

Somerset West 7129, South Africa

ISSN 2226-8057

ISBN 978-3-943701-13-5

European Radiation Dosimetry Group e. V.

Imprint

© EURADOS 2016

Issued by: European Radiation Dosimetry e.V. Postfach 1129 D-85758 Neuherberg Germany [email protected] www.eurados.org

The European Radiation Dosimetry e.V. is a non-profit organization promoting research and

development and European cooperation in the field of the dosimetry of ionizing radiation. It is

registered in the Register of Associations (Amtsgericht Braunschweig, registry number VR 200387)

and certified to be of non-profit character (Finanzamt Braunschweig-Altewiekring, notification

from 2008-03-03).

Liability Disclaimer

No liability will be undertaken for completeness, editorial or technical mistakes, omissions as well

as for correctness of the contents.

Members of the organising group

This irradiation campaign was organised by the following people, most of them are members of

EURADOS Working Group 11 “High Energy Radiation Fields”:

A. Buffler Local spokesperson

University of Cape Town (UCT) Department of Physics Rondebosch 7700, South Africa

F. Wissmann EURADOS spokesperson

Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany

D. Geduld University of Cape Town (UCT)

University of Cape Town (UCT) Department of Physics Rondebosch 7700, South Africa

S. Röttger R. Nolte

Physikalisch-Technische Bundesanstalt (PTB) Bundesallee 100, 38116 Braunschweig, Germany

F. D. Smit iThemba Laboratory for Accelerator Based Sciences (iThemba LABS) Somerset West 7129, South Africa

Members of the editorial group

This report was prepared by the following members of EURADOS Working Group 11 “High Energy

Radiation Fields”:

A. Buffler

G. Reitz

University of Cape Town (UCT) Department of Physics Rondebosch 7700, South Africa German Aerospace Center, Radiation Biology Linder Hoehe, 51147 Koeln, Germany

S. Röttger Physikalisch-Technische Bundesanstalt (PTB) Bundesallee 100, 38116 Braunschweig, Germany

F. D. Smit iThemba Laboratory for Accelerator Based Sciences (iThemba LABS)National Research Foundation Somerset West 7129, South Africa

F. Wissmann Physikalisch-Technische Bundesanstalt (PTB) Bundesallee 100, 38116 Braunschweig, Germany

List of contributors to the participants’ reports

S. Röttger

F. Wissmann

I. Ambrožová K. Pachnerová Brabcová

J. Kubančák

Physikalisch-Technische Bundesanstalt (PTB) Bundesallee 100, 38116 Braunschweig, Germany Department of Radiation Dosimetry Nuclear Physics Institute (NPI) AS CR Na Truhlarce 39/64, 180 00 Prague, Czech Republic

L. Hager

M. Caresana

F. Trompier

S. Mayer

Public Health England (PHE) CRCE, Chilton, Didcot, OXON OX11 0RQ, United Kingdom Dipartimento di Energia Sezione di Ingegneria Nucleare - CESNEF Politecnico di Milano (POLIMI) Via Ponzio 34/3 20133 Milano, Italy Institute de Radioprotection et de Sûreté Nucléaire (IRSN) 31, avenue de la Division Leclerc 92260 Fontenay-aux-Roses, France Paul Scherrer Institute, Radiation Safety and Security, 5232 Villingen, Switzerland

C. Domingo

M.-T. Romero Departament de Física Univ. Autònoma de Barcelona (UAB) Edifici C, Campus UAB, E-08193, Bellaterra, Spain

S. Burmeister

G. Reitz

University Kiel, 24118 Kiel, Germany German Aerospace Center, Radiation Biology, 51147 Köln, Germany

Acknowledgements

We highly appreciate the effort by the accelerator group of iThemba LABS which delivered an excellent beam during the two weekends in October 2011.

- i -

Content

Content ............................................................................................................................................................................... i

Abstract ............................................................................................................................................................................. iv

1 Introduction ............................................................................................................................................................ 1

1.1 EURADOS ............................................................................................................................................................ 1

1.2 High-Energy Neutrons ................................................................................................................................... 2

1.2.1 Secondary Cosmic Radiation .................................................................................................................. 2

1.2.2 Particle Therapy Facilities ......................................................................................................................... 4

1.3 High-Energy Accelerators ............................................................................................................................. 5

1.4 High-energy Quasi-monoenergetic Neutron Beams .......................................................................... 6

1.4.1 Overview ........................................................................................................................................................ 6

1.4.2 Neutron Facility at iThemba LABS......................................................................................................... 7

2 Irradiation Campaign ......................................................................................................................................... 10

2.1 Organisation ................................................................................................................................................... 10

2.2 Irradiations ...................................................................................................................................................... 10

3 Neutron Beam Metrology ................................................................................................................................. 11

3.1 Beam Characterisation ................................................................................................................................ 11

3.2 Spectral Fluence ............................................................................................................................................ 12

3.3 Spatial Beam Profile ..................................................................................................................................... 14

3.4 Determination of Dosimetric Quantities .............................................................................................. 17

3.5 Analysis of the Monitor Count Rates and the Determination of Fluence ................................. 18

4 Reports of the Participants ............................................................................................................................... 21

4.1 Participant Report: PTB ............................................................................................................................... 21

4.1.1 PTB Reference Instrument - πDOS ..................................................................................................... 21

4.1.2 Measurements........................................................................................................................................... 22

4.1.3 Results .......................................................................................................................................................... 22

4.2 Participant Report: NPI ................................................................................................................................ 25

4.2.1 Detectors description and analysis .................................................................................................... 25

4.2.1.1 Thermoluminescent Detectors (TLD) ...................................................................................... 25

4.2.1.2 Plastic Nuclear Track Detectors (PNTD) .................................................................................. 25

4.2.1.3 Liulin .................................................................................................................................................... 26

4.2.1.4 Tissue Equivalent Proportional Counter HAWK .................................................................. 27

4.2.2 Experiment ................................................................................................................................................. 27

- ii -

4.2.3 Results .......................................................................................................................................................... 27

4.2.3.1 Thermoluminescent Detectors .................................................................................................. 27

4.2.3.2 Plastic Nuclear Track Detectors ................................................................................................. 28

4.2.3.3 Liulin .................................................................................................................................................... 30

4.2.3.4 HAWK .................................................................................................................................................. 33

4.3 Participant Report: PHE ............................................................................................................................... 36

4.3.1 Objectives ................................................................................................................................................... 36

4.3.2 Materials ...................................................................................................................................................... 36

4.3.3 Analysis ........................................................................................................................................................ 37

4.3.4 Experimental set-up ........................................................................................................................... 37

4.3.5 Results and Discussion ...................................................................................................................... 37

4.3.6 Future Work ........................................................................................................................................... 42

4.4 Participant Report: POLIMI ........................................................................................................................ 43

4.4.1 Material ........................................................................................................................................................ 43

4.4.2 Response function ................................................................................................................................... 43

4.4.3 Calibration .................................................................................................................................................. 44

4.4.4 Irradiation and results ............................................................................................................................ 44

4.5 Participant Report: IRSN ............................................................................................................................. 46

4.5.1 Objectives ................................................................................................................................................... 46

4.5.2 Materials ...................................................................................................................................................... 46

4.5.3 Results .......................................................................................................................................................... 47

4.5.4 Conclusions ................................................................................................................................................ 48

4.6 Participant Report: PSI ................................................................................................................................. 49

4.6.1 Objectives ................................................................................................................................................... 49

4.6.2 Materials and Methods .......................................................................................................................... 49

4.6.3 Measurements and Results .................................................................................................................. 50

4.6.4 Conclusion .................................................................................................................................................. 50

4.7 Participant Report: UAB .............................................................................................................................. 51

4.7.1 SRAM SEU Based Neutron Detector .................................................................................................. 51

4.7.2 PADC based dosemeter ......................................................................................................................... 52

4.7.3 Results for the SRAM SEU detector .................................................................................................... 54

4.7.4 Results for the PADC based dosemeters ......................................................................................... 55

4.8 Participant Report: DLR/Kiel ...................................................................................................................... 58

- iii -

4.8.1 The Flight Radiation Environment Detector (FRED) .................................................................... 58

4.8.1.1 Instrument description ................................................................................................................. 58

4.8.1.2 Analysis of the measurements ................................................................................................... 59

4.8.2 The Phoswich Instrument for Neutrons and Gammas (PING) .................................................. 62


4.8.2.2 Analysis of the measurements ................................................................................................... 63

4.8.3 Radiation Assessment Detector (RAD) ............................................................................................. 65


4.8.3.2 Neutral Particle Measurement ................................................................................................... 66

4.8.3.3 -Background Detector ................................................................................................................ 67

4.8.3.4 Experimental Setup ....................................................................................................................... 67

4.8.3.5 Data evaluation and Results ....................................................................................................... 69

5 Summary ................................................................................................................................................................ 70

6 References ............................................................................................................................................................. 71

- iv -

Abstract Secondary high-energy neutrons are produced when high-energy particles (of several GeV) interact

with matter as it is the case for cosmic radiation impinging on the Earth’s atmosphere, during ion-

beam therapy or on shielding at high-energy particle accelerators. Especially, after traversing a

large amount of matter, the residual neutron energy spectrum exhibits two energy regions which

mainly contribute to the total ambient dose equivalent: around 1 MeV (evaporation peak) and

around 100 MeV. Particle detectors and dosemeters used in such a radiation environment need a

full characterization which requires measurements in neutron reference radiation fields. Up to 20

MeV in energy such fields are maintained by PTB in Germany, IRSN in France or NPL in United

Kingdom. At higher neutron energies the neutron facility at iThemba LABS together with the

neutron metrology by PTB provides the possibility to characterize instruments at neutron energies

around 100 MeV. This was the aim of the EURADOS campaign in 2011.


EURADOS Report 2016-02 1

1 Introduction

1.1 EURADOS

The European Radiation Dosimetry Group (EURADOS) coordinates research, development and

harmonization in the area of radiation dosimetry. The activities include a long record of

intercomparisons of dosemeter systems and numerical procedures, in order to harmonize dose

assessment in Europe.

During the last years, EURADOS has played a crucial role in research and development of radiation

protection procedures related to the exposure to cosmic radiation at civil aviation altitudes. This

topic is of special importance because the accumulated effective doses of air crew members mostly

exceed the annual limit for normal population of 1 mSv. Exposures can reach, in the absence of

solar particle events, up to about 7 mSv per year depending on the extent of the solar activity.

During extremely large solar particle events hitting the earth even higher exposure levels,

delivered in a couple of hours, may occur. Thus, this group receives the largest exposures of all

occupationally exposed persons. Since standard procedures of radiation protection cannot be

applied readily in the special case of cosmic radiation, the European Union has funded several

research projects focussing on the investigation of the radiation field encountered at aviation

altitudes as well as on the development of instruments suitable for monitoring the radiation

exposure during routine flight operations. These efforts led to a compilation of measured data

obtained during 2000-2004, which was published by the European Commission [EU 2004]. In the

meantime, the regulators in each EU member state have mostly implemented what is required by

the Directive 96/29/Euratom. This implementation process has significant differences depending

on the country [Thierfeldt 2009]. Scientist working in this field developed regular monitoring

activities and some of them cooperate in the framework of EURADOS. These projects have almost

covered a complete solar cycle since its last minimum around year 1996.

In a recent publication [Bottollier 2012], eleven codes were compared by using real flight routes

crossing the globe and covering a wide range of latitudes. In some European countries, these

codes are partly used for legal dose assessment for aircrew members or are used for purely

scientific purposes. The conclusion of this comparison was that

“The overall agreement between the codes, however, was better than ±20 % from the median.”

This conclusion can be attributed to the results of a large number of measurements carried out

during the last years and which have been used to validate some of the codes. Furthermore, one

has to emphasise that, based on the cooperation within the former EURADOS working group 5,

most of the measurement procedures, the characterizations and calibrations of instruments were

harmonized by intercomparisons in well defined reference fields or by joint in-flight

measurements. One good example is the DOSMAX project [DOSMAX 2004] which was funded by

the European Commission with 7 partners from 7 European countries which are all EURADOS

members.

Irradiations at the High-Energy Neutron Beam at TLABS

2 EURADOS Report 2016-02

1.2 High-Energy Neutrons

1.2.1 Secondary Cosmic Radiation

Ionizing radiation in the atmosphere is a result of the interaction of primary cosmic radiation with

the atoms of the Earth’s atmosphere. Cosmic radiation consists of two components, galactic cosmic

radiation (GCR) and solar cosmic radiation (SCR). GCR consist of 98 % hadrons and 2% electrons.

The hadronic component comprises 88% protons, 10% helium ions and 2% heavier ions. GCR enter

the heliosphere continuously from all directions. Their energy spectrum is modulated by the solar

activity leading to a lower flux of particles below a few GeV at high solar activity. SCR is a result of

solar flares and coronal mass ejections and consists mainly of protons and a small amount of

heavier ions. Solar particles reach energies up to several GeV with flux and energy spectra which

differ considerably and cannot be predicted. Large solar particle events hitting the Earth are called

Ground Level Events (GLE) and are very rare. The interaction of the primary particles with the atoms

of the atmosphere result in secondary particles such as photons, electrons and positrons, all being

part of the so-called electromagnetic component, a hadron component consisting of pions,

protons, neutrons and heavier particles as well as a meson component. The hadronic and the

electromagnetic showers are coupled by the decay of π0 into two photons and by photonuclear

reactions [Gaisser 1990, Grieder 2001]. The number of primaries entering the upper atmosphere

depends on the magnetic latitude. At polar latitudes particles of all energies can enter, whereas in

equatorial regions only high energy particles have access due to the Earth’s magnetic field. The

deeper particles penetrate the atmosphere, the more are lost through interactions, but causing a

build-up of secondary particles in the atmosphere. The intensity increases until it reaches a

maximum (Pfotzer-Maximum). After that the intensity decreases since the cascade dies out due to

absorption of the primaries. At the Earth’s surface muons dominate the rather low cosmic radiation

exposure.

The equivalent dose in the atmosphere strongly depends on geomagnetic latitude and altitude

(Fig1.2.1a, Schrewe). A high fraction of the dose equivalent is contributed by neutrons (see

Fig. 1.2.1.b) [Grieder 2001]. At flight altitudes of about 10 km the total dose equivalent comprises

about 40 % from neutron-, about 45 % from electron/positron and photon- (indicated in Fig. 1 by

”e-”), as well as 10 % from proton and pion-, and only 5 % from muon interactions. It is the neutron

component of this complex radiation field which poses the greatest challenge for dosimetric

systems since the neutron contribution can be only measured via the linear energy transfer of

secondary charged particles produced in their interaction with the detector material. From the so-

called “recoil spectra” the energy spectra of the neutron energy spectra can be calculated via an

inversion method. A prerequisite is the knowledge of the detector response as a function of

neutron energy.



Figure 1.2.1a: Rates of H*(10) estimated by adding IC and the NM 500X dose readings in dependence of altitude and latitude. Every data point corresponds to a measuring time of 300 s.

The importance of investigating dosimetry systems at high neutron energies is evident from

Fig. 1.2.2, where the red line indicates the spectral distribution of the neutron component at

aviation altitudes. The pronounced peak structure around 1 MeV is due to evaporated neutrons

from target fragmentation, whereas the second peak at about 100 MeV is attributed to knock-on

reactions. This is due to the broad minimum of the relevant neutron total cross sections at this

energy. It is, therefore, obvious that the response of any instrument to neutrons with energies

around 100 MeV is an important consideration for the dosimetry of secondary cosmic radiation.

An appropriate instrument for this purpose is the Tissue Equivalent Proportional Counter (TEPC)

[Lindborg 1999, Schrewe 1999]. The technique of in-field calibration of other dosemeters using a

TEPC as reference instrument needs a rather precise knowledge of the TEPC response to

photons/electrons and especially to neutrons [Schrewe 1999a, Schrewe 2000]. This can only be

achieved in well-defined neutron beams with known neutron fluence and spectral distributions.

Smaller and more robust instruments are needed to meet the operational aspects at flight

altitudes.



1.2.2 Particle Therapy Facilities

During the last decade a new topic emerged with the operation of high-energy ion-beam therapy

facilities with ion beam (proton and 12C) energies up to 450 MeV/u. The secondary radiation field

produced during radiation therapy leads to an additional but unwanted exposure of the patients.

An example is shown in Fig. 1.2.3. The measured neutron spectra were produced by a 12C ion beam

hitting a water phantom leading to neutron energies up to a few hundreds of MeV [Gunzert-Marx

2009]. In order to determine the ambient dose equivalent in such a high energy field, dosemeter

systems with a high sensitivity at neutron energies up to 400 MeV are required. Here, neutrons are

not the only contributions to the emitted field. Also light ions like deuterons, tritons and He ions

are emitted and produce a large fraction of the dose outside the water phantom. This makes the

dose measurements even more complicated since the response of the dosemeter systems at these

energies is also not well known.

Figure 1.2.1b: Calculated contributions to the dose equivalent as a function of the altitude. At ground level 80% of the total radiation exposure is due to muons. At flight altitudes (about 10 km) the contribution of neutrons is of the order of 40 %.



Figure 1.2.2: The neutron spectra of secondary cosmic ray neutrons measured by a Bonner Sphere spectrometer at ground level and flight altitudes (the two top curves) in comparison with the neutron spectrum measured at GSI within the CONRAD project (see text) behind think concrete shielding

Figure 1.2.3: Neutron energy spectra measured at different angles for neutrons produced by a 200 MeV/u 12C ion beam irradiating a water phantom [Gunzert-Marx 2009].

1.3 High-Energy Accelerators

A further radiation protection problem has arisen in the last years, i.e. the radiation protection at

workplaces at high-energy accelerators (with beam energies of several GeV) providing high-



intensity beams. Even outside of the extensive/thick shielding around these facilities the

monitoring of stray radiation fields is very difficult. As in the case of dosimetry at flight altitudes,

neutrons contribute the largest fraction of the dose equivalent. The monitoring of such a radiation

environment can be achieved using active detectors, passive dosemeters, or a combination of the

two. A recent review of the techniques and instrumentation used for area monitoring can be found

for example in Refs. [Silari 2007] and [Agosteo 2008]; more extensive information is given in Ref.

[Bilski 2006] and in the references quoted therein.

A significant effort has been made within EURADOS to coordinate an intercomparison at the

heavy-ion accelerator facility of GSI (Darmstadt, Germany), the so-called CONRAD project [CONRAD

2009a, CONRAD 2009b, and CONRAD 2009c] which was partially funded by the European

Commission. In Fig. 1.2.2 the neutron spectra were measured outside the thick concrete shielding

are compared to the cosmic ray neutron spectra at flight altitudes and at ground level. The

dosemeter concepts developed for dosimetry at flight altitudes can be also applied at high-energy

accelerators, if the pulse structure of the particle beams does not create timing problems for the

active, i.e. particle counting, dosemeter systems. But again, the response of the different systems to

these high-energy neutron fields is not well known and needs a more detailed investigation. This

was one of the main conclusions of the CONRAD project:

„… the need for well-characterised high-energy neutron calibration facilities. Calibration

facilities providing nearly-monoenergetic neutron beams with energy up to about 20 MeV are

available, whereas the few existing laboratories offering high-energy neutrons are not really

metrology laboratories (TSL in Uppsala, Sweden, iThemba LABS in Cape Town, South Africa, RCNP

in Osaka, Japan). There are intrinsic limitations for the production of quasi-monoenergetic high-

energy beams, and cost issues involved, but the availability of a reference facility delivering well-

characterised beams over a wide of energies is certainly deemed useful. Of similar interest is the

general availability of facilities for ‘‘in-situ’’ calibrations, i.e. facilities delivering high-energy, mixed

radiation fields similar to those encountered at workplaces. “

1.4 High-energy Quasi-monoenergetic Neutron Beams

1.4.1 Overview

In all three cases described in the previous chapter, i.e. the dosimetry of cosmic radiation, of stray

radiation fields at high-energy accelerators and secondary radiation produced during ion beam

therapy, the radiation field consists to a large extent of neutrons with energies ranging from

thermal up to a few hundreds of MeV. Therefore, comparable measuring methods can be applied.

In continuation of the EURADOS strategy, the characterization of different dosemeter systems at

neutron energies up to 200 MeV is a major step forward in the harmonization of dosimetry of high-

energy mixed radiation fields.

Existing high-energy neutron facilities are discussed in a recent EURADOS report [Pomp 2013]. The

details of neutron metrology in common and especially at these high energies are summarized in a

special issue of the journal Metrologia, i.e. in Ref. [Thomas 2011] and Ref. [Harano 2011],

respectively.



1.4.2 Neutron Facility at iThemba LABS

The Separated-Sector Cyclotron (SSC) of the iThemba Laboratory for Accelerator-Based Sciences

(iTL) accelerates protons in the energy range from 25 MeV to 200 MeV. A beam pulse selector can

suppress a chosen fraction of proton bunches to enlarge the time interval between pulses, which

allows time-of-flight measurements to be carried out. Typical currents for 100 MeV are about 5 mA

in unselected mode and 500 nA at the repetition rate of 2.5 MHz. The time spread of a proton

bunch is about 1 ns.

The 7Li(p,n)7Be reaction is often employed to produce quasi-monoenergetic neutrons from 25 MeV

to 200 MeV. The 7Li(p,n)7Be reaction proceeds only by the transition to the ground state and first

excited state of 7Be since all higher levels are unstable. A quasi-monoenergetic neutron emission

which is strongly forward peaked is obtained in this way. Breakup reactions in lithium cause a low

energy tail to the monoenergetic peak. Also (p,xn) reactions in the more massive nuclei of the

target holder generate neutrons of lower energies. The neutron emission from these reactions can

be roughly approximated by a phase space distribution with smaller angular dependence.

The iThemba LABS high-energy neutron facility is outlined in Figure 1.4.1 and extensively

described in Refs. [Mosconi 2010], [Harano 2011] and [Pomp 2013]. A 2 m thick steel collimator with

openings at 0º , 4º , 8º , 12º and 16º shapes 10 cm × 10 cm2 beams at 8 m. The collimator is lined by

a layer of borated wax and polyethylene (see Fig. 1.4.1). The 16º position is used to directly measure

the neutrons produced by the nearly isotropic breakup reactions. Two neutron production targets

can be mounted simultaneously on a remotely controlled target ladder placed in an evacuated

scattering chamber. Targets of natC and natBe have already been used at iTL to produce beams with a

continuous energy distribution and different quasi-monoenergetic beams, respectively.

Background measurements can be carried out by bombarding an empty position of the target

ladder at the same focusing employed for the measurement. A cleaning magnet deflects proton

beams into the well shielded beam stop, where a Faraday cup measures the beam current. The

experimental area has a width of about 9 m and a depth of 3 m in the direction of the beam. An

annex, 3 m long and 1.3 m wide, exists in this area in front of the 0º exit of the collimator. Thus,

flight paths between 4 m and 11 m in length are possible.

Figure 1.4.1: Outline of the high-energy neutron beam line at iThemba LABS [Mosconi 2010].



For the neutron energies under considerations only the neutron facility at iThemba TLABS provides

traceability to the primary standards in Europe. This was achieved at PTB by making use of two

fission chambers which were compared to the primary standard for neutron fluence at a neutron

energy of 15 MeV [Mosconi 2010, Nolte 2012]. The agreement of the three instruments is within 1

% as shown in Fig. 1.4.2.

Figure 1.4.2: The calibration factor of the PTB long counter monitor with respect to the recoil proton telescope RPT1 (PTB), the primary standard at PTB for neutron fluence, the PTB fission chamber H21 (238U-FC (PTB) and the iThemba LABS fission chamber (TLABS FC) as the transfer instrument [Mosconi 2010, Nolte 2012].

For the campaign reported on in this document, primary proton beams of 66 MeV or 100 MeV

impinged on a 6 mm thick natLi target to produce quasi-monoenergetic neutron beams. At a point

8m from the target the neutron beam is 10 cm × 10 cm and is shaped by holes at 0° and 16°

through a 2 m thick steel wall. Sufficiently thick graphite blocks were placed on the target side of

the holes to stop stray protons from the target area reaching the experimental side of the steel

wall. On the experimental side of the wall there are also layers of borated wax and/or borated

polythene as indicated in Figure 1.4.1.

The planned section indicated in Figure 1.4.1 has not beam implemented. However the 77 cm wide

passage behind the borated polythene door to the target area, pictured at the bottom in Figure

1.4.1, was narrowed in size to 37 cm by polythene blocks 40 cm × 100 cm in an effort to reduce the

low energy neutrons reported on by [Mosconi 2010].

The 0° neutron beam time-of-flight spectrum has a strong quasi-monoenergetic peak and a broad

low energy tail (Fig. 1.4.3). At 16° the neutron time-of-flight spectrum has a greatly reduced quasi-

monoenergetic peak as well as a broad low energy tail of similar shape to that at 0° (Fig. 1.4.3). By

subtracting the two spectra one is largely left with the quasi-monoenergetic contribution, which is

the motivation for irradiating the detectors at these two angles. Here the energy loss of the protons

in the Li target has the greatest contribution to the width of the quasi-monoenergetic peak and is

the sacrifice made for an increase in the neutron fluence.



Figure 1.4.3: Measured neutron fluence distribution at 0° (black lines) and 16° (red lines) for 66 MeV protons (left) and 100 MeV protons (right).



2 Irradiation Campaign

2.1 Organisation

The organization of the beam time started more than one year in advance of the irradiation

campaign. In October 2010 a proposal was submitted to the local Program Advisory Committee

(PAC) at iThemba LABS and presented during the PAC meeting by the local spokesperson. The

beam time was attributed and scheduled for two successive weekends in October 2011. In April

2011 the beam time was officially announced within EURADOS and a deadline for registration was

set to July 31, 2011. A total of 9 institutions participated in that campaign: 4 of them only sent their

dosemeters to TLABS, the other 5 attended as “active” participants.

All unattended dosemeters were sent directly to TLABS and the organising group took over the

responsibility for the irradiations. These groups also included a clear description of the irradiations

conditions: total dose, on phantom or free in air.

As active participants of the irradiations, 9 people from 5 institutions attended the two weekends.

They were responsible for their own instruments and also supported the organising group during

the irradiations.

In addition to the attending people, the metrology group consisted of 4 people from UCT, iTL and

PTB and was responsible for the beam characterisation and monitoring during the entire beam

time.

2.2 Irradiations

The irradiations of the dosemeters were split into two groups: the passive dosemeters and the

active dosemeters. The passive ones were exposed to neutron doses of several mSv, the active

devices at much lower dose rates. Mostly, both beam lines, i.e. the 0°-beam line and the 16°-beam

line, were used simultaneously.

Each beam time started on Friday afternoon with the characterisation of the neutron beam which

required between 16 h and 20 h. From Saturday morning until Monday morning 6:00 h the beam

could be used for the irradiations, together with beam optimizations and background

measurements.



3 Neutron Beam Metrology

3.1 Beam Characterisation

Quasi-monoenergetic neutrons are produced by (p,n) reactions with Li targets. The energy

distribution of the neutron beam exhibits a high-energy peak resulting from transitions to the

ground state and low-lying excited states of the product nuclei as well as an adjacent low-energy

continuum which is produced by neutrons emitted in break-up reactions. The yield of high-energy

neutrons is strongly forward-peaked while the dependence of the break-up continua on neutron

emission angles is much less pronounced. Neutron beams with different spectral distributions can

therefore be produced simultaneously using different neutron emission angles. As outlined in the

earlier work [Nolte 2002], this feature can in principle be used to discriminate between the effects

caused by the continuum neutrons from those of the high-energy neutrons by difference

measurements.

For the present experiments, quasi-monoenergetic neutrons beams were produced by

bombarding a 6 mm thick natLi target with 66 MeV and 100 MeV protons, respectively. The neutron

fields were collimated by iron collimators, 2 m in length which had square openings with a cross

section of 4.75 cm × 4.75 cm. The exit of the collimator is at a distance of 4 m from the Li target. The

angles of the collimators relative to the direction of the proton beam were 0° and 16°. Figure 3.1.1

shows the layout of the facility and the position of the instruments used for beam characterisation.

Figure 3.1.1: Experimental set-up at the iTL neutron beam facility. The scintillation detector and the 238U fission ionisation chamber used for the fluence measurement are denoted by SC and FC1, respectively. The monitor ionisation chamber and the NE102 monitor detector are marked by FC2 and NE102, respectively.

A 238U fission ionisation chamber and a thin NE102 scintillation detector were permanently placed

in the 0° neutron beam as transmission monitor detectors to relate the measurements carried out

for the characterisation of the beam to the irradiation of the device undergoing tests. The count

rates from these detectors together with the beam current were continuously recorded in intervals

of 60 s during the whole experiment. As in the earlier work, the beam pulse selector installed at the



iTL separated sector cyclotron (SSC) was used to adapt the repetition frequency of the proton

beam to the requirements of time-of-flight (TOF) measurements.

A measurement of the relative spectral fluence (ΦE/Φ) with high energy-resolution was carried out

using a fast liquid scintillation detector at a beam current of about 55 nA (66 MeV) and 25 nA

(100 MeV). These measurements were performed at the two neutron emission angles of 0° and 16°.

The low-energy cut-off of the TOF measurements was about 9 MeV. Below this energy a constant

extrapolation of the relative spectral fluence was used, which is motivated by the results of

measurements [Nolte 2002] using a Bonner sphere spectrometer and a quasi-monoenergetic

60 MeV neutron beam at the neutron beam facility of the Université Catholique de Louvain (UCL) in

Louvain-la-Neuve, Belgium. The spectral fluence (ΦE/M) per unit monitor count M at a neutron

emission angle of 0° was measured using 238U fission chambers (IRMM-FC total 238U mass 241(1) mg

and H22, total 238U mass 871.8(5) mg) with a slower time response. These measurements were

performed at beam currents of about 575 nA (66 MeV) and 445 nA (100 MeV) and with the same

beam parameters as for the irradiation of the devices undergoing tests.

3.2 Spectral Fluence

When the high energy-resolution measurements at low currents were compared to the low

energy-resolution measurements at higher currents, it was found that the continuum-to-peak ratio

had to be multiplied by factors of 1.11 and 1.15 for the 66 MeV and 100 MeV data, respectively. This

effect could be explained by parasitic neutrons produced either in the target holder or in structures

in the beam line in front of the Li target which are hit by the halo of the proton beam. This

additional neutron fluence contribution varies with beam current and focusing conditions. If the

parasitic neutrons were produced in the beam line they would only contribute to the spectral

distribution measured at 0° since the 16° collimator shields neutrons produced in the beam line.

The relative spectral distributions at the neutron emission angles of 0° and 16° are shown in Figures

3.2.1 and 3.2.2. The 0° distributions were corrected for the effect discussed above. Identical

corrections were applied to the 16° spectra, i.e. it was tentatively assumed that no parasitic

neutrons originate somewhere else in the beam line. The most important parameters

characterizing the spectral fluence distributions are given in Table 3.2.1. The mean energy of the

high energy neutrons as calculated from the target thickness dLi and the proton energy Ep are

indicated as E0,calc and ΔEcalc. The energies E0,exp and E1 denote the experimental mean energy of the

high-energy peak and the cut-off energy at the low-energy side of the peak, respectively (see

equation (3.2.1)). The peak fluence Φ0 is determined by integration of the spectral fluence Φ above

E1:

EE

E d1

0 (3.2.1)



Figure 3.2.1: Spectral fluence (ΦE ΦE /Q) normalized to beam charge at a distance of 8 m from the Li target for neutron emission angles of 0° (black) and 16° (red) and a proton energy of Ep = 65.93 MeV. The fluence ratio between 0° and 16° is (Φ0°/Φ16°) = 1.638.

Figure 3.2.2: Spectral fluence (ΦE ΦE /Q) normalized to beam charge at a distance of 8 m from the Li target for neutron emission angles of 0° (black) and 16° (red) and a proton energy of Ep = 99.35 MeV. The fluence ratio between 0° and 16° is (Φ0°/Φ16°) = 1.640



For the 0° neutron beam, the peak fluence Φ0 per unit monitor count at a reference distance of 8 m

from the Li target was determined from the measurement using the fission ionisation chamber.

The uncertainty does not include a measured uncertainty of the 238U(n,f) cross section but one

which was taken from [Carlson 1997]. The distance of 8 m from the target was taken as the

reference position, because at this distance the neutron beam area is sufficiently large for most

detectors, whereas the fluence rate is still high and the distance from the walls is large enough to

reduce the in-scattering of neutrons. Most irradiations took place at this reference position.

Irradiations at other positions are corrected for this distance effect. Table 3.2.2 shows the results for

the monitors, beam charge Q, number of events MNEf from the NE102 monitor (fast anode signal)

and number of events MNEs from the NE102 monitor (slow dynode signal).

Table 3.2.1: Parameters of the spectral fluence distributions. The mean energy of the high energy neutrons as calculated from the target thickness dLi and the proton energy Ep are indicated as E0,calc and ΔEcalc. The energies E0,exp and E1 denote the experimental mean energy of the high-energy peak and the cut-off energy at the low-energy side of the peak, respectively.

Ep dLi E0,calc Ecalc E0,exp E1 (0°/) (0°/16°)

MeV mm MeV MeV MeV MeV

65.93 6 62.77 2.73 63.43 61.0 0° 0.467

16° 1.638

99.35 6 96.53 2.01 97.73 92.4 0° 0.454

16° 1.640

Table 3.2.2 Peak fluence Φ0 above the cut-off energy E1 per monitor count at a reference distance of 8 m from the Li target for the beam charge monitor (Q) and the NE102 monitor (MNEf, MNEs). For the NE102 monitor, the fast anode signal (f) and the slower dynode signal (s) was used.

E0,exp (0/Q) (0/MNEf) (0/MNEs)

MeV cm-2 nC-1 cm-2 cm-2

63.43 4.6(4) 25.3(20) 1.76(14)

97.73 4.8(4) 181(15) 1.70(13)

3.3 Spatial Beam Profile

The spatial beam profile was determined using image plates mounted behind a 2 mm PMMA plate

(see Figure 3.3.1). The distance of the image plates to from the Li-target was about 8 m for both the



66 MeV and 100 MeV runs. The vertical and horizontal gradients of the neutron fluence distribution

are shown in Figure 3.3.2 and Figure 3.3.3.

The characterization of the beam profile for all energies measured is summarised in Table 3.3.1. The

reference fluence is determined at 8 m from the target for a virtual detector with a radius of 25 mm.

To apply the beam profile correction one first has to determine the size of the detector, at a

distance of 8 m from the target, geometrically and then to interpolate the correction factor for the

appropriate radius of the detector undergoing tests.

Figure 3.3.1: Image plate picture as colour plot showing the beam intensity and dimension for a proton energy of Ep = 99.35 MeV at 0°. The blue coloured circle shows the position and size of the reference detector with a radius of 25 mm at a distance of 8 m from the target



Figure 3.3.2: Projection of the image plate intensity to show the vertical gradient of the neutron fluence distribution as shown and with the parameters given in Figure 3.3.1.

Figure 3.3.3: Projection of the image plate intensity to show the horizontal gradient of the neutron fluence distribution as shown and with the parameters given in Figure 3.3.1.



Table 3.3.1: Relative beam profile correction factor for a virtual reference detector at 8 m from the target with a radius of 25 mm.

rdet ref / ref / ref / ref /

mm 66 MeV, 0° 66 MeV, 16° 100 MeV, 0° 100 MeV, 16°

5 0.993 1.003 0.992 0.999

10 0.990 1.003 0.988 0.998

15 0.991 1.003 0.993 1.000

20 0.993 1.003 0.997 1.000

25 1.000 1.000 1.000 1.000

30 1.009 0.999 1.003 1.000

35 1.018 0.999 1.007 1.000

40 1.026 0.999 1.010 1.001

45 1.046 1.001 1.015 1.004

50 1.083 1.027 1.038 1.030

3.4 Determination of Dosimetric Quantities

The calculation of dosimetric quantities D is done using the information on (0/M), (E/0) and M

from the present analysis together with the fluence-to-dose conversion coefficients d available

from the literature for h*(10) [ICRP 1990] [Ferrari 1998]. For hp(10) the angular distribution of the

20 MeV values has been taken and extrapolated with the energy dependence of the h*(10)

fluence-to-dose conversion coefficients.

EMdMD E d00 (3.4.1)

Resulting conversion coefficients are given in Tables 3.4.1 and 3.4.2 for the 66 MeV neutron field

and in Tables 3.4.3 and 3.4.4 for the 100 MeV neutron field.

Table 3.4.1: For the 66 MeV neutron beam <h*(10)> and <hp(10)> conversion coefficients for the total spectrum as shown in Figure 3.2.1 in units of pSv·cm2 are shown. <hp(10)> is given for the standard set of angles for neutron incidence to the detector.

66 MeV <h*(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)>

pSv·cm2 pSv·cm2 pSv·cm2 pSv·cm2 pSv·cm2 pSv·cm2 pSv·cm2

beamline 0° 0° 15° 30° 45° 60° 75°

0° 413.49 416.89 413.89 430.35 427.06 427.66 389.81

16° 439.19 443.71 440.64 458.09 454.41 454.41 412.94



Table 3.4.2: For the 66 MeV neutron beam <h*(10)> and <hp(10)> conversion coefficients for the peak area (60.9 MeV – 67.4 MeV) of the spectrum as shown in Figure 3.2.1 in units of pSv·cm2. <hp(10)> is given for the standard set of angles for neutron incidence to the detector.

66 MeV <h*(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)>


beamline 0° 0° 15° 30° 45° 60° 75°

0° 357.37 357.37 354.39 368.68 366.30 368.68 339.50

16° 359.59 359.59 356.59 370.97 368.58 370.97 341.61

Table 3.4.3: For the 100 MeV neutron beam <h*(10)> and <hp(10)> conversion coefficients for the total spectrum as shown in Figure 3.2.2 in units of pSv·cm2. <hp(10)> is given for the standard set of angles for neutron incidence to the detector.

100MeV <h*(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)>


beamline 0° 0° 15° 30° 45° 60° 75°

0° 342.17 344.07 341.42 355.05 352.32 352.62 321.15

16° 363.56 366.05 363.28 377.76 374.74 374.58 340.27

Table 3.4.4: For the 100 MeV neutron beam <h*(10)> and <hp(10)> conversion coefficients for the peak area (92.6 MeV – 104.4 MeV) of the spectrum as shown in Figure 3.2.2 in units of pSv·cm2. <hp(10)> is given for the standard set of angles for neutron incidence to the detector.

100MeV <h*(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)> <hp(10)>


beamline 0° 0° 15° 30° 45° 60° 75°

0° 288.72 288.72 286.31 297.86 295.94 297.86 274.28

16° 291.80 291.80 289.37 301.04 299.10 301.04 277.21

3.5 Analysis of the Monitor Count Rates and the Determination of Fluence

Four monitor signals were recorded in 60 s intervals during the experiment: beam charge Q, count

rates from the slow dynode MNEs and the fast anode signal MNEf of the NE102 monitor as well as

number of counts MFC from the fission ionisation chamber. The fission chamber is rather photon

insensitive however the count rate was low; the NE102 detector is sensitive to neutrons and, to a

lesser extent also to photons. The beam charge has to be used carefully, in particular at low current,

due to leakage currents and focusing effects. This is why the ratio of the different monitors has to

be monitored carefully. In addition, time-of-flight signals from the fast anode of the NE102 detector

were recorded in parallel to the characterisation and irradiation measurements. With these time-of-

flight matrices two additional monitoring values have been calculated, one for the non-



monoenergetic background, and one for the purely monoenergetic neutrons (an example of a

matrix is given in Figure 3.5.1 with the corresponding monitor logger values in Figure 3.5.2). The

analysis of all monitoring signals was used to check for the stability of the accelerator during the

measurement campaign and to determine the assigned uncertainty of the irradiations.

Table 3.5.1: Uncertainty budget for the relative uncertainties of the quasi monoenergetic peak fluence per charge, the peak fluence and total fluence

rel. uncertainty of the peak fluence per unit charge (0/Q)

N0: 0.029 number of fission events in the TOF peak

f: 0.030 fission cross section

: 0.016 ff detection efficiency k2: 0.004 neutron transport

k3: 0.019 PH-threshold

k4: 0.001 TAC deadtime

k5: 0.001 distance

k6: 0.005 air attenuation

k7: 0.005 profile correction

fQ: 0.019 fit of calculated TOF spectrum

(0/Q): 0.053 E > 32.5 MeV

rel. uncertainty of peak fluence

N/Q: 0.02 variance of monitor ratios0: 0.06 E > 32.5 MeV

rel. uncertainty of total fluence

Nt/N0: 0.038 stat. uncertainty of TOF measurementt/0: 0.015 rel. efficiency of the DE detector

fcont,Mn1/fcont,

Mn2:0.005 stability of the spectral distribution

(t/0): 0.041 E < 32.5 MeV

Using the uncertainty budget given in Table 3.5.1, together with the monitor values, yields the

monitor calibration factors with their assigned uncertainties which are used to determine the

fluence. The values are:

Φp/Q = 4.7(4) cm-2 nC-1

Φp/TOFtotal = 1.71(14) cm-2

Φp/TOFback = 6.2(5) cm-2



Φ p/TOFpeak = 3.10(25) cm-2

Φp/NE102fast = 180(19) cm-2

Φp/NE102slow = 1.67(18) cm-2

Φp/FC = 1260(140) cm-2

This set of calibration factors is used to determine the average fluence rate during the irradiation of

active devices and the total fluence during the irradiation of passive devices. The fluence or the

fluence rate is used to calculate the dose or dose rate, respectively, using the dose conversion

coefficients given in Tables 3.4.1 to 3.4.4.

Figure 3.5.1: Example for the time-of-flight monitor matrix of time-of-flight spectra per irradiation run during the irradiation runs with En = 100 MeV.

Figure 3.5.2: Example for the monitor logger values normalized to charge integrator signal during the irradiation runs with En = 100 MeV, including the monitoring values calculated from the time-of-flight spectra.



4 Reports of the Participants

4.1 Participant Report: PTB

4.1.1 PTB Reference Instrument - πDOS

For many years PTB has maintained a reference dosimetry system to measure the ambient dose

equivalent at aviation altitudes [Wissmann 2006]. The central instrument of the PTB Dosimetry

System (πDOS) is a 2’’ tissue-equivalent counter (TEPC) filled with tissue-equivalent gas at a gas

pressure of about 40 hPa which corresponds to a tissue volume of 4 μm in diameter. The TEPC is

surrounded by an active veto-counter, the cylindrical multi-wire proportional counter CACS. Thus,

the lineal energy y measured with the TEPC can be classified as due to a charged or neutral primary

particle. The detectors, all electronic modules required for their operation and the data acquisition

and power supply are installed in a cabin baggage size suitcase. Therefore, dose measurements

onboard aircraft are simple to perform. πDOS has been in routine operation for more than 10 years.

The results obtained so far, were the basis for the development of the program code FDOScalc

[Wissmann 2010] to calculate the dose rate at aviation altitudes around the globe.

The calibration of the TEPC system is threefold:

1. The measured energy depositions are calibrated in terms of lineal energy in keV/μm by

using neutrons from an AmBe source and the proton edge, defined as the maximum energy deposition by recoiling protons. The latter can be calculated by the SRIM code.

2. The absorbed dose for each detected event is calculated by using the gas density inside the

TEPC cavity and accumulated as a function of the lineal energy. Based on the assumption that the measured lineal energy y is an approximation of the linear energy transfer L, the

obtained spectrum is folded with the quality factor Q(L y) as defined by ICRP 103 [ICRP 2007] resulting in the dose equivalent HTEPC.

3. The dose calibration in terms ambient dose equivalent is carried out in mono-energetic

neutron reference fields up to 20 MeV and in high-energy photon reference fields between 4 MeV and 7 MeV, as, for example, provided by PTB. Since the πDOS-TEPC is surrounded by a large amount of material, it is therefore well justified to use two dose-calibration factors: a low-LET calibration factor klow determined in high-energy photon fields and a high-LET calibration factor khigh determined in mono-energetic neutron fields up to 20 MeV [Wissmann 2006].

The ambient dose equivalent (or its rate) is calculated according to

H*(10)�DOS = klow Hlow + khigh Hhigh , (4.1.1)

where Hlow and Hhigh are the measured dose equivalents for y ≤ 10 keV/μm and y > 10 keV/μm,

respectively. From previous measurements in the afore mentioned reference fields the obtained

calibration factors were klow = (0.675±0.022) and khigh = (0.893±0.058).



4.1.2 Measurements

At both 0° and 16° beam lines, πDOS was setup at distances from the neutron target of

approximately 8 m in accordance with the reference point for the neutron fluences. For small

deviations from this distance the measured doses were corrected along the 1/r2-law.

The lineal energy y measured with the TEPC was converted into an absorbed dose D(y) and leading

to a microdosimetric yd(y)-spectrum as shown in Fig. 4.1.1. The total spectrum is shown in black,

the spectrum obtained in the neutral mode of πDOS, i.e. no signal in the CACS, is shown in red and

the charged mode, i.e. TEPC and CACS in coincidence, is plotted in blue. At 66 MeV the TEPC mainly

measured doses from neutral particles at both irradiation positions. At 100 MeV the contributions

owing to neutral and charged particles are comparable.

The data acquisition system of πDOS measured in 5 minutes time intervals in which the single

events are accumulated in three energy spectra corresponding to three different amplifier gains.

This leads to a wide dynamical range and covers an energy range from 0.67 keV to 2.7 MeV which

corresponds to a y-range from 0.25 keV/μm to 1000 keV/μm. The proton beam current was chosen

to provide an ambient dose equivalent between 100 μSv/h and 200 μSv/h. For each irradiation

about 7 to 10 such 5-min.-measurements were analysed and the uncertainty of the final results was

obtained from the standard uncertainties of these 5-min.-measurements.

Figure 4.1.1: The yd(y)-spectra (normalized) measured during the 66 MeV (top row) and 100 MeV (bottom row) irradiations at 0° (left) and 16° (right), respectively. The spectrum obtained in the charged mode is coloured in blue, in the neutral mode in red and the total spectrum in black

4.1.3 Results

The conversion factor was determined as:



h*(10) = H*(10)πDOS / Φn . (4.1.2)

Here, the neutron fluence Φn was evaluated for different irradiation conditions: the total fluence of

the neutron spectrum at the 0°- and 16°-beam line, or the peak fluence at the peak neutron energy.

The dose values measured for the total neutron spectra were attributed to the mean neutron

energy <En> which was evaluated according to

<En> = ( ∑Φn(Ei)· Ei )/ ∑Φn(Ei), (4.1.3)

where Ei is the energy of the i-th channel of the fluence spectrum. The obtained values are given in

Tab. 4.1.1 for the two proton beam energies 66 MeV and 100 MeV.

The mono-energetic conversion factors at 62.4 MeV and 96.4 MeV were obtained by the

subtraction method which assumes that the neutron spectrum at the 16°-beam line describes the

background neutron spectrum at the 0°-beam line quite well and can thus be subtracted. Because

even the 16°-spectrum exhibits the elastic high-energy peak, this contribution is taken into account

by the peak area ratio obtained from the normalized fluence spectra as described in this report.

Thus, the mono-energetic ambient dose equivalent was obtained using the following equation,

H*(10,Epeak)DOS = ( H*(10,0°)DOS - H*(10,16°)DOS )∙ f0/16 , (4.7.4)

with the peak ratio f0/16 = 1.338 and f0/16 = 1.260 for the 62.4 MeV and 96.4 MeV neutron peak

energy, respectively, obtained from the neutron fluence spectra provided by the metrology group.

Since the two measurements at 0° and 16° were not made simultaneously, the 16°-measurement

was normalized to the same neutron fluence rate as during the 0°-measurements.

The final results are summarized in Tab. 4.1.1 in comparison with the values recommended by ICRP

folded with the neutron energy spectrum as given in this report. The experimental data are within

an interval of ±30 % which means that πDOS is an ideal instruments to measure the ambient dose

equivalent in high-energy neutron radiation fields.



Table 4.1.1: Results of the πDOS measurements of the conversion factor h*(10) in accordance with the irradiation conditions: “Total” denotes the entire neutron spectrum at the respective beam line (0° or 16°) with a fluence averaged neutron energy <En> at the incident proton energy Ep; “Peak” denotes the mono-energetic values obtained by the subtraction method according to Eq. (4.7.4). The last column gives the reference value according to ICRP 74 [ICRP 1996].

Ep / MeV Spectrum Beam line <En> / MeV h*(10) / pSv cm-2 ICRP 74

66 Total 16° 39.3 434±27 442.3

66 Total 0° 46.9 387±25 415.8

66 Peak 0°-16° 62.4 315±58 357.7

100 Total 16° 63.4 409±33 364.4

100 Total 0° 73.6 334±29 342.4

100 Peak 0°-16° 96.4 213±60 288.2



4.2 Participant Report: NPI

4.2.1 Detectors description and analysis

4.2.1.1 Thermoluminescent Detectors (TLD)

Thermoluminescent detectors are usually used to measure absorbed dose particularly from

particles with low linear energy transfer (LET) (photons, high-energy protons, etc.); for particles

with higher values of LET (about >10 keV/μm, depending on the type of TLD), their relative

response decreases and can be only several tens of percent of the response to photons. The aim of

this experiment was to investigate the response of several types of TLD to high-energy neutrons.

Since the TLD are often used in combination with plastic nuclear track detectors in various mixed

radiation field (such as onboard spacecraft), where high-energy neutrons can be present, it is

important to know their sensitivity to neutrons of several tens/hundreds of MeV.

Several types of TLD were used – CaSO4:Dy [Guelev 1994] [Yamashita 1971], Al2O3:C [Akselrod

1990], LiF:Mg,Cu,P enriched with 6Li and 7Li (MCP6 and MCP7) [Bilski 2006a]. All TLDs are about 1

mm thick and have a diameter of 4.5 mm. For the evaluation of irradiated detectors two TLD

readers are used: TOLEDO 654 and RA-04. Information about the detectors and their evaluation is

summarized in Table 4.2.1. All TLD used were calibrated with photons from a 137Cs reference

radiation source, in terms of air kerma and then converted to the absorbed dose in water. For more

information about the detectors and their evaluation see for example [Spurný 2004] [Spurný 2008].

Table 4.2.1: Detectors information and processing conditions.

Detector CaSO4:Dy Al2O3:C LiF:Mg,Cu,P (MCP 6, MCP 7)

Annealing cycle 380oC, 10 min 700oC, 20 min 240oC, 10 min

Cooling Rate fast cooling on Al table

fast cooling on Al table

fast cooling on Al table

Heating Rate 10oC/s 10oC/s 10oC/s

Reading Annealing

280oC, 30 s; 280oC, 30 s

320oC, 20 s; No

240oC, 25s;

Preheat 150oC (22s) No 100oC (10 min)

4.2.1.2 Plastic Nuclear Track Detectors (PNTD)

The principle of PNTD can be briefly outlined as follows. A heavily ionizing particle passing through

a medium causes extensive ionization along its path and leaves a narrow trail of damage (latent

track). If the material containing latent tracks is exposed to certain chemical agent (etchant),

chemical reactions are more intensive in these damaged regions. Thus after a certain time of

etching, a tiny etch cone or cylinder (track) is created that can be observed under an optical

microscope. A relation between the parameters of etched tracks and LET of the particles can be



determined via calibration; from the knowledge of LET, other dosimetric characteristics (such as

absorbed dose or dose equivalent) can be then calculated.

As plastic nuclear track detector we used a 0.9 mm thick HARZLAS TD-1 manufactured by Fukuvi

Chemical Industry, Japan; it is a polyallyldiglycolcarbonate with density 1.3 g/cm3. The detectors

were cut to 2 x 2 cm2 pieces and placed together with TLD against the polycarbonate holders (Fig.

4.2.1). After the irradiation, the detectors were etched in 5N NaOH at 70oC for 18 hours; these

etching conditions correspond to the removal of a layer of about 14 μm on each side of the

detector. The etched detectors were then processed with a high speed microscope system HSP-

1000 and HspFit [Yasuda 2005] software. From the parameters of the etched tracks (major and

minor axes) it is possible to determine the LET of the detected particles and to calculate the

absorbed dose and dose equivalent as

D = ∫ (dN / dL) L dL,

H = ∫ (dN / dL) L Q(L) dL,

where dN/dL is the number of tracks in a certain LET interval, L is LET value, and Q(L) is the ICRP 60

[ICRP 1990] quality factor.

Using our evaluation conditions, we can determine the LET from about 7 keV/μm up to more than

1000 keV/μm. More information about the evaluation conditions, calibration, and procedure of LET

determination including analysis of uncertainties, can be found for example in [Jadrnickova 2010]

[Pachnerová 2013].

PNTD can register only charged particles with LET above the detection threshold. Neutrons of

energy between 1 and 20 MeV are detected via tracks from recoil protons formed as a result of

interactions between the neutron and hydrogen in the polymeric detector. Higher energy neutrons

are detected via target fragmentation reactions with the C and O nuclei of the detector, or in target

materials in close proximity to the detector.

4.2.1.3 Liulin

Liulin is an active planar-type silicon semiconductor detector with a 300 μm thin HAMAMATSU

S2744 PIN diode with area of 2 cm x 1 cm as a sensitive volume [Dachev 2012]. Energy imparted to

the sensitive volume of the photodiode is transformed into an electric signal which is consequently

Figure 4.2.1: Holder with TLD and PNTD.



amplified using a charge amplifier, shaped by a shaping amplifier and converted using a 12 bit

analogue-to-digital converter. The digitized signal is further processed by a microcontroller and

stored in the SRAM memory. Data from SRAM are processed by several other microcontrollers and

finally stored in a flash memory. A Liulin can measure energy deposition spectra, fluxes and

absorbed doses in mixed radiation fields by detecting the energy imparted to its active volume in a

single energy deposition event. Calibration of the Liulin spectrometer-dosimeter was performed in

proton and heavy ion beams [Uchihori 2002]. Liulin type detectors can be also used to estimate the

value of the H*(10) on-board aircraft after a proper calibration [Ploc 2011]. More detailed

information about this device can be found for example in [Dachev 2002] [Dachev 2010].

Two Liulin detectors were used in this experiment, MDU-1 and MDU-2. The construction of both

units is the same; the only difference was that MDU-1 had a 2mm thick PE layer in front of the

sensitive Si diode.

4.2.1.4 Tissue Equivalent Proportional Counter HAWK

HAWK is a TEPC designed by Far West Technology Inc. for research purposes onboard commercial

and military aircraft. Its sensitive volume is a 12.6 cm sphere with 0.2 cm walls made from tissue

equivalent A-150 plastic and filled with pure propane at low pressure of 933.2 Pa representing a

tissue of about 2 μm. The sphere with the electronics is contained in a cylindrical stainless steel

container of 0.1 cm wall thickness. HAWK records lineal energy spectrum y from 0 to 1525.5

keV/μm by 1.5 keV/μm, and detailed spectrum from 0 to 25.55 keV/μm by 0.1 keV/μm. A

combination of the spectra is used to calculate dose and dose equivalent.

Assuming a wall thickness of 2.1 mm, it measures the dose and dose equivalent at 2.1 mm depth in

a 4.2 mm diameter tissue equivalent sphere. It differs from the dose and dose equivalent measured

at 10 mm depth in a 30 cm ICRU sphere. The values of H*(10) can be calculated combining a

corrected low and high LET component as described in [Lillhök 2007]. The calibration factors are

different for low-LET and high-LET components. The low-LET factor is 1.11 and was determined

from the calibration with 60Co and 137Cs sources and the high-LET factor of 0.8 from the calibration

with a 241Am-Be source.

4.2.2 Experiment

The detectors were irradiated at TLABS in October 2011 in 66 and 100 MeV quasi-monoenergetic

beams. The information about the exposures (spectra, reference values of , H*(10) and

dH*(10)/dt) were provided by the organizers. For both beams (66 and 100 MeV) the detectors were

irradiated in air (no phantom) and at two positions – at 0 and 16 degrees.

4.2.3 Results

4.2.3.1 Thermoluminescent Detectors

TLD were evaluated as described above. The background (non-irradiated detectors travelling

together with exposed ones) was subtracted from the measured signal.

The measured signal was quite low (sometimes only at the level of the background); therefore the

uncertainties of measured absorbed dose are relatively large. The response (expressed as

measured absorbed dose related to reference neutron fluence and reference H*(10)) of all TLD to



66 and 100 MeV neutrons is presented in Tables 4.2.2 and 4.2.3. The measured signal is a little

higher for MCP detectors – these types of TLD are supposed to be ultra-high sensitivity detectors.

The differences between 0˚ and 16˚measurements are within the uncertainties, with the exception

of 100 MeV and MCP detectors. The response of all TLDs to high-energy neutrons is very low (only

several percent). When used in mixed radiation fields, the TLD signal to neutrons of several

tens/hundreds of MeV could be considered as negligible.

Table 4.2.2: Absorbed doses per unit neutron fluence measured with various TLD in 66 and 100 MeV neutron beams.

Exposure

Energy, position

D [nGy/cm2]

CaSO4:Dy Al2O3:C MCP-6 MCP-7

66 MeV, 0o 0.31 ± 0.08 0.22 ± 0.04 0.76 ± 0.24 0.87 ± 0.13

66 MeV, 16o 0.19 ± 0.07 0.11 ± 0.03 0.52 ± 0.09 0.57 ± 0.11

100 MeV, 0o 0.54 ± 0.10 0.32 ± 0.06 1.07 ± 0.12 0.94 ± 0.17

100 MeV, 16o 0.37 ± 0.08 0.25 ± 0.05 0.35 ± 0.08 0.27 ± 0.04

Table 4.2.3: Response (absorbed doses per reference H*(10)) measured with various TLD in 66 and 100 MeV neutron beams.

Exposure

Energy, position

D [mGy/mSv]

CaSO4:Dy Al2O3:C MCP-6 MCP-7

66 MeV, 0o 0.035 ± 0.009 0.025 ± 0.005 0.086 ± 0.027 0.098 ± 0.015

66 MeV, 16o 0.032 ± 0.012 0.018 ± 0.005 0.087 ± 0.014 0.095 ± 0.018

100 MeV, 0o 0.072 ± 0.013 0.042 ± 0.008 0.141 ± 0.016 0.125 ± 0.022

100 MeV, 16o 0.074 ± 0.016 0.050 ± 0.010 0.072 ± 0.017 0.056 ± 0.008

4.2.3.2 Plastic Nuclear Track Detectors

After the irradiation, the detectors were etched and then processed with the high speed

microscope system HSP-1000 and HspFit software. An area of about 0.5 cm2 was analysed (this

corresponded to about 3000 and 2000 analysed tracks for 0 and 16 degree irradiations,

respectively). From measured tracks, the LET was calculated using the calibration function

described in [Pachnerová 2013]. LET spectra (normalized to the fluence of primary neutrons) are

shown in Figure 4.3.2. For exposures in the beam (0o) there is higher contribution of particles with



lower LET (< 100 keV/μm). When we compare the responses to 66 and 100 MeV beams, no

significant differences (within the uncertainties) in LET spectra were observed, with the exception

of the lowest LET region (< 15 keV/μm) where the number of measured tracks is higher for 66 MeV

neutrons.

Figure 4.2.2: LET spectra for 66 (left) and 100 (right) MeV neutrons (normalized to the fluence of primary neutrons)

From LET, absorbed dose and dose equivalent were then calculated; the results are shown in

Tables 4.2.4 (per unit fluence) and 4.2.5 (per reference H*(10)). In the Tables are shown not only

total values but also values for LET < 100 keV/μm. The tracks with LET below about 100 keV/μm are

supposed to be produced mainly by recoil protons. Measured absorbed dose and dose equivalent

due to particles with LET < 100 keV/μm are about a half of the total measured absorbed dose and

dose equivalent.

It should be mentioned that no corrections, such as critical angle of track registration, were applied

so the secondary particle production is only based on visible tracks (particles incident on the

detector with larger angles than so-called critical angle are not registered). This means that the

contribution of secondaries can be much higher. To correctly estimate the “invisible” component, it

would be necessary to know the angular distribution of the radiation field (produced secondary

particles). If we assume a isotropic radiation field (as is usually done in space dosimetry, but not the

case for accelerator experiments), the total number of secondaries could even be almost three

times higher and the absorbed dose equivalent higher by a factor of about 1.2.



Table 4.2.4 Measured number of particles, absorbed dose D and dose equivalent H per unit neutron fluence measured with PNTD.

Exposure N

NLET<100 keV/m D

[nGy/cm2]

DLET<100

keV/m

[nGy/cm2]

H

[nSv/cm2]

HLET<100

keV/m

[nSv/cm2]

66 MeV,

0o

(3.14 ± 0.051) *

10-3

(2.84 ± 0.051) *

10-3

0.28 ±

0.021

0.15 ±

0.011

4.75 ±

0.311

2.51 ±

0.091

66 MeV,

16o

(2.50 ± 0.05) *

10-3

(2.18 ± 0.05) *

10-3

0.24 ± 0.02 0.13 ± 0.01 4.38 ± 0.33 2.16 ± 0.09

100 MeV,

0o

(2.69 ± 0.05) *

10-3

(2.33 ± 0.04) *

10-3

0.28 ± 0.01 0.14 ± 0.01 5.02 ± 0.30 2.47 ± 0.08

100 MeV,

16o

(2.30 ± 0.04) *

10-3

(1.93 ± 0.03) *

10-3

0.26 ± 0.02 0.13 ± 0.01 4.76 ± 0.22 2.16 ± 0.06

1 the uncertainties are statistical only

Table 4.2.5 Measured number of particles, relative absorbed dose D and relative dose equivalent H related to reference H*(10) measured with PNTD.

Exposure

N

[cm-2/

mSv]

NLET<100

keV/m

[cm-2/mSv]

D

[mGy/mSv]

DLET<100 keV/m

[mGy/mSv]

H

HLET<100

keV/m

66 MeV, 0o 355 ± 61 321 ± 6 0.032 ± 0.0021 0.017 ± 0.001 0.54 ± 0.031 0.28 ± 0.01

66 MeV, 16o 416 ± 9 362 ± 8 0.040 ± 0.003 0.021 ± 0.001 0.73 ± 0.05 0.36 ± 0.01

100 MeV, 0o 357 ± 6 309 ± 6 0.037 ± 0.002 0.019 ± 0.001 0.67 ± 0.04 0.33 ± 0.01

100 MeV, 16o 467 ± 9 393 ± 6 0.053 ± 0.003 0.026 ± 0.001 0.97 ± 0.05 0.44 ± 0.01

1 the uncertainties are statistical only

4.2.3.3 Liulin

The results from both Liulin detectors are summarized in Tables 4.2.6 and 4.2.7. In the Tables are

shown not only total values but also values for deposited energy < 1 MeV. It is assumed that

detector’s signal bellow 1 MeV is caused by low LET particles (electrons – including those from

photon interactions, positrons, muons, and high energy protons and alphas) and above 1 MeV by

neutrons or neutron-like particles (low energy protons and alphas). The total uncertainties of dose

were estimated to be about 10% (statistical uncertainties were below 1%).



Table 4.2.6: Measured number of particles, N, and measured absorbed dose in Si, DSi per unit neutron fluence

MDU-1 MDU-2

Exposure N

N<1 MeV

DSi

[pGy/cm2]

DSi, <1MeV

[pGy/cm2]

N

N<1 MeV

DSi

[pGy/cm2]

DSi, <1MeV

[pGy/cm2]

66 MeV, 0o 0.301 0.179 642 46 0.218 0.137 489 35

66 MeV, 16o 0.237 0.161 401 35 0.170 0.123 289 28

100 MeV, 0o 0.290 0.193 562 52 0.223 0.147 481 40

100 MeV, 16o 0.258 0.189 395 43 0.192 0.143 315 34

Table 4.2.7: Measured number of particles, N, and measured absorbed dose in Si, DSi related to reference H*(10)

MDU-1 MDU-2

Exposure

N

[cm-2/

mSv]

N<1 MeV

[cm-2/

mSv]

DSi

[Gy/m

Sv]

DSi, <1MeV

[Gy/mSv]

N

[cm-2/

mSv]

N<1 MeV

[cm-2/

mSv]

DSi

[Gy/mSv]

DSi, <1MeV

[Gy/mSv]

66 MeV, 0o 33973 20257 72.6 5.1 24616 15520 55.2 4.0

66 MeV, 16o 41901 28533 70.9 6.2 30031 21682 51.1 5.0

100 MeV, 0o 38467 25562 74.7 6.9 29562 19544 63.9 5.3

100 MeV, 16o 51874 38068 79.6 8.7 38584 28711 63.5 6.8

Spectra of deposited energy are then shown in Figures 4.2.3 and 4.2.4. The deposited energy is

clearly higher for exposures in the beams (0o). When we compare MDU-1 and MDU-2, the response

is higher for MDU-1. This unit had a PE layer in front of the Si diode, which increased the response

to neutrons due to recoil protons.

A 2 mm thick PE layer in front of the diode increased the Liulin response mainly in the region of

deposited energy below 9 MeV; the increase in both number of particles and absorbed dose in Si is

a little higher for 66 MeV beam. For MDU-1 (with PE) the response is about by a factor of 1.2 – 1.4

higher than for MDU-2.

Number of particles with low-LET (energy deposit below 1 MeV) represents 60-70% of all detected

particles; absorbed dose in Si due to low-LET particles is only about 10% of total absorbed dose.



Figure 4.2.3: Spectra of deposited energy (normalized to the fluence of primary neutrons) in 66 MeV beam.

Figure 4.2.4: Spectra of deposited energy (normalized to the fluence of primary neutrons) in 100 MeV beam.



4.2.3.4 HAWK

The measured linear energy spectra were converted to microdosimetric yd(y) spectra, where d(y) is

dose contributions per dy intervals. The spectra, shown on Figure 4.2.5, are scaled to yield unit

integral.

Figure 4.2.5: Microdosimetric yd(y) spectra measured in and outside neutron beams of 66 MeV (top) and 100 MeV (bottom) with TEPC HAWK.

The spectra for the beam positions should be corrected due to the fact that the sensitive volume of

HAWK has a larger profile than the beams at 8 m. HAWK volume is the sphere with diameter of

r=6.285 cm while the beam profile was considered as a square with sides of b=10 cm as is shown

on Figure 4.2.6. The tops outside of the HAWK sensitive volume can be approximated by a circular

cone with volume Vt according to . The volume of a spherical cup Vsc is one sixth

of the volume difference between the HAWK sphere and a cube of the beam sides b without

volumes of 8Vt. Then, accordingly, the volume irradiated inside the HAWK is 82 .

For the calculation of the mean chord length, the area Sirr of the irradiated volume Virr is needed. It

comprises of two areas Ssc corresponding to upper sphere of Vsc volumes, and four circles, each

with area , where √2 .Ssc can be approximated as one sixth of the



surface difference between the HAWK sphere and eight bases of circular cones with diameter (ri-r).

Thus, the area corresponding to the irradiated volume is 2 4 .

The lineal energy measured in the beam position were corrected by a factor l/lirr = 0.72, where l is

mean chord length of HAWK volume and lirr is the mean chord length of the irradiated volume,

both calculated according to 4V/S, 4Virr/Sirr, respectively. The corrected microdosimetric yd(y)

spectra are compared with the uncorrected in Figure 4.2.7. The real spectra would correspond to

an intermediate state, since the uncorrected spectra cover the case for the irradiation of the total

HAWK volume with a uniform beam and the corrected spectra cover the case of ideal beams with

no penumbra.

Figure 4.2.6: Beam view of HAWK (diameter r) and the beam (diameter b) with tops of Vt

exceeding HAWK sensitive volume; Vsc is auxiliary volume of spherical cup as explained in the text.

Figure 4.2.7: Microdosimetric yd(y) spectra measured in neutron beams of 66 MeV (left) and 100 MeV (right) with TEPC HAWK; the comparison with the spectra corrected for the real size of the irradiated volume.

The calculated values H*(10) corrected for the size of irradiated volume were compared to the

reference values, as shown in Table 4.2.9.



Table 4.2.9: The dose equivalent measured with HAWK compared to the reference values.

Exposure H*(10)ref

[μSv]

H*(10)

[μSv] H*(10)low/ H*(10) H*(10)/ H*(10)ref

66 MeV, 0o 73.73 52.48 0.04 0.71

66 MeV, 16o 41.08 28.29 0.04 0.69

100 MeV, 0o 493.86 385.13 0.05 0.78

100 MeV, 16o 61.93 50.48 0.05 0.82



4.3 Participant Report: PHE

4.3.1 Objectives

The calibration of the PHE neutron personal dosemeter at high energies is required because it is

used for measurements in aircraft at civil aviation altitudes and also for measurements in low earth

orbit on board the International Space Station, where neutrons at much higher energy than normal

terrestrial workplace fields exist. It may also be used around high energy research and medical

accelerator facilities where high energy neutrons can be present. The CERN/EU reference field

facility, CERF, has been used in past years for comparative purposes, but accurate measurements in

monoenergetic and quasi-monoenergetic fields forms the basis of the dosemeter calibration,

hence the importance of well characterized fields such as those at iThemba.

4.3.2 Materials

The PHE neutron dosemeter comprises a sensitive detector element of poly-allyl-diglycol

carbonate (PADC) contained within a holder (Fig. 4.3.1). The PADC detector is of rectangular shape

27 x 39 mm, of thickness 500 μm. The chemical formula is C12H18O7 and the molecular weight 274.

The density of the monomer is 1150 kg m-3 and of the polymer 1310 kg m-3. In manufacture of the

polymer, 3% by mass of the initiator IPP (diisopropyl peroxydicarbonate) is added. The cure cycle is

4 hours at each of 45°C, 49°C and 56°C followed by 2 hours at each of 65°C and 85°C. The polymeric

form is a clear rigid material of good mechanical stability and chemical resistance. The detector is

manufactured by Instrument Plastics Ltd, Maidenhead, Berkshire, England. The detectors are cut by

laser beam from larger sheets. Each detector is encoded by laser-cut holes with a unique BCD

(binary coded decimal) number in an array of 8 x 4 locations. The general form of the holder is that

of a shallow box. The PADC element sits in the front half into which the back half clips, secured by

`one-way' locking tabs. The holder is manufactured by Magma Moulding Ltd, Plymouth, Devon,

England, and is made of nylon-6, polyamide of composition (C6H11ON)n, elemental percent weight

H:9.8%; C:63.7%; N:12.4%; O:14.1%. The colour is dark green.

Figure 4.3.1: PADC detector elements (background and dosed) and dosemeter holder



4.3.3 Analysis

The dosemeter is subject to an electro-chemical etch process which develops the traces of

radiation damage to the point where they are visible as etched pits. The process is 11.5 hours at

40°C using 5 mol/litre (20%) Sodium Hydroxide solution followed contiguously by a further 8 hours

at 40°C using 5 mol/litre (20%) Sodium Hydroxide solution but with an electric field of 23.5 kVcm-1

applied across the detector. The processed detector is scanned using a Nikon LS5000 slide scanner

and the image acquired using Silverfast software produced by Lasersoft, Germany. Proprietary

image analysis software is used to count the etched pits, over a total read area of 1.767 cm2. To

improve the reliability of the overall measurement result two PADC sheets from separate

production batches were used, and three independent sets of exposures were made for each

energy and angle position, and the results averaged.

4.3.4 Experimental set-up

Dosemeter sets of four dosemeters per set were mounted onto thin cardboard in a close 2 x 2

dosemeter array so that the dosemeters should be within the beam width. The card with

dosemeters was affixed to the front face of a 30cm x 30cm x 15cm PMMA phantom, with no extra

build up used. Exposures were made at the 0⁰ and 16⁰ positions for the nominal 66 MeV and 100

MeV beam energies

4.3.5 Results and Discussion

For the quasi-monoenergetic neutron fields at iThemba, the fluence response characteristic for the

fluence peak was determined by an iterative fitting method to match the experimentally measured

value to a calculated value. For each of the two neutron fields, 66 MeV and 100 MeV, but starting

with the lowest peak neutron energy, that is 66 MeV, a first rough estimate of the peak energy

fluence response characteristic was taken from the difference in the response between the in-

beam (0°) and out of beam (16°) exposures. Together with the results of the lower neutron energy

response determinations, a full set of energy response characteristics were constructed and folded

with the total energy distributions for 0° and 16°. The dosemeter reading calculated thus was

compared to the experimentally measured value. Adjustments were made in the peak energy

fluence response value until the folded result matched the measured value as closely as possible,

for both 0° and 16° exposures. The same procedure was then performed for the 100 MeV field.

Once the 100 MeV response was partially optimised, the whole process was started again going

back to the 66 MeV field. Several iterations were carried out to obtain a final value of the peak

response for both fields which, when included in the fluence response characteristic file, resulted in

an agreement between calculated and observed dosemeter readings for the full spectra. The mean

energies of the fluence peaks were taken as 63.4 MeV and 98.0 MeV.

To obtain the calculated values a folding process was used which is limited to 200 data lines

maximum. The supplied fluence energy distributions contained significantly more lines than this

and hence were reduced to enable the folding to work. Hence there were fewer energy bins, but

with higher contents per bin. The revised energy binning was still satisfactory and not expected to

have any significant impact on the folding results.

The statistical (type A) uncertainties for the dosemeter readings have been combined in

quadrature with the total standard uncertainties on the neutron fluences to give the standard



uncertainties shown (1 σ). Three independent repeat measurements were made at each energy

and angle, hence the uncertainties are much reduced. For each measurement, detectors from two

separate production batches of PADC plastic made by the current supplier, Instrument Plastics Ltd,

Maidenhead, were used.

The measured data is given in Table 4.3.1 and Table 4.3.2 for 66 MeV and 100 MeV respectively.

Table 4.3.1. Measured data for the nominal 66 MeV full energy spectrum

0° 16°

Set No Time (mins)

Net corrected pits

Φtot0° (106

cm-2) Time (mins) Net corrected

pits Φtot16° (106 cm-

2)

1 17 499 ± 15 5.31 ± 0.43 31 530 ± 15 5.99 ± 0.49

2 17 495 ± 52 5.13 ± 0.42 17 268 ± 32 3.15 ± 0.26

3 17 466 ± 11 5.38 ± 0.44 17 304 ± 19 3.11 ± 0.25

Table 4.3.2. Measured data for the nominal 100 MeV full energy spectrum

0° 16°

Set No Time (mins)

Net corrected pits

Φtot0° (106

cm-2) Time (mins) Net corrected

pits Φtot16° (106

cm-2)

1 23 504 ± 34 6.90 ± 0.56 24 371 ± 16 4.20 ± 0.34

2 23 464 ± 14 6.55 ± 0.53 23 341 ± 18 4.08 ± 0.33

3 24 474 ± 42 5.96 ± 0.48 22 334 ± 23 3.91 ± 0.31

The difference in the measured response between the in-beam (0°) and out of beam (16°)

exposures is taken as the first rough estimate of the fluence response in the peak, RF(P), which is

calculated by the following method using the data in Tables 1 and 2

∅ ~0 16 ∅ 0 ∅ 16

As an example, using the data in Table 4.3.1 for 66 MeV, set 1, gives

∅ ~

499 1517

530 1531

5.31 6 0.43 617

5.99 6 0.49 631



∅ ~29.4 0.9 17.1 0.5

3.12 5 0.25 5 1.93 5 0.16 5

∅ ~103.4 6

The above method is used for all three measurement sets and the mean value taken. For 66 MeV

the mean value of all three sets was 94 e-6 cm2. For 100 MeV the mean value of all three sets was 58

e-6 cm2. These are the starting values for the iterative process described above. During the process,

the calculated result from the folding is compared to the mean experimentally determined values

for the full spectra shown in Table 4.3.3 to Table 4.3.6. Adjustments were made in the peak energy

fluence response value until the calculated folded result matched the measured value, for both, the

0° and 16°exposure, as closely as possible.

Table 4.3.3. Measured data at 0⁰ for the nominal 66 MeV full energy spectrum

Set No Net corrected pits Fluence in peak, Φ0 0⁰ (106 cm-2)

Φ0 0⁰/ Φtot 0⁰ R / Φtot 0⁰ (10-6 cm2)

1 499 ± 15 2.48 ± 0.20 0.467 94.0 ± 8.2

2 495 ± 52 2.39 ± 0.20 0.467 96.5 ± 0.13

3 466 ± 11 2.51 ± 0.21 0.467 86.7 ± 7.4

Mean 92.4 ± 5.6


Set No Net corrected pits

Fluence in peak, Φ0 0⁰ (106 cm-2)

Φ0 0⁰/ Φtot 0⁰ Φtot16⁰ / Φtot 0⁰ R / Φtot 16⁰ (10-6 cm2)

1 530 ± 15 4.58 ± 0.37 0.467 0.6105 88.5 ± 7.6

2 268 ± 32 2.41 ± 0.20 0.467 0.6105 85.0 ± 12.4

3 304 ± 19 2.38 ± 0.19 0.467 0.6105 98.0 ± 10.0

Mean 90.5 ± 5.9


Set No Net corrected pits Fluence in peak, Φ0 0⁰ (106 cm-2)

Φ0 0⁰/ Φtot 0⁰ R / Φtot 0⁰ (10-6 cm2)

1 504 ± 34 3.13 ± 0.25 0.454 73.1 ± 7.7

2 464 ± 14 2.97 ± 0.24 0.454 70.8 ± 6.1

3 474 ± 42 2.71 ± 0.22 0.454 79.4 ± 9.5

Mean 74.4 ± 4.6




Set No Net corrected pits

Fluence in peak, Φ0 0⁰ (106 cm-2)

Φ0 0⁰/ Φtot 0⁰ Φtot16⁰ / Φtot 0⁰ R / Φtot 16⁰ (10-6 cm2)

1 371 ± 16 3.13 ± 0.25 0.454 0.6094 88.5 ± 8.1

2 341 ± 18 3.04 ± 0.25 0.454 0.6094 83.4 ± 8.1

3 334 ± 23 2.91 ± 0.24 0.454 0.6094 85.5 ± 9.1

Mean 85.8 ± 4.9

The closest matching of folded and experimental results was obtained by using the peak fluence

responses given in Table 4.3.7, and also shown on Figure 4.3.2. Table 4.3.8 compares the

experimentally measured and calculated fluence response values when considering the full energy

distributions for both 66 MeV and 100 MeV, and position either in the beam 0° or out of the beam

16°. The calculated values are obtained by using the monoenergetic values from Table 4.3.7, in the

dosemeter fluence energy response characteristic file and folding with the supplied full fluence-

energy distribution for the nominal neutron energies of 66 MeV and 100 MeV. It can be seen that

there is good agreement between the experimentally determined values and the calculated values,

for both energies, and for in beam or out of beam locations. Table 4.3.9 shows the personal dose

equivalent response of the dosemeter in the full energy distributions, obtained by dividing the

measured dosemeter fluence response by the fluence to Hp(10) conversion coefficient values

provided with the energy distributions (Figure 4.3.2).

Table 4.3.7. Assessed monoenergetic fluence responses (at the location of the peak)

Neutron energy (MeV) Fluence response (cm2) x 10-6

63.4 80.0 (5.1)#

98.0 70.2 (4.5)#

#values in brackets are 1 standard uncertainty



Figure 4.3.2 Fluence response characteristic of the PADC dosemeter showing the values assessed in this report for peak energies of 63.4 MeV and 98.0 MeV

0.00E+00

2.00E‐05

4.00E‐05

6.00E‐05

8.00E‐05

1.00E‐04

1.20E‐04

1.40E‐04

1.60E‐04

1.80E‐04

0.01 0.1 1 10 100

Response per fluence (cm

2)

Neutron Energy (MeV)



Table 4.3.8. Comparison of measured and calculated fluence response (using the full energy distributions)

Fluence response (10-6 cm2)

Nominal 66 MeV full energy distribution

Nominal 100 MeV full energy distribution

0° 16° 0° 16°

Experimental measurement 92 (6)# 91 (6)# 74 (5)# 86 (5)#

Calculated (using folding program) 90 95 79 82

# values in brackets are 1 standard uncertainty

Table 4.3.9. Measured fluence and dose equivalent responses (using the full energy distributions)

Nominal 66 MeV full energy

distribution Nominal 100 MeV full

energy distribution

0° 16° 0° 16°

Measured R (10-6 cm2) 92 (6)# 91 (6)# 74 (5)# 86 (5)#

Hp(10) (pSv cm2) 416.89 (2.1) 443.7 (2.2) 344.1 (1.7) 366.1 (2.2)

RHp(10) (mSv-1) 221 (14) 205 (14) 215 (15) 235 (14)

# values in brackets are 1 standard uncertainty

4.3.6 Future Work

In terms of the PHE neutron personal dosemeter, for its use in assessing doses at aviation and low

earth orbit altitudes, there will be a periodic requirement to repeat measurements such as those

reported here, as well as when new PADC plastic suppliers are investigated.

The method used in this report, namely using a folding technique to optimise the peak fluence

response estimate does involve some degree of uncertainty because the nearest other data point is

at 19 MeV. Interpolation by the folding program between 19 MeV and 63 MeV could be introducing

significant errors if the interpolation does not match the true response of the dosemeter. Hence,

other methods of extracting the peak fluence response from the full spectrum response may need

to be investigated.

It would be useful to also have neutron calibrations in the future at an energy of 30MeV to 40 MeV,

and around 80 MeV.



4.4 Participant Report: POLIMI

4.4.1 Material

The POLIMI neutron detector is based on PADC (poly-allyl-diglycol carbonate) detectors supplied

by Intercast. They have a surface area of 2.5 x 25 cm2, are 1.5 mm thick and 1.31 g·cm-3 in density.

PADC are coupled to a PMMA [composition (C5O2H8)n] radiator 1cm thick (See figure 4.4.1). The

neutron interaction in PMMA provides recoil proton via (n,p) elastic scattering and heavier

secondary charged particles produced by inelastic scattering. Secondary particle production via

elastic scattering dominates up to a neutron energy of around 7–8 MeV. For higher neutron

energies, elastic and inelastic scattering cross sections of oxygen and carbon become important.

The effect on the passive detectors is that tracks are not only produced by recoil protons but by

heavier charged secondary particles.

The chemical treatment after irradiation is made by etching the PADC in a NaOH aqueous solution

6.25 mol at 98°C for 90 minutes. Under this etching condition the bulk attack velocity, measured

with the fission fragments technique, is VB= 10±0.2 μm/h.

The PADC analysis is made with a commercial reader called Politrack [Caresana 2010], developed at

the Politecnico di Milano and marketed by MiAm s.r.l. (Italy).

4.4.2 Response function

The main goal in assessing the response function of the PADC detectors is to make it independent

of the neutron energy. The simplest response function is the track density, but especially in case of

high energy neutron tracks are produced both by recoil protons and heavier charged particles (2H, 3H, 3He, 4He) or even recoil carbon and oxygen. Of course tracks produced by these particles are

associated with a higher energy deposition and a simple evaluation of the track density results in a

decrease in sensitivity. In order to properly account for this effect the response function used is

given by equation 4.4.1)

i

n

i i

iLETQ

LETH

______

1

______

6

cos10602.1

1

(4.4.1)

where iLET______

is the average LET of the i-th particle expressed in keV·μm-1, n is the number of tracks

detected per square cm, is the PADC density expressed in g·cm-3 and i is the particle impinging

angle with respect to the normal to the detector surface. The numerical factor permits one to have

H expressed in mSv. The impinging angle, as well the average LET can be calculated starting from

the morphological characteristic of the track. H can be regarded as an estimator of both personal

and ambient dose equivalent. However, PADC detectors have a detection threshold in terms of LET

around 10 keV/μm. Below that threshold the radiation damage is too low for the particles to be

detected. Also the limiting angle plays a key-role in the PADC sensitivity, for particles hitting the

detector surface with an high impinging angle the penetration depth into the detector bulk can be

lower than the detector layer removed during the etching bath. If so the etching erases the track.

For a detailed explanation of what is described in this paragraph see [Caresana 2012].



4.4.3 Calibration

Two kind of calibrations are needed to characterize the detector, on calibration in terms of LET and

one calibration in terms of ratio between the reference dosimetric quantity and the response

function given by eq. 4.4.1.

The former one is obtained by irradiating the bare detector with an electroplated 252Cf source. 252Cf

decays through spontaneous fission (about 3% branching ratio) and alpha emission (97%

branching ratio with energy 6.1 MeV). Fission fragment are used for Vb measurement while alpha

particles are used for LET calibration [Caresana 2012].

The latter one has been performed at PTB with the monochromatic neutron beams listed in Table

4.4.1

Table 4.4.1 results of the detector calibration with monochromatic neutron beams

Beam energy H (Eq. (4.4.1))

mSv

H*(10)ReF /mSv Response in

(in terms of H)

565 KeV 1.79 3.67 0.49

8 MeV 1.75 4.90 0.36

14 MeV 3.49 6.90 0.51

19 MeV 1.84 2.90 0.64

4.4.4 Irradiation and results

The irradiations were performed in air with the front face of the detector covered by 1 cm of PMMA

(poly-methyl-methacrylate) (see Figure 4.4.1).

Reported in Table 4.5.2 are the irradiation results, i.e. the measured doses in the PADC vs. the

reference dose (H*(10)Ref). A full explanation of the irradiations can be found in [Caresana 2014].

Figure 4.4.1: Schematic diagram of the setup used for irradiations.

PMMA PADC

Neutrons



Table 4.4.2: Detector response in terms both of H and track density.

Irradiation conditions

H (Eq. (4.5.1))

mSv

H*(10)ReF /mSv

Response in

(in terms of H)

Response

(in terms of track density)

cm-2·mSv-1

66 MeV 0 deg. 2.38 4.4 0.535 247

66 MeV 16 deg 1.72 3.2 0.537 354

100 MeV 0 deg. 1.52 2.36 0.643 387

100 MeV 16 deg. 1.75 2.83 0.626 386

No data analysis has been done to subtract the irradiations at 16°and at 0°, because the aim of this

research group was not to assess the sensitivity to an ideal mono-energetic beam of high energy

neutrons. The four different irradiation conditions have been regarded as four different irradiations

with a known energy distribution and intensity in terms of H*(10). The main aim was so

demonstrate that the response function calculated according to equation 4.4.1 permits one to

obtain a sensitivity almost independent of the neutron energy.

By comparing Table 4.4.1 and Table 4.4.2 it can be proved that this feature holds in a wide energy

range (0.565–100MeV). This is particularly interesting for personal and environmental dosimetry

applications, since this neutron energy range is wide enough for these detectors to be successfully

used in many practical situations.



4.5 Participant Report: IRSN

4.5.1 Objectives

The aim of this study was to investigate and compare the response of different types of

Polyallydiglycol carbonate (PADC) track etched detectors in the high energy neutron range that

could possibly be used in routine monitoring or for specific dosimetric requirements, such as

around hadron therapy facilities or onboard aircrafts. These experiments were performed in the

frame of a larger study that includes also experiments at CERN/EU reference field facility with mono

energetic beams. The aim is to define energy correction factors to be applied for the case of

irradiation with high energy neutron (above 20 MeV).

4.5.2 Materials

Two types of Polyallydiglycol carbonate (PADC) detectors were tested by IRSN during the

EURADOS campaign at the iThemba LAB facility. PADC from TASL (UK) and from Technol (Japan)

were considered for these tests. Both PADC detectors are of rectangular shape 20 x 25 mm, of

thickness 1.5 mm, in order to fit in the IRSN holder used for routine monitoring (Fig. 4.5.1). This

holder insures the role of radiator or converter for neutron. Only one face is in contact with the

holder.

Figure 4.5.1: Pictures of the IRSN holder for PADC sheet.

For both types of dosimeters, prior to irradiation, a pre-treatment with ethanol and methanol is

performed to polish the surface and to remove alpha particle tracks and scratches.

The TASL PADCs were tested for all irradiation configurations (0° and 16 °, 66 MeV and 100 MeV).

Irradiations were performed free in air and on a slab phantom for 5 mSv. PADCs from Technol were

tested only in two configurations (66 MeV, 0° and 16°), because of the limited number of

dosimeters available. For each irradiation a maximum of six PADCs were exposed as shown in

Figure 4.5.2, in order to fit in the homogeneous region of the beam. PADCs were placed on a thin

aluminium sheet. For each configuration and dose, a minimum of 12 dosimeters was exposed.



Figure 4.5.2: Pictures of the configuration of irradiation for the dosimeter exposed in the “free in air” configuration.

The same chemical procedure for etching was used for the both types of PADC. It consists in a

chemical etching with NaOH at 6.25 N for 15 hrs at 70°C. Only the total number of tracks is reported

in order to easily compare the two PADC types. The background track density was estimated with

control dosimeters and was taken into account into the analysis.

4.5.3 Results

Table 4.5.1 displays the average track density measured in air and on a slab phantom with the TASL

PADC for the four configurations of irradiation. Each value corresponds to the average value

obtained of 12 dosimeters irradiated together. The error bars are the standard deviation of the

measured track density of 12 dosimeters.

Table 4.5.1: Track density measured with TASL PADC for the different irradiation configurations.

66 MeV, 0° 66 MeV, 16 ° 100 MeV, 0° 100 MeV, 16°

On phantom 152±13 157±6 133±8 154±1

In air 154±4 153±1 136±7 156±3



For the configuration with the highest average energy for 100 MeV, 0°, a 15 % decrease in the track

density is observed compared to all the other configurations. A decrease in terms of track density

(24 %) was also reported by PSI with the TASL PADC in the same configuration of irradiations and

similar analysis conditions [Trompier 2014].

These results seem to indicate that the energy dependence of the track density response is

relatively weak in this energy range. In Boshung et al. (2008), a 15% variation of the track density

was observed for TASL dosimeter between AmBe neutron source and CERN-CERF irradiation.

Additional mono-energetic neutron irradiation above 5 MeV will be necessary to finish

characterizing the energy response.

Moreover, no significant difference is observed in terms of track density irrespective of whether the

dosimeters are exposed on a slab phantom or in air. A unique calibration coefficient could be

derived for ambient and personal dose equivalent calibration for high neutron energies. This result

was expected, because of the very small difference between <h*(10)> and <hp(10)> conversion

coefficient at 66 and 100 MeV (see Table 3.4.1 and 3.4.3) and due to the weak impact of the slab

phantom on the neutron backscattering at these energies.

In addition to the TASL PADCs, a few PADCs from Technol were also tested. The comparison of

Technol PADCs to those from TASL is shown in Table 4.5.2.

Table 4.5.2: comparison of track density measured with TASL and Technol PADC for irradiations in air at 66 MeV.

66 MeV, 0° 66 MeV, 16 °

TASL 154±4 153±1

Technol 182±24 169±12

Taking into account uncertainties in track density, no significant difference is observed in terms of

track density between the PADC from Technol and TASL. More than the materials, the etching

conditions, the radiator properties and the track analysis methods have larger influence on the

track density. As an example with the same PADC (TASL), PSI has reported for similar irradiations a

track density of about 58 track/cm2/mSv [Trompier 2014].

4.5.4 Conclusions

The response of PADC dosimeters was investigated for high energy neutrons. The variation of the

track density response was not found to be significant in the energy range investigated. Additional

experiments at lower neutron energy but above 20 MeV would be needed to draw a conclusion on

the relevance of applying an energy correction factor above 20 MeV.



4.6 Participant Report: PSI

4.6.1 Objectives

The aim of this study was to measure and compare the response of two neutron dosemeter

designs based on PADC (poly allyl diglycol carbonate) used at the Paul Scherrer Institute (PSI). The

so-called original PSI design has been in use since 1998. The original design (i.e. holder) was later

changed to the so-called new PSI design and a new evaluation method based on a microscope

scanning technique has been introduced. The experiments at high energies at iThemba LABS

yielded necessary data points for establishing the response curve of the two PSI dosemeter

designs.

4.6.2 Materials and Methods

Original PSI design

The original personal neutron dosemeter design consists of an outer case holding together two

units of a PADC detector covered on both sides with 2-mm radiators. One half of the radiator of

each side is made of polyethylene to produce protons from fast neutron elastic scattering and the

other half is of polyethylene containing ~1% of lithium of natural abundance to produce tritons

through the 6Li(n,)3H reaction from incident thermal neutrons.

New PSI design

The new PSI neutron dosemeter design consists of a housing of hydrogenous material (~10%

hydrogen) and a PADC detector. The housing has two dips to hold two standard LiF chips (TLD600)

with dimensions 3 x 3 x 0.9 mm3 in such a way that the LiF chips are at the same location on both

sides of the detector. Here the sensitivity to fast neutrons is achieved via recoil protons produced in

the housing itself and to thermal neutrons with the use of the tritons (3H) produced in the nuclear

reaction of 6Li in the LiF chip.

In both designs the PADC detector material is manufactured by Track Analysis Systems Ltd (TASL),

which is known under the trade name TASTRAKTM. These detectors are of thickness of 1.5 mm and

size of 20 x 25 mm2.

Figure 4.6.1: Pictures of the PSI personal neutron dosemeter designs.



Etching procedure and evaluation

Before the PADC detectors can be evaluated, they have to undergo a chemical etching process. The

detectors were etched for 2 h 50 min with 6.25 N sodium hydroxide at 85°C. The etching process

was terminated via a neutralisation for 15 min in a weak hydrochloric acid solution (0.1 N HCl) and a

cleaning with hot and cold distilled water. After this process the detectors were evaluated on the

evaluation system manufactured by TASL. This system acquires images of the etched tracks by a

high magnification microscope, which are then analysed by a software algorithm. Each measured

track is characterised by a multitude of separate parameters, such as covering size, shape, position,

optical density, noise discrimination or quality of measurement. These parameters are then used in

the neutron exposure algorithm to enable optimum noise discrimination, sensitivity calibration

and dose calculation.

4.6.3 Measurements and Results

At iThemba LABS the irradiations were carried out in neutron fields with nominal peak energies of

66 and 100 MeV of the spectral energy fluence. In order to apply a difference method, the

measurements were carried out at 0° and 16° scattering angle. For all irradiations the peak fluence

at 0° has been used for normalisation of experimental results. The ratio of peak fluence to total

fluence, and the ratio of 0°–16° total fluence are well known and given by the experimental setup.

The resulting dose equivalent Hp(10) for each irradiation has been determined by numerically

integrating the fluence spectrum weighted by interpolated fluence-to-dose conversion coefficients

and integrating over the peak region.

The sensitivity of the different dosemeter designs in terms of track density per cm2 was then

related to the sensitivity of 241Am–Be irradiated dosemeters of each design. Adding the

measurement results of iThemba LABS to the measurement results obtained with radionuclide

sources and monoenergetic neutron irradiations at PTB, a response function over an energy range

from 24 keV to 100 MeV could be determined. The results can be found in the RPD paper

“Determination of the response function for two personal neutron dosemeter designs based on

PADC” [Mayer 2014] and [Trompier 2014].

4.6.4 Conclusion

At iThemba LABS the response of two PSI neutron dosemeter designs based on PADC was

investigated in the provided high energy neutron fields. The measurements were part of a series of

measurements in reference fields for the establishment of the response curves of the PSI personal

neutron dosemeters and thus played a key role for the dosimetric characterisation of the

dosemeters.



4.7 Participant Report: UAB

4.7.1 SRAM SEU Based Neutron Detector

The detector is based on the relation between Single Event Upsets (SEU) in digital SRAM memories

and the known neutron fluence. This device is actually a grid of detectors which is a memory cell of

a digital semiconductor device. Each cell of the memory stores a logic level (bit) that can be

changed due to the interaction of a particle with the materials of the component if enough

ionization is produced from the subsequent processes to change the logic level.

The memory components were selected to have a borophosphosilicate glass (BPSG) dielectric layer

in the chip layout in such a way as to make them highly sensitive to thermal neutrons. The neutron

capture cross section of 10B, leading to alpha particle emission, has a 1/v behaviour where v stands

for the neutron velocity, therefore this process is not dominant whenever fast neutrons are

involved (above several MeV). At higher energies, around 10 MeV, alpha particle production

through (n,) reactions becomes important in Silicon.

The system is controlled from a computer through a serial link. Before each irradiation the memory

content is set to a fixed pattern. Afterwards the memory content is written and the amount of SEUs

is computed. Previous tests conducted with the system showed that the number of events has

repeatability (relative standard deviation) of around 2% for typical irradiations inside a LINAC

treatment room. Due to the large size of system memory available, occupancies are always below

3x10-5, and thus the probability of SEU pile-up is of the order 10-9.

The response of two different prototypes was studied (see Fig. 4.7.1). The first one (A1) consists of a

total of 64 MiB with 8 boards with 8 MiB memory per board inside a PVC box, while the second one

(SN1) is a planar version inside an aluminium box with 512 kiB size. These memories were selected

in order to retain the sensitivity to thermal neutrons. It was previously found that the detector A1

was sensitive to neutrons of a few MeV (up to 20 MeV) due to the presence of a Silicon layer

(containing 28Si) through the (n,) reaction [Domingo 2010], but no information was available

about its behaviour to higher energies.

Figure 4.7.1: Prototypes (left: A1, right: SN1) of the SEU based neutron detector.



4.7.2 PADC based dosemeter

The Universitat Autònoma de Barcelona etched track neutron Poly-Allyl-Diglycol-Carbonate

dosimeter (UAB-PADC) consists of a 1 mm thick PADC (Intercast Europe S.r.l., Parma) layer covered

with several converters:

i. polyethylene (3 mm), to discriminate protons from fast neutrons;

ii. Makrofol (300 μm), to flatten the energy response of the dosimeter;

iii. and Nylon (100 μm), to make it sensitive to thermal neutrons via the 600 keV protons

originating from neutron interaction with its Nitrogen nuclei.

A 5 mm thick methacrylate holder, placed behind the PADC layer, is used as support for the

converter + PADC set. Figure 4.7.2 schematically shows the structure of the dosemeter.

The surface covered by each dosemeter was (4 cm x 4 cm), using 4 pieces of 2 cm x 2 cm pre-cut

PADC. Irradiations took place free-in-air for H*(10) response as well as on top of a 15 cm thick 30 cm

x 30 cm methacrylate phantom (for Hp(10) response) for both the 0° and the 16° irradiations. In

each irradiation condition, one dosemeter was irradiated “as is” while another was covered with a

~ 5 mm thick lead layer to enhance the response to high energy neutrons through the (n,xn)

reaction.

Figure 4.7.2: Schematic representation of the UAB PADC-based dosemeter.

Once irradiated, the UAB-PADC dosimeters were electrochemically etched. We used our standard 3

step electrochemical etching procedure [Bouassoule 1998], choosing KOH 6M as etching solution

for the front side of the PADC sheets and a low concentration KOH 0.25M solution to close the

electric circuit for the back side. The etching conditions are summarised in Table 4.7.1. The main

effect of the 1st step is to preserve the tracks originating from sparsely ionising particles, the 2nd

step has the effect of magnifying the tracks preserved by the first step, while post-etching has the

effect of rounding the tips of the etched tracks so that a small spot will be visible at their centre,

helping to distinguish overlapping tracks if necessary. Finally, washing with tap water removes the

etchant and stops etching. The electrochemical etching process is automatically controlled with a

specifically designed software application using Labview, through a computer-controlled

multifunction DAQ system (National Instruments, model USB-6341) and a high-speed high-voltage

power amplifier (Trek, model 10/10b-HS) (see figure 4.7.3).



Figure 4.7.3: Electrochemical etching system at UAB.

Table 4.7.1: Summary of the standard UAB electrochemical etching conditions

Step Chemical solution Time Temperature Electric field strength Frequency

1st step 6 M KOH 5 h 60 ºC 20 kV·cm-1 50 Hz

2nd step 6 M KOH 1 h 60 ºC 20 kV·cm-1 2 kHz

Post-etching 6 M KOH 15 min 60 ºC

Final wash Tap water 15 min ambient

Table 4.7.2: Summary of the results obtained with the digital devices.

Energy

(MeV)

Position

(beam line)

SEU count

A1 device

SEU count

SN1 device

66 0° 786 ± 28 347 ± 19

16° 504 ± 22 261 ± 16

100 0° 983 ± 31 391 ± 20

16° 760 ± 28 350 ± 19

Table 4.7.3: Fluence and H*(10) response of the A1 and SN1 units to 66 MeV and 100 MeV monoenergetic neutrons

Unit Energy

(Mev) Net counts in peak

Fluence response

(x 10-5 cm-2)

H*(10) response

(mSv-1)

A1 66 257 ± 71 6.6 ± 2.0 186 ± 55

100 199 ± 99 4.3 ± 2.2 151 ± 77

SN1 66 73 ± 40 2.0 ± 1.1 55 ± 31

100 30 ± 50 0.7 ± 1.1 23 ± 38



The combination of the structure of the dosemeter with the electrochemical etching technique

used makes it sensitive to a wide neutron energy range. Its response has already been validated

experimentally up to 20 MeV with ISO sources and realistic neutron fields as specified by the ISO

standards, as well as in quasi-monoenergetic neutron beams at the Institute for Reference Materials

and Measurements from the European Joint Research Center JRC-IRMM (Gëel, Belgium) Van der

Graaf accelerator.

Electrochemically etched tracks recorded in a detector are large enough not to require

sophisticated high magnification devices to analyse and count them. In our case, digitised images

of the etched PADC plates were obtained using a commercial photographic scanner (Canon, model

700f) with high optical resolution (up to 9600 x 9600 dpi). The scanned area covers a minimum of

0.4 cm x 0.4 cm. A software application (“ContaTracks”) to perform track counting was developed

using MATLAB. This application is specifically designed to be able to adequately separate and

count overlapping tracks. We have not observed saturation for track densities up to 15

000 tracks/cm2.

4.7.3 Results for the SRAM SEU detector

Table 4.7.2 displays the SEU counts for the two prototypes (A1 and SN1) of the digital device

utilised in this experimental campaign. From the spectral information of the neutrons present in

each of the irradiations and the peak fluences provided, it was possible to estimate the response of

these devices to monoenergetic neutrons of 66 and 100 MeV. Table 4.7.3 summarises the results of

this evaluation. The column “net counts in peak” displays the

counts on each device due to neutrons with energies in the specified peak, while the columns

“fluence response” and “H*(10) response” are evaluated taking into account the reference fluence

and H*(10) values provided for the different irradiations. The uncertainties provided were obtained

from the propagation of the statistical uncertainties of the measurement and, in the case of the

SN1 unit, are so large that the results become almost meaningless. The fluence response of the A1

device to monoenergetic neutrons is displayed in figure 4.7.4, combining the results obtained in

this irradiation (red) with those obtained in previous exposures with lower energy neutrons

[Domingo 2010].

The high response value of the device to thermal neutrons displayed was already explained as a

consequence of the 10B content of the SRAM memories. The rise in sensitivity starting at a few MeV

and extending to over 100 MeV is due to the presence of Silicon (including 28Si) in the SRAM

memories, and originates from charged particles (protons and alphas) via the (n,p), (n,Xp), (n,)

and (n,Xreactions that are concurrently able to produce SEUs concurrently. The cross sections

of these reactions in 28Si (ENDF/B-VII.1) are displayed in Figure 4.7.5 superimposed on the fluence

response of the device. It is clear that the rise in response overlaps with the rise in cross sections,

while at higher energies the response decreases because the protons and alpha particles produced

are able to escape from the sensitive volume and therefore not able of create SEUs.



Figure 4.7.4: Fluence response of the A1 device to monoenergetic neutrons. Results from the present irradiation are displayed in red.

Figure 4.7.5: Fluence response of the A1 device (excluding thermal neutrons) superimposed to the (n,p), (n.Xp), (n,α) and (n,Xα) cross-sections in 28Si (ENDF/B-VII.1).

4.7.4 Results for the PADC based dosemeters

A summary of the results for the irradiation of PADC dosemeters (bare and covered with lead) is

shown in Table 4.7.4 for free in air irradiations and in Table 4.7.5 for on phantom irradiations

(except for results from one of the irradiations, due to damage to the PADC during etching). In both

cases, responses are evaluated from the net track densities and the reference values provided for

H*(10) or Hp(10). Uncertainties in track density are the combination of counting uncertainties,

statistical uncertainties from averaging readings from 4 PADC plates for each dosemeter, and the

background/transport uncertainty. Uncertainties in response are evaluated through standard

propagation methods. It should be emphasised that the response values reported do not represent



the response to monoenergetic neutrons of the energy displayed, but to the complete spectrum

provided in each irradiation situation, including the peak energy and the corresponding lower

energy tail.

Table 4.7.4: Response of the UAB PADC dosemeters irradiated free in air without and with a 5 mm thick lead cover

Line Lead cover 5 mm thick

Energy (MeV)

Net track density (cm-2)

Reference H*(10) (mSv)

H*(10) response (cm-2·mSv-1)

0° NO 66 622 ± 25 4.45 ± 0.36 140 ± 13

100 486 ± 23 4.02 ± 0.32 121 ± 11

16° NO 66 410 ± 31 3.20 ± 0.26 128 ± 14

100 546 ± 53 2.97 ± 0.24 146 ± 18

0° YES 66 669 ± 31 4.45 ± 0.36 151 ± 14

100 535 ± 34 4.02 ± 0.32 133 ± 14

16° YES 66 397 ± 30 3.20 ± 0.26 124 ± 14

100 546 ± 53 2.97 ± 0.24 184 ± 23

Table 4.7.5: Response of the UAB PADC dosemeters irradiated on phantom without and with a 5 mm thick lead cover.

Line Lead cover

5 mm thick

Energy

(MeV)

Net track density

(cm-2)

Reference Hp(10)

(mSv)

Hp(10) response

(cm-2·mSv-1)

0° NO 66 637 ± 36 4.64 ± 0.38 137 ± 14

100 644 ± 67 4.58 ± 0.37 141 ± 19

16° NO 66 365 ± 23 3.08 ± 0.25 118 ± 12

100 376 ± 24 2.72 ± 0.22 138 ± 14

0° YES 66 --- ± -- 4.64 ± 0.38 --- ± --

100 550 ± 39 4.58 ± 0.37 120 ± 13

16° YES 66 432 ± 22 3.08 ± 0.25 140 ± 13

100 396 ± 27 2.72 ± 0.22 145 ± 15

It is difficult to extract information on the variation of the H*(10) or Hp(10) response variability as a

function of the different individual variables involved (beam peak energy, beam line used and

presence or not of the lead layer) as the differences in response between in (0˚) and out (16˚) of



beam are comparable to the uncertainties associated with the measurement. It may appear that

the average values of response for the dosemeters covered with a lead layer are slightly higher

than those for the uncovered ones, although this may not be conclusive. For the lead thickness

employed, the gain due to (n,Xn) reactions may just compensate for absorption process. Further

analysis is still needed if the response to monoenergetic neutrons of 66 MeV and 100 MeV is to be

evaluated from the spectral differences present in both (0° and 16°) irradiation angles.

Figure 4.7.6 displays the response of the UAB PADC based dosemeter to Hp(10) at 66 and 100 MeV

neutrons (evaluated from the present experiment), compared to that obtained in a previous work

[Domingo 2013] for lower energy neutrons. The older dosemeters evaluated in that case were

based on 500 μm thick Pershore PADC sheets, which may have a slightly different response than

the Intercast PADC used here and were much more unstable, brittle, irregular in thickness, and had

a high variation in the background levels recorded. This is the reason why the uncertainties are

much bigger for the measurements at lower energies than those of the present irradiation.

Figure 4.7.6: Response to Hp(10) of the UAB PADC based dosemeter for epithermal, fast and high energy neutrons. Dosemeters used by Domingo et al. (2013) were based on Pershore 500 μm thick plates, while those used in this irradiation are based on Intescast 1 mm thick plates. All electrochemical etching conditions were the same for both cases, including the electric field strength, implying that the high voltage applied was duplicated for the Intercast plates.



4.8 Participant Report: DLR/Kiel

4.8.1 The Flight Radiation Environment Detector (FRED)

4.8.1.1 Instrument description

FRED is a particle telescope consisting of segmented silicon semiconductor detectors which was

developed to measure neutral and charged particles. Figure 4.8.1 shows a sketch of the Detector

Stack of FRED, that is made up of four segmented silicon semiconductor detectors which are

arranged in telescope geometry with an opening angle of 120 degrees. All detectors have a

thickness of 300 μm. The active areas are squares with a side length of 30 mm for the outer

segment and 22 mm for the inner segment.

Figure 4.8.1: Schematic of the detector stack of the Flight Radiation Environment Detector. A to D denote segmented solid state detectors. The opening angles on 60 degrees and 120 degrees are defined by the inner and outer segments of the detector A and the detector Stack BCD, respectively (Möller, 2013a).

The detectors B, C, and D are glued together as close as possible to form a sandwich detector.

Detectors B and D and the outer segment of detector C are used as an anticoincidence in order to

separate neutral and charged particles. Thus, particles loosing energy in the inner part of detector

C (red area in Figure 4.8.1) can be identified as neutral, if no signal would be measured in the

anticoincidence detectors. Every detector segment has two gain levels. The energy deposit range

for the high gain is from 75 keV up to 17 MeV and for the low gain from 1 MeV up to 270 MeV. That

means we can measure the energy loss in silicon from 75 keV up to 270 MeV, which corresponds to

a Linear Energy Transfer (LET) range in water from 0.1 up to 429.8 keV/μm [Möller 2013a]. A

detailed description of the Flight Radiation Environment Detector can be found in [Möller 2013b].



4.8.1.2 Analysis of the measurements

Measurements were taken at two different positions the first one was inside the beam line (0°

position) and the second one was outside the beam to measure the background (16° position).

Figure 4.8.2 shows the 66 MeV (left panel) and the 97 MeV (right panel) neutron low gain energy

loss spectra measured in the inner segment of the C-detector. These are only signals in the neutral

channel and none in the anticoincidence. The black curves show the primary measurement inside

the beam line (0° position) and the red curves show the corresponding background measurement

on the 16° position. The right panel reveals no significant difference between the 97 MeV primary

neutron measurements and the background measurement. This means FRED could not measure

the 97 MeV neutrons. The very small difference can be caused by the background because the

background at 16° and 0° is only approximately identical.

Figure 4.8.2: The high-energy neutron measurement measured in the low gain. The black curves show the measurement at the 0° position for 66 MeV (left side) and for 97 MeV (right side). The red curves show the corresponding background measurements at the 16° position (Möller, 2013b).

The 66 MeV neutron measurement and its background measurement show a significant difference.

Figure 4.8.3 shows the corrected energy loss spectrum for the 66 MeV measurements. For

correction the background measurement was scaled by the ratio of the PTB fluence values for the

0° and 16° measurement before the correction was applied. The difference spectrum shows a

significant number of counts.



Figure 4.8.3: The corrected 66 MeV neutron energy loss spectrum (Möller, 2013b).

One possible explanation for these observations is that the high-energy neutrons interact with

silicon by compound reaction and spallation processes. The secondary particles produced in the

detector can trigger the anticoincidence, which is a simple explanation why it is not possible to see

the 97 MeV neutrons and a large part of the 66 MeV neutrons hits. Figure 4.8.4 shows this behavior

in a 2D histogram for the 66 MeV (left) and 97 MeV (right) runs. It shows the high gain of the inner

segment of detector C versus the inner segment of detector D (high gain). In addition, it was

considered that there were no hits in the B-detector nor in the outer segment of C as further trigger

condition. One can see that many secondary particles, produced in the C-detector triggered the D-

detector, which is part of the anticoincidence. These particles formed a cloud shaped structure due

to the random energy loss in the two detector segments.

Figure 4.8.4: The 2D histogram of the inner segment of detector C (high gain) and inner segment of detector D (high gain). In the left panel the 66 MeV neutron run and in the right panel the 97 MeV run is shown [Möller 2013b].

The measured absorbed dose in silicon and the number of particles NSi for the 66 MeV runs

measured with the C-detector are given in Table 4.8.1 and Table 4.8.2. While Table 4.8.1 shows the



results for the measurement with anticoincidence conditions Table 4.8.2 gives the same

measurement without anticoincidence condition thus values are only from the single C-detector.

Table 4.8.1: Results for the 66 MeV neutron measurement measured in neutral channel (inner segment of the C-detector) with anticoincidence conditions.

Position Time

[min]

Ref.

106 [cm-2]

Ref.

H*(10)

[mSv]

NSi DSi

[mGy]

0° 28 4.51 2.520 85734 0.191

16° 29 4.52 2.513 62289 0.115

Table 4.8.2: Results for the 66 MeV neutron measurement measured in neutral channel (inner segment of the C-detector) without anticoincidence conditions.

Position Time

[min]

Ref.

106 [cm-2]

Ref.

H*(10)

[mSv]

NSi DSi

[mGy]

0° 28 4.51 2.520 122384 0.353

16° 29 4.52 2.513 82414 0.187

Table 4.8.1 and Table 4.8.2 show the results from the 97 MeV runs thereby the absorbed dose and

the number of particles, NSi measured in the C-detector with anticoincidence conditions is shown in

Table 4.8.3. The same measurements without the anticoincidence condition are displayed Table

4.8.4

Table 4.8.3: Results for the 97 MeV neutron measurements measured in neutral channel (inner segment of the C-detector) with anticoincidence conditions.

Position Time

[min]

Ref.

106 [cm-2]

Ref.

H*(10)

[mSv]

NSi DSi

[mGy]

0° 146 7.06 4.922 103575 0.231

16° 87 4.713 2.306 61934 0.108



Table 4.8.4: Results for the 97 MeV neutron measurements measured in neutral channel (inner segment of the C-detector) without anticoincidence conditions.

Position Time

[min]

Ref.

106 [cm-2]

Ref.

H*(10)

[mSv]

NSi DSi

[mGy]

0° 146 7.06 4.922 419025 0.694

16° 87 4.713 2.306 188726 0.243

4.8.2 The Phoswich Instrument for Neutrons and Gammas (PING)


The Phoswich Instrument for Neutrons and Gammas (PING), which has been developed in the

dissertation of Esther M. Dönsdorf [Dönsdorf 2014], has been irradiated at the iThemba Laboratory

for Accelerator Based Sciences, Somerset West, South Africa in October 2011. The measurements

with 66 MeV and 100 MeV neutrons were performed for the purpose of energy calibration of the

instrument only.

The instrument is a Phoswich detector which consists of two different scintillators: a BC-412-plastic

scintillator and a sodium-doped ceasium iodide crystal (in the following referred to as CsI(Na)). The

plastic scintillator is dedicated to measure neutral particles, especially neutrons, whereas the

CsI(Na) serves as an anti-coincidence. In 4.8.5 a sketch of the sensor head of PING as well as the

direction of the beam is shown. The long side of the sensor head was placed parallel to the beam,

so that the incident neutrons could traverse the full 8 cm of the plastic scintillator. The Fig 4.8.6

shows a picture of PING at the beam line.

Figure 4.8.5: Sketch of the sensor head of PING and its position with respect to the beam (adopted from Dönsdorf, 2014).



Figure 4.8.6: PING set up in the neutron vault at the iThemba Laboratory for Accelerator Based Sciences; in this picture the neutron beam comes from the left side.

4.8.2.2 Analysis of the measurements

The 66 MeV and 100 MeV neutron measurements were used for the energy calibration of the

events in the plastic scintillator. Measurements at the 0° as well as at the 16° position were

performed.

In order to use these measurements for the energy calibration, first the events occurring exclusively

in the plastic scintillator had to be separated from the rest of the events via pulse shape analysis

and a pulse height correction was done [Dönsdorf, 2014]. After that a spectrum of these events was

generated for each neutron energy (66 MeV and 100 MeV) and each position. The two

measurements at the 0° position show a typical edge in the spectrum which results from the

interaction of neutrons depositing their full energy in the detector. These edges were fitted with an

error function to determine their position. For 66 MeV the edge is at 872.3 ± 0.5 mV and for 100

MeV it is at 1141.7 ± 1.0 mV [Dönsdorf 2014]. The measurements at the 16° position can be used to

determine the background radiation. For the energy calibration the background was not taken into

account, because the effect of the background subtraction regarding the positions of the neutron

edges is below 0.5 % for the 66 MeV as well as the 100 MeV measurements [see Dönsdorf, 2014].

The spectra of both measurements at the 0° position feature an additional peak below the neutron

edge. It is assumed that these peaks are the result of the reaction C(n,d) (Röttger, 2012) in the

instrument. In Figure 4.8.7 the measurements at the 0° and 16° position are shown for 66 MeV as

well as 100 MeV.



Figure 4.8.7: Measurements with 66 MeV and 100 MeV neutrons at the 0º position and at the 16º position (adopted from [Dönsdorf, 2014])

The beam time in October 2011 at the iThemba Laboratory for Accelerator Based Sciences was very

successful regarding the measurements with PING. The results were essential for the energy

calibration of the instrument. In addition the results were very valuable because it could be

validated that it is possible to measure 100 MeV neutrons with this instrument. Figure 4.8.8 shows

the calibration curve for neutrons included those received as monoenergetic beam at PTB. It shows

the dependence of the signal at the Analogue-Digital Converter (ADC) in dependence of neutron

energy. For details see the dissertation of Doensorf [Dönsdorf 2014].



Figure 4.8.8: Neutron calibration curve: measured voltage at ADC in dependence of incident neutron energies.

4.8.3 Radiation Assessment Detector (RAD)


The Radiation Assessment Detector (RAD), onboard the Mars Science Laboratory rover Curiosity,

measures energetic particles, and the radiation dose rate on the surface of Mars. The RAD

instrument can measure charged as well as neutral particles and is described in detail in [Hassler

2012]. A sketch of the instrument is shown in Figure 4.8.9.

The iThemba measurements focused on understanding the neutral particle measurements.

Gamma rays and neutrons are measured by two scintillators, D and E, which are fully surrounded

by detectors that are used in active anticoincidence (AC) to reject charged particles. The D

scintillator, which consists of caesium iodide, is highly sensitive to gamma rays. The E detector,

which consists of plastic, is highly sensitive to neutrons. To exclude charged particles from the

neutral particle measurement, B, C, and F are used as anticoincidence. The energy analysis is done

via pulse-height analysis on board.



Figure 4.8.9 Schematic view of the Radiation Assessment Detector, consisting of three silicon detectors (A, B, C), a cesium iodide scintillator (D) and a plastic scintillator (E). Both scintillators are surrounded by a plastic anticoincidence (F). For detecting charged particles, A, B, C, D, E are used as a telescope. Neutral particles are detected in D and E using C and F as anticoincidence.

4.8.3.2 Neutral Particle Measurement

The charged particles that are stopped by the scintillators in RAD deposit their full energy. In

contrast, neutral particles can create an energy deposit which is randomly distributed and ranges

from zero up to their incident energy. This makes the measurement of neutral particles very

difficult. Further problems arise from the fact that the D detector is not only sensitive to gamma

rays, but also to neutrons, albeit to a lesser degree. Similarly, E is also sensitive to gamma rays.

Therefore, the neutral particle measurements in D and E do not reflect the incident gamma and

neutron spectra. In [Köhler, 2011 and Köhler, 2012] we demonstrated that the incident

gamma/neutron spectra can be obtained via an inversion method.

An essential factor of this inversion is the detector response function (DRF) which is obtained using

the GEANT4 Monte-Carlo code. The simulation is based on a detailed model of instrument and

includes effects such as electronic and optical noise in D, E, F. However, the fidelity of GEANT4 in

describing the response of the detector to neutral particles correctly has to be verified in

calibration runs. Parameters which need to be determined are:

Calibration values (MeV/ADC) Optical noise in the scintillators Quenching in the scintillators The correct physics models/list to use in the GEANT4 simulation.



The iThemba neutron beam measurements mainly addressed the last point and were used to

compare the QGSP_BIC_HP and QGSP_BERT_HP physics list, which use the BINary and the BERTini

intranuclear cascade model to calculate cross section for high energy nucleons.

The instrument which was used for the iThemba LABS measurements is a prototype model of the

instrument which is currently on Mars.

4.8.3.3 -Background Detector

The -Background detector was developed to provide an additional, independent neutral particle

measurement, and to have a reliable background measurement for RAD. It features two detectors,

one cesium iodide and one gadolinium oxyorthosilicate scintillator. The first scintillator is identical

to the D detector in RAD. The second scintillator was chosen to have an additional detector

material for validation with GEANT4. A CAD view of the detector is shown in Figure 4.8.10.

Figure 4.8.10 CAD view of the Ɣ-backound detector. The cesium iodide scintillator is shown in orange, the gadolinium oxyorthosilicate scintillator is shown in blue.

4.8.3.4 Experimental Setup

4.8.3.4.1 100 MeV Neutron Beam

Table 4.8.5: RAD was placed in the 0° beam position for the following three runs:

date Start stop start stop 106 cm-2

run no. # #

logger time

8 2011-10-15 11:34:39 12:19:39 1434 1479 2700 0,289 ± 0,024

9 2011-10-15 12:26:39 13:19:39 1486 1539 3180 0,347 ± 0,028

10 2011-10-15 13:25:39 14:09:39 1545 1589 2640 0,287 ± 0,024



Table 4.8.6: The Ɣ-Background detector was placed in the 16° beam position for the following two runs:

date start stop start stop / 106 cm-2

run no. # #

logger time

11 2011-10-

15 14:24:39 15:24:39 1604 1664 3600 0,391 ± 0,032

12 2011-10-

15 15:32:39 16:29:39 1672 1729 3420 0,374 ± 0,031

4.8.3.4.2 66 MeV Neutron Beam

Table 4.8.7: RAD was placed in the 0° beam position for the following two runs:

date Start stop start stop Φ0 / 106 cm-2

run no. # #

logger time

10 2011-10-08 16:33:00 17:15:00 1186 1228 2520,00 0,121 ± 0,010

11 2011-10-08 17:28:00 18:27:00 1241 1300 3540,00 0,161 ± 0,014

Table 4.8.8: The Ɣ-Background detector was placed in the 0° beam position for the following three runs:

date Start stop start stop Φ0 / 106 cm-2

run no. # #

logger time

13 2011-10-08 18:47:00 19:45:00 1320 1378 3480,00 0,153 ± 0,013

14 2011-10-08 19:53:00 19:54:00 1386 1387 60,00 0,0047 ± 0,0006

15 2011-10-08 19:59:00 20:58:00 1392 1451 3540,00 0,157 ± 0,013

Table 4.8.9: The Ɣ-Background detector was placed in the 16° beam position for the following two runs:

date start stop start stop Φ0 / 106 cm-2

run no. # #

logger time

17 2011-10-08 21:27:00 22:26:00 1480 1539 3540,00 0,141 ± 0,012

18 2011-10-08 22:39:00 00:45:00 1552 1678 7560,00 0,283 ± 0,024



4.8.3.5 Data evaluation and Results

For each measurement, histograms of the energy deposits in RAD detectors D and E were

generated. Those were corrected by the background measurements and compared to GEANT4

simulations.

Due to technical problems the 66 MeV neutron measurements are polluted by electronic noise.

However, these problems were solved prior to the 100 MeV neutron measurements.

The GEANT4 simulation using the Bertini cascade model showed significantly better agreement

with the measurements than the simulations using the Binary cascade. An example of a

comparison between simulation and measurement is shown in Figure 4.8.11. Based on these

results, and on measurements at The Svedberg Laboratory and at the Physikalisch-Technische

Bundesanstalt, the Bertini cascade model was selected for describing the detector response for

high energy neutrons.

Figure 4.8.11 Example of neutron beam measurements and simulation in the RAD D detector. The left Figure shows a simulation using the Binary cascade, the right figure shows a simulation using the Bertini cascade.



5 Summary The aim of the campaign was to achieve response functions for different devices and to

characterize them at neutron energies around 60 and 100 MeV. In order to receive mono-energetic

conversion factors at 62.4 MeV and 96.4 MeV a subtraction method should be applied which

assumes that the neutron spectrum at the 16°-beam line describes the background neutron

spectrum of the 0°-beam line quite well and can thus be subtracted. A further goal was to identify

more robust instruments which are needed for example to meet operational aspects in air flight

operations.

The subtraction method could be only successful applied for one instrument, namely the πDOS

instrument, which is a 2’’ tissue-equivalent counter. The instrument FRED a silicon detector with

anticoincidence also provided data for the different angles of the beam, but no conversion factors

were given for the two peak regions. The silicon detector is so thin that almost all of the recoils

escape and trigger the anticoincidence; therefore the instrument is not capable of measuring high

energy neutrons. Liulin silicon detectors gave varying responses which did not allow the use of the

subtraction method and the determination of conversion factors. The instrument PING, basically a

scintillation counter with anticoincidence, provided excellent results, since a calibration function

for neutrons up to 100 MeV could be established.

Of the passive devices plastic nuclear track detectors could be applied successfully in so far as the

response function energies have been proven to be independent of the neutron energies between

1 and 100 MeV. With these experiments, the participants had the possibility to independently use

high-energy neutron beams to calibrate their detectors or to validate specific methods for dose

estimation. Based on these data the dosimetry performances of PADC will be evaluated in future

benchmarking experiments. It was shown that the application of thermoluminescent detectors

(TLD) in high energy neutron fields is very limited and no further experiments are recommended.

Staff and passengers in airliners as well as astronauts in space are constantly exposed to neutrons

with these energies. Furthermore proton therapy facilities producing secondary neutrons of these

energies are on the increase. The wide and independent spectrum of choice displayed in this

report as to how neutron dose measurements at high energies can be performed, and where

similar detectors and methods were used, the variation in results, is indicative of a field far from

maturity. Clearly there is a lot of scope for development of detectors and detector systems,

especially with respect to personal dosimetry and area monitoring, for the measuring of neutron

doses reliably at these energies



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