Characterization of a Neutron Source

for Fission Yields Studies

Andrea Mattera

Licentiate Thesis

Department of Physics and AstronomyDivision of Applied Nuclear Physics

2014

Characterization of a Neutron Source for Fission Yields Studies

Andrea Mattera

Department of Physics and Astronomy

Division of Applied Nuclear Physics

Uppsala University

2014

ABSTRACT

In this thesis the design and the characterization of a neutron source for measurements of

neutron-induced �ssion yields are presented.

Fission yields, i.e. the probability of a certain �ssion product to be produced in a �ssion

event, are of interest in several �elds. In basic science they are important for the under-

standing of both the nuclear physics behind the �ssion process and the nucleosynthesis in

nuclear astrophysics.

Detailed knowledge of this quantity is also relevant for applications, such as nuclear energy

production. In particular, a good estimation of the inventory of �ssion products at all

stages of the fuel cycle is important, e.g., for reactivity calculations, for the estimation of

decay heat or for the storage of spent nuclear fuel. The present knowledge of �ssion yields

is generally su�cient for the safe operation of the current generation of nuclear reactors.

However, more and better data, especially at high incoming neutron energies, are needed

in view of Generation IV fast reactors or Accelerator Driven Systems.

The present work was developed in the framework of the AlFONS project and proposes

to measure neutron-induced independent �ssion yields at the IGISOL-JYFLTRAP facil-

ity, University of Jyväskylä, Finland. For this purpose, a proton-neutron converter was

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designed that uses the 30 MeV proton beam of the MCC30/15 cyclotron recently installed

at IGISOL to produce a white neutron �eld.

The converter consists of a 5 mm-thick water-cooled beryllium plate. The neutron source

was simulated using di�erent Monte Carlo codes (MCNPX and FLUKA), that predict

a �ux of neutrons above 1 MeV larger than 1012 neutrons on target per second, at the

nominal current of the MCC30/15.

The high-energy part of the neutron spectrum was characterized in a measurement at

the The Svedberg Laboratory facility, Uppsala, Sweden. The results of the measurement

show that the FLUKA simulation tends to underestimate the neutron yield at the highest

energies, while MCNPX shows a better agreement with the experimental data.

A prototype of the proton-neutron converter was installed at the IGISOL-JYFLTRAP

facility for a test-run in March, 2014. During this experiment no �ssion was induced,

but the neutron source was tested for the �rst time and a pre-study was performed for

the estimation of the neutron �ux and energy spectrum. Some preliminary results of the

absolute neutron yield obtained in this measurement will be shown, as well as the plans

for di�erent con�gurations of the neutron source, that will allow the measurement of the

energy dependence of �ssion yields.

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Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of Papers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Fission yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 Status of Independent neutron-induced �ssion yields Nuclear Data . . . . . 6

1.2.1 Measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.2 Available data and uncertainties . . . . . . . . . . . . . . . . . . . . 7

1.2.3 Ongoing and planned experiments . . . . . . . . . . . . . . . . . . . 8

1.3 IGISOL-JYFLTRAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.1 Measurement of �ssion yields at IGISOL-4 . . . . . . . . . . . . . . 10

1.3.2 Towards neutron-induced �ssion yield measurements . . . . . . . . 13

2 The Neutron Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 A neutron source for IGISOL-JYFLTRAP . . . . . . . . . . . . . . . . . . 15

2.2 The early design: a full-stop target . . . . . . . . . . . . . . . . . . . . . . 16

2.2.1 Heat removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3 A thin beryllium target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.3.1 n-production reactions . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3.2 Existing data on Be sources . . . . . . . . . . . . . . . . . . . . . . 20

2.3.3 Monte Carlo calculations of the neutron yield . . . . . . . . . . . . 21

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3 Validation measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1.1 Measurement positions . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.2 TOF detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.2 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.2.1 Selection of the neutron events . . . . . . . . . . . . . . . . . . . . 32

3.2.2 Choice of the Pulse Height threshold(s) . . . . . . . . . . . . . . . . 33

3.2.3 Correction for the e�ciency of the detector . . . . . . . . . . . . . . 33

3.2.4 Proton current correction and subtraction of the background . . . . 35

3.3 Results and Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4 Current & Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1 Installation and �rst characterization of the converter at IGISOL . . . . . 41

4.2 Other targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2.1 Thin target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2.2 Moderated neutron source . . . . . . . . . . . . . . . . . . . . . . . 43

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

A Monte Carlo simulation of the scintillator detector . . . . . . . . . . . . 49

A.1 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

A.2 Conversion from Energy to Light Output . . . . . . . . . . . . . . . . . . . 49

A.3 Extraction of the detector e�ciency . . . . . . . . . . . . . . . . . . . . . . 49

A.3.1 Light Output resolution . . . . . . . . . . . . . . . . . . . . . . . . 50

A.4 Validation and selection of the libraries . . . . . . . . . . . . . . . . . . . . 52

A.4.1 Comparison with experimental data (Drosg) . . . . . . . . . . . . . 52

A.4.2 Comparison with previously calculated detector e�ciencies for theNordball detector (Arnell et al.) . . . . . . . . . . . . . . . . . . . . 53

A.5 Extraction of the interaction depth . . . . . . . . . . . . . . . . . . . . . . 58

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B Measurement of the neutron �ux with TFBC detectors . . . . . . . . . 59

B.1 Functioning principle of TFBCs . . . . . . . . . . . . . . . . . . . . . . . . 59

B.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

B.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

B.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Paper I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Paper II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

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List of Papers

This thesis is partly based on the following papers:

Paper I

Characterization of a Be(p,xn) neutron source for �ssion yieldsmeasurements

A. Mattera, M. Lantz, S. Pomp, V. Rakopoulos, A. Solders, P. Andersson, A. Hjalmars-son, J. Valldor-Blücher, A.V. Proko�ev, E. Passoth, D. Gorelov, H. Penttilä, S. Rinta-Antila, R. Bedogni, A. Gentile, D. Bortot, A. Esposito, M.V. Introini and A. Pola.

Nuclear Data Sheets, Volume 119, May 2014, Page 416

My contribution: I participated in the experiment and performed the analysis. I alsowrote the paper.

Paper II

Measurement of the energy spectrum from the neutron source plannedfor IGISOL

A. Mattera, R. Bedogni, V. Rakopoulos, M. Lantz, S. Pomp, A. Solders, A. Al-Adili, P.Andersson, A. Hjalmarsson, J. Valldor-Blücher, A. Proko�ev, E. Passoth, A. Gentile, D.Bortot, A. Esposito, M.V. Introini, A. Pola, H. Penttilä, D. Gorelov and S. Rinta-Antila

Proceedings of the ERINDA Workshop, CERN, Geneva, Switzerland, 1-3 October 2013,CERN-Proceedings-2014-002

My contribution: I participated in the experiment and performed the analysis of theTime of Flight data. I also wrote the sections of the paper related to the Time of Flightmeasurement.

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Other papers not included in the thesis:

A ROOT-based analysis tool for measurements of neutron-induced �ssion prod-ucts at the IGISOL facility

A. Mattera, D. Gorelov, M. Lantz, B. Lourdel, H. Penttilä, S. Pomp and I. Ryzhov

Physica Scripta, Volume T150, 2012, Page 14025

Accurate Fission Data for Nuclear Safety

A. Solders, D. Gorelov, A. Jokinen, V.S. Kolhinen, M. Lantz, A. Mattera, H. Penttilä, S. Pomp,V. Rakopoulos and S. Rinta-Antila

Nuclear Data Sheets, Volume 119, May 2014, Page 338

Target thickness dependence of the Be(p,xn) neutron energy spectrum

V. Rakopoulos, M. Lantz, P. Andersson, A. Hjalmarsson, A. Mattera, S. Pomp, A. Solders, J.Valldor-Blücher, D. Gorelov, H. Penttilä, S. Rinta-Antila, R. Bedogni, D. Bortot, A. Esposito, A.Gentile, E. Passoth, A.V. Proko�ev, M.V. Introini and A. Pola

EPJ Web of Conferences, Volume 66, March 2014, Page 11032

Design of a high intensity neutron source for neutron-induced �ssion yieldstudies

M. Lantz, D. Gorelov, A. Jokinen, V.S. Kolhinen,A. Mattera, H. Penttilä, S. Pomp, V. Rakopou-los, S. Rinta-Antila and A. Solders

Individual Reports. Annex to Compendium of Neutron Beam Facilities for High Precision Nuclear

Data Measurements, IAEA-TECDOC-1743, July 2014, Page 234

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Independent Isotopic Product Yields in 25 MeV and 50 MeV Charged ParticleInduced Fission of 238U and 232Th

H. Penttilä, D. Gorelov, P. Karvonen, V.-V. Elomaa, T. Eronen, J. Hakala, A. Jokinen, A.Kankainen, I.D. Moore, J. Parkkonen, S. Rahaman, S. Rinta-Antila, J. Rissanen, V. Rubchenya,T. Sonoda, J. Äystö, M. Lantz, A. Mattera, V.D. Simutkin, S. Pomp and I. Ryzhov

Nuclear Data Sheets, Volume 119, May 2014, Page 334

Design of a neutron converter for �ssion studies at the IGISOL facility

M. Lantz, D. Gorelov, A. Mattera, H. Penttilä, S. Pomp, D. Rados, I. Ryzhov and the IGISOLgroup

Physica Scripta, Volume T150, 2012, Page 14020

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List of acronyms

ADC Analog-to-Digital ConverterAlFONS Accurate FissiOn data for Nuclear SafetyAUP Actual User PositionBSS Bonner Sphere SpectrometerCERN Organisation Européenne pour la Recherce Nucléaire

(European Organization for Nuclear Research)DAQ Data AcQuisitionENDF Evaluated Nuclear Data FilesFALSTAFF Four Arm cLover for the Study of Actinide Fission FragmentsFLUKA A fully integrated particle physics Monte Carlo simulation packageFR Fast ReactorFWHM Full Width at Half MaximumFY Fission YieldsGANIL Grand Accélérateur National d'Ions Lourds

(Large Heavy Ion National Accelerator)GSI GSI Helmholtzzentrum für Schwerionenforschung GmbH

(GSI Helmholtz Centre for Heavy Ion Research)IAEA International Atomic Energy AgencyIGISOL Ion Guide and Isotope Separator OnLineILL Institut Laue-LangevinJYFL Jyväskylän Yliopisto Fysiikan Laitos

(University of Jyväskylä Department of Physics)JYFLTRAP Penning trap at JYFLLANSCE Los Alamos Neutron Science CEnterLWR Light Water ReactorLO Light OutputMC Monte CarloMCA Multi-Channel AnalyserMCNPX Monte Carlo N-Particle eXtendedNDL Nuclear Data LibraryNFS Neutrons for ScienceNIM Nuclear Instrumentation ModulePH Pulse HeightPSD Pulse Shape DiscriminationPMT Photo-Multiplier TubeRF Radio Frequency

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RMS Root Mean SquareSC Shadow ConeSOFIA Studies On FIssion with AladinSPIDER Spectrometer for Ion DEtermination in �ssion ResearchSTEFF SpectromeTer for Exotic Fission FragmentsTDC Time-to-Digital ConverterTFBC Thin Film Breakdown CounterTOF Time of FlightTSL The Svedberg LaboratoryVAMOS VAriable MOde SpectrometerWA Wrap Around (or Frame Overlap)WRENDA World REquest list for Nuclear DAta

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Preface

This thesis focuses on the characterization measurement of a neutron source for neutron-induced �ssion yield studies.

The work was developed in the framework of the AlFONS project (Accurate FissiOn datafor Nuclear Safety) for the measurement of neutron-induced �ssion yields for applications.For this reason, a broader discussion will be presented, involving some background on themotivation for new experiments to obtain accurate nuclear data and - more speci�cally -�ssion yields data. Some de�nitions of �ssion yields and the current status of the techniquesavailable for measuring them will be brie�y introduced. Some emphasis will be put on thedescription of the IGISOL-JYFLTRAP facility, where the AlFONS project proposes tomeasure neutron-induced �ssion yields in the near future. The technique used at IGISOL-JYFLTRAP for the identi�cation of �ssion yields will be described and put into the broaderpicture of available techniques. All this will be the subject of Chapter 1.

IGISOL-JYFLTRAP has, so far, been used to study proton-induced �ssion yields. A ma-jor upgrade of the facility, concluded in 2013, saw the installation of a new high-currentcyclotron. This opened up new possibilities for the facility, such as the production of neu-trons, through a proton-neutron converter, that could be used to measure neutron-induced�ssion yields. The design of the neutron source, a responsibility of Uppsala University,started in 2010. The various steps of the development work through simulation of theneutron yield and of the mechanical properties of the proton-neutron converter will bediscussed in Chapter 2.

At the end of the development phase, when a design for the proton-neutron converterwas chosen, a characterization measurement was performed at The Svedberg Laboratory(TSL), in Uppsala. The analysis of these data constitutes the main part of this thesis. Theexperimental setup, the analysis and the results of this work will be presented in Chapter3 and compared with the simulations.

The measurement at TSL was performed in 2012 and, since then, much work has been doneat the IGISOL facility to prepare the beam-line and the cyclotron for new experimentalruns. In March 2014, a prototype of the proton-neutron converter was installed at IGISOLfor a test-run. This run was not used for �ssion-related measurements, but only as a �naltest of the latest prototype and as a pre-study for to determination of the absolute neutronoutput. This ongoing work and the �rst results of the analysis of the data collected in thetest-run will be shown in Chapter 4. In addition, some proposals for a di�erent neutronsource design to be used in a second phase of the �ssion yield measurements will also bediscussed.

1

Chapter 1

Introduction

This chapter discusses the background and the motivation for Fission Yields (FY) mea-surements. The current status of evaluated Nuclear Data Libraries (NDL) and of ongoingor planned experiments will be described.

In particular, the plans for measurements of neutron-induced FY at the IGISOL -JYFLTRAPfacility in Jyväskylä, Finland, will be presented in some detail.

1.1 Fission yields

Once the process of �ssion was recognized in 1938 by Hahn, Strassmann and Meitner, aseries of experiment started in order to understand its features. Among others, was themeasurement of �ssion products, in terms of their kinetic energy and production yield.

In particular, �ssion yields, the amount of a certain nucleus produced after �ssion, were rec-ognized as an important parameter to be measured very early-on and large radiochemistrylaboratories were set-up in the framework of the Manhattan project. There, Anderson,Fermi and Grosse were among the �rst to measure what were then referred to as branchingratios of 235U [8].

In the exit channel of the �ssion reaction, �ssion fragments are often produced in a highlyexcited state. They can release their internal excitation energy emitting particles (such asprompt neutrons or prompt γs) and �nally decaying to less neutron-rich nuclei, movingtowards the valley of stability. This usually happens through β−-decay, but it can alsoinvolve di�erent processes (e.g. emission of a neutron, in this case referred to as a delayedneutron).

Due to the complexity of this process and because of the time-dependence of the amountof a certain isotopic species present, it is useful to de�ne di�erent types of �ssion yields(adapted from Ref. [9]):

� Independent �ssion yields: number of atoms of a speci�c nuclide pro-duced directly in the �ssion process (i.e. not via radioactive decay of pre-cursors).

� Cumulative �ssion yields: total number of atoms of a speci�c nuclideproduced (directly and via decay of precursors).

3

235U

132

A = 132

A = 100

Fig. 1.1: Nuclides produced in the n-induced �ssion of 235U. Fissionable nuclides are de-picted in green, while �ssion products are in pink in the neutron-rich half of the chart ofnuclides. Nuclides marked in blue are stable or long-lived at the end of a decay chain. Theshape of the Fission Yields distribution changes with the incoming neutron energy (inset).

� Total chain yields: de�ned as the sum of cumulative yield(s) of the last(stable or long-lived) chain member(s).

� Mass number yields: de�ned as the sum of all independent yields of aparticular mass chain and are in this way distinguished from chain yields.Some of the most modern methods to measure �ssion yields provide sets oftruly independent yields which - at summation - will produce mass numberyields rather than chain yields.

Some experimental methods allow the measurement of yields prior to promptneutron emission. These yield distributions are designated as fragment yields.Measurements of yields posterior to prompt neutron emission are instead re-ferred to as product yields.

In a �ssion event (e.g. thermal �ssion of 235U), more than 140 nuclei can be produceddirectly after �ssion with a probability (independent fragment yield) larger than 0.1% [10].They populate the lower part of the chart of nuclides, corresponding to the neutron-richnuclei (Fig. 1.1). The shape of the FY distribution depends not only on the �ssioningnuclide, but also on the energy available for the reaction that - for n-induced �ssion -depends on the energy of the incoming neutron (cfr Fig. 1.1) [11].

4

Already in the �rst measurement of the FY of 235U, the mass asymmetry of the �ssionproducts was identi�ed. This appears in the mass number FY curve as a double-humpeddistribution, with maxima around mass number A = 95 and A = 140 [12].

Less than 4 years after the �ssion process was identi�ed and the possibility to start achain reaction theorized, the �rst self-sustaining reactor was built in a Squash Court onthe campus of the University of Chicago. The reactor, with natural uranium as fuel andmoderated by graphite, was slowly brought to criticality on December, 2nd 1942. It wasproved that the power level of the nuclear pile could be easily controlled by means ofneutron absorber strips inserted in the reactor core [13].

Twelve years later, the �rst nuclear power plant was connected to the electricity grid. Itwas the Soviet APS-1 reactor at Obninsk [14]. Since then, the use of nuclear power forelectricity production has increased remarkably all over the world, reaching a total installedcapacity in 2014 of 376 GW(e) [15].

Fission yields are among the quantities that are needed for the operation of nuclear powerplants and accurate nuclear data on FY are important in applications for several reasons.

Talking about nuclear power, when �ssion products are mentioned, the thought goesstraight to the issue of long-term storage of nuclear waste, where they constitute the largepart of the radioactivity and heat in the �rst hundred years of storage, or to the inventoryof �ssion products that - in case of severe accidents - could be released from the reactorcore. These are both legitimate reasons for better nuclear data for FY, but probably notthe main.

Today, 60 years of operation of nuclear reactors prove that the knowledge in terms of nucleardata is su�cient for predictions of the reactors' parameters for the everyday operation ofpower plants. The inventory of �ssion products in a Light Water Reactor (LWR), the kindused on large scale all over the world, are generally known with an accuracy that is enoughto ensure their safe operation and storage of the fuel.

This might not be true if the fuel burnup1 is to be increased beyond the current levels, sothat even �ssion products that have a relatively low probability to be produced (becausethey are in the tails of the FY-curve or produced in �ssion of minor actinides) couldbecome important in absolute value. This will a�ect not only the �ssion product inventoryfor storage, but also reactor kinetics, since a good knowledge of the delayed neutronsprecursors and of neutron poisons2 is required for safe reactor operation.

The requirements in terms of nuclear data are similar for the new concepts of GenerationIV Fast Reactors (FR) and Accelerator Driven Systems (ADS), but since in these designsthe neutron energy is higher, data become scarce.

Measurement of �ssion yields, however, is not only about applications. Fission yields are

1Burnup measures the use of uranium in a reactor (GWd/Metric t of U) and depends on the time thefuel is kept in the reactor core and what power the reactor is operated at.

2A neutron poison is a nuclide with a large cross section for neutron absorption. When the amount ofpoisons becomes too large, the reactivity of the reactor is a�ected [16].

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also of great interest for the basic understanding of a complex phenomenon such as �ssionitself, for which a complete theoretical model is still missing and in nuclear astrophysics forthe understanding of the r -process, that is responsible for the synthesis of heavy elementsabove Germanium in the cosmos [17].

1.2 Status of independent n-induced FY Nuclear Data

1.2.1 Measurement techniques

Several di�erent techniques exist for the measurement of �ssion yields. Some are moreapplicable to the production of chain yield curves, while others - thanks to their shortmeasurement time - can be used to obtain independent FY. Here follows a brief review ofthe available methods, with some references to notable experiments and an overview of thestrong and weak points of each technique.

Radiochemical separation

Measurements of �ssion products started in the 1940s during the Manhattan project andwere performed using radiochemical methods. The technique consists in dissolving thelongest lived isotope of a radioactive chain after a waiting time that has allowed all precur-sors to decay and then measure its activity. In the early days, before the development ofdetectors for high-resolution γ-ray spectroscopy, β-radiation was usually measured. Thisrequired many corrections and a di�cult analysis procedure that resulted in large uncer-tainties on the �nal product yields. This methodology is now abandoned in favour of γ-rayspectroscopy, and the old data are heavily scrutinized by evaluators before they are insertedinto nuclear data libraries [9].

Mass spectrometry

Mass spectrometry, as the name suggests, measures the mass of the nuclides in an evapo-rated and ionized sample and is usually a very precise technique. The need for a normal-ization step to go from relative to absolute yields is the main source of uncertainty [9].

A disadvantage with this technique, is that large �ssile targets and long irradiation times arerequired to obtain su�cient amounts of �ssion products for the analysis, possibly resultingin changes in the �ssion yield distribution due to neutron capture in the �ssion products.All things considered, the typical uncertainty of this technique for measurement of chainyields is below a few per cent [18].

This technique can also be applied to measurements of independent �ssion yields, where�ssion products should be measured as quickly as possible after their production. In thiscase, the �ssion target is usually connected directly to the ion source needed to produce

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the evaporated and ionised target for the analysis. The problems connected with long irra-diation times do not apply, but other precautions must be taken in order not to introducebiases due to, e.g., di�erent volatilisation properties of di�erent elements.

Direct γ ray spectroscopy

This technique identi�es the �ssion products by analysing their γ lines. The data analysisis quite complicated and requires a good knowledge of the decay properties of the nuclidesthat are to be measured. The big advantage of requiring only milligram-amounts of the�ssile nuclide, makes this technique largely applied for exotic �ssionable targets of limitedavailability [9].

The uncertainty in chain yield determination is typically not better than 10% and �ssionproducts with very low yields are not accessible without further chemical-separation.

Measurement of unstopped fragments

All techniques that measure �ssion fragments at their full kinetic energy can be includedin this category. The basic principle of this technique is to calculate the masses of thetwo fragments from measurement of their kinetic energy and velocity using momentumconservation. One important requirement is for the �ssion target to be as thin as possible,to limit energy loss inside the target that could hinder an accurate determination of velocityand/or energy. [9, 19]

Notable examples of this technique are the Lohengrin mass separator [20] and the HI-AWATHA instrument [21], that separate and identify single fragments, and Cosi fan tutte,that instead was used to measure both fragments in coincidence [22].

Mass spectrometers as the ones mentioned above provide in principle only the mass of the�ssion products. If such a system is equipped with a detector able to discriminate thenuclear charge, also independent �ssion yields, and not only chain yields, can be provided.The nuclear charge is typically measured estimating the stopping power dE

dxof the ions.

This technique allows to identify single nuclei up to Z = 47 [9].

The possibility to measure more than one parameter at the same time (e.g. mass andkinetic energy of the fragments) is another advantage of this experimental technique. Thesetup could be complemented with further detectors to measure, e.g. the prompt neutronand γ multiplicities.

1.2.2 Available data and uncertainties

Despite the fact that most of the �ssion yields used in application are known to a level thatmakes safe operation of nuclear power plants possible, the �Requests for FY measurements

7

Fig. 1.2: Mass yield distribution of 235U neutron induced �ssion at thermal energy (blacksquares) from the ENDF/B-VII.1 library [24] together with relative uncertainties of thethermal yields from the same library (red bars) [4].

- supplement to WRENDA 93/94� published by the IAEA states that, for what concernsindependent �ssion yields, practically all �ssioning systems need further measurements [23].

Where measurements of independent yields are available, the uncertainties and the discrep-ancy between di�erent experiments make the need for new accurate measurements muchstringent, especially for what concerns the validation of nuclear models.

It is interesting to notice that even for thermal �ssion of 235U, by far the most studiedreaction in terms of independent �ssion yields, only 106 out of 998 individual yield ratiostabulated in the ENDF/B-VII.1 evaluation have a relative uncertainty smaller than 10%(Fig. 1.2) [4, 24].

The situation worsens for other �ssioning nuclides and higher neutron energies, so thatonly 4 mass chains have been measured for 241Pu at high energies (En = 14 MeV) [23].

1.2.3 Ongoing and planned experiments

Perhaps as an answer to the call for new experimental data from the International NuclearData Committee of the IAEA [23], the last decades saw a larger and larger interest aimed atthe measurement of �ssion products, so that we are witnessing a renaissance of experimentson �ssion yields [25].

Here follows a short overview of some of the ongoing and planned project in this regard,as it emerged from a recent workshop on FIssion Experiments and Theoretical Advances3

3http://t2.lanl.gov/�esta2014/workshop.shtml

8

(Santa Fe, Sept. 2014).

γ-ray spectrometry

Since 2008, a new e�ort in the measurement of �ssion products has been undertaken atthe ILL (Institut Laue-Langevin) facility, where the Lohengrin mass separator and a high-resolution ionization chamber is installed. This setup was complemented with a movingimplantation-tape and γ-spectrometry station to extend the measurement of isotopic yieldsto masses A > 42 [26].

Measurement of unstopped fragments

Many of the new experimental e�orts in the measurement of �ssion products yields areaimed at a multi-parametric measurement of the unstopped �ssion fragments. These usethe principle described in the previous section to determine the mass A and the charge Zof the �ssion products.

Since �ssion products are identi�ed on an event-by-event basis, this opens the way tomulti-parameter experiments, where the mass of the �ssion products are correlated withother observables (such as γ or neutron multiplicities). This is the case, for example, of theSTEFF setup. Here the nuclei are identi�ed measuring their velocity and energy (2E, 2vtechnique) and the emitted prompt-γ energy and multiplicity are measured in coincidence.Experimental runs have already been performed at ILL, showing a mass resolution of4 AMU. Work is now being performed to improve this �gure and new experiments areplanned at the new Experimental Area 2 of nTOF, at CERN.

New experimental setups are also being built at Los Alamos Neutron Science CEnter (LAN-SCE) - with the SPIDER spectrometer, a development based on the 2E,2v technique thatwas behind Cosi fan tutte [27] - and at the NFS facility, at GANIL, where the FALSTAFFspectrometer will be installed [28].

Experiments in inverse kinematics

New experimental programs are also being developed exploiting the availability of newheavy ions beams to perform experiments in inverse kinematics. In this concept, a beamof actinides or pre-actinides is impinged on a heavy mass target, where it �ssions.

The SOFIA (Study On FIssion with Aladin) experiment at GSI [29] and the VAMOSspectrometer at GANIL [30] both aim at extensive and systematic studies of �ssion in amass range spanning from pre-actinides (A = 180-210) up to heavy actinides (A > 238).

The great advantage of this technique is the possibility to study the �ssion of very exoticand/or radioactive actinides, that are usually impossible or very di�cult to produce inamounts and purities suitable for a neutron-irradiation experiment.

9

1.3 IGISOL-JYFLTRAP

In the framework of the AlFONS (Accurate FissiOn data for Nuclear Safety) project, theIGISOL-JYFLTRAP facility at the Department of Physics, University of Jyväskylä (JYFL)was proposed as the location for n-induced �ssion yield measurements of various actinides.

The University of Jyväskylä has a long history of measurements of nuclear data and nuclearphysics-related quantities. In particular, the Ion Guide and Isotope Separator OnLine(IGISOL) technique has been applied to FY studies.

Separation of �ssion products with the IGISOL technique is a standard method to selectisobaric chains that can be then analysed with gamma-ray spectroscopy for isotope iden-ti�cation. This is, however, a very long an tedious method that is very sensitive to theaccuracy with which decay schemes of �ssion products are known. In the early 2000s, thepossibility to couple the IGISOL technique with a high resolution mass spectrometer, thePenning trap, was investigated and tested at JYFL for proton-induced �ssion [31].

1.3.1 Measurement of �ssion yields at IGISOL-4

In 2010, the IGISOL-JYFLTRAP beam-line was closed for a major upgrade and in thesummer of 2013, the new IGISOL-4 line has been used again for the �rst time since theupgrade. During this campaign, p(25 MeV)-induced �ssion yields of 232Th were measuredand veri�ed the successful operation of the new setup [32].

The upgrade included the installation of a new cyclotron dedicated to IGISOL-JYFLTRAP,the MCC30/15. The cyclotron is able to deliver proton beams between 18 and 30 MeVat a maximum current of 200 µA [33]. Also deuterons can be accelerated to half thoseenergies at 60 µA. The �rst experiment using the new MCC30/15 was performed in early2014 (some details of this experiment will be discussed in chapter 4).

The measurement of �ssion products proceeds as follows (numbers correspond to the ele-ments of the IGISOL beam-line highlighted in the sketch in Fig. 1.3):

1O Fission is induced in an actinide target inside the �ssion chamber either with a directproton beam or with a neutron beam produced in a proton-neutron converter.

2O A �ow of helium gas is used to slow down the �ssion products and guide them towardsthe extraction electrode. Helium is also used to recombine the highly ionised nuclidesso that most of them reach a +1 state.

3O The ion beam, where still all masses are present, is then accelerated with a voltageof 30 kV towards a dipole magnet. It is here that the �rst selection based on thecharge-to-mass ratio Q

Ais performed. The mass resolving power of the dipole magnet

is M∆M≈ 5× 102, and is enough to isolate an isobaric chain.

10

Fig. 1.3: The IGISOL-JYFLTRAP upgraded beam-line at the University of Jyväskylä.1O: �ssion chamber. 2O: extraction electrode. 3O: dipole magnet. 4O: RFQ coolerand buncher. 5O: JYFLTRAP Penning trap. 6O: micro channel plate detector. Themeasurement procedure is described more in detail in section 1.3.1

4O The beam of isobars is then accelerated towards the Radio FreQuency (RFQ) coolerand buncher. This step is needed to prepare the beam for the analysis in the Penningtrap. The ions are accumulated over a period of a few hundred ms and during thistime, they lose energy in the helium contained in the RFQ cooler and buncher. Theenergy spread of the beam is reduced down to a few eV. Up until this point, the�ssion products travel as a continuous beam from the �ssion chamber, only after theRFQ cooler and buncher they travel as a bunch. A short bunch is needed for thecapture in the Penning trap and a low energy spread is important to increase theprecision of the measurement.

5O In the trap, a sequence of dipole and quadrupole excitations selects the nuclidesbased on their Q

Aratio with a maximum mass resolving power of ≈ 8 × 105. This

mass resolution is enough to select single nuclides based on their mass and, in somecases, also isomeric states of the same nuclide.

6O The selected ions are ejected from the trap and counted in a Micro Channel Platedetector. The mass-dependent quadrupole excitation frequency in the Penning trapis varied to cover the mass range of interest (usually corresponding to the isobaricchain selected by the dipole magnet), and frequency spectra can be produced witheach peak corresponding to a speci�c nuclide (see Fig. 1.4).

The main advantage of this technique is the possibility to unambiguously identify single

11

Frequency (Hz)

1064.94 1064.96 1064.98 1065 1065.02 1065.04

310×0

200

400

600

800

1000

1200

1400

1600

Frequency Scan - A =101

47-Ag-10141-Nb-101

8545 / 6448-Cd-10140-Zr-101

7131 / 64

Nr.

of

ions

Fig. 1.4: Example of a frequency scan obtained with the JYFLTRAP Penning trap atmass A = 101 in the p-induced �ssion of 232Th. The peaks of 101Zr and 101Nb have beenidenti�ed. No peak is observed for 101Cd and 101Ag since these two isotopes are in theproton-rich half of the chart of nuclides and are not produced in �ssion. The positionof the peaks as identi�ed from the banner is calculated from a rough frequency-to-masscalibration. Where a peak is found, the banner shows also the number of counts in thepeak (e.g. 7131 for 101Zr) and the number of times the scan was repeated to obtain enoughstatistics (64).

nuclides within a few hundred milliseconds from their production, thus obtaining actualmeasurements of Independent Fission Yields also for short-lived nuclides.

The selection of the nuclides with the dipole magnet and the subsequent measurement inthe Penning trap are chemistry-independent, making it possible to measure any elementin the periodic table.

The background for this measurement is very low, as can be seen in Fig.1.4, so that evennuclides with very low yield can be measured, making this a competitive technique formeasurements of �ssion products in the tails or the valley of the FY distribution. Thepossibility to automatically scan masses with the dipole magnet and in the Penning trapalso allows a systematic measurement of all masses of interest within the same experimentand in a relatively short time.

The main di�culty is the correct estimation of the e�ciency with which �ssion productsare stopped in the helium bu�er inside the �ssion chamber. It has to be veri�ed that no

12

bias is introduced depending on the mass, energy or chemistry of the �ssion products, sothat the number of ions measured in the Penning trap can be assumed proportional to theions produced in the �ssion events. This e�ect is currently being investigated and moredetails can be found in reference [34].

Another disadvantage with this technique is that it is not possible to measure more thanone observable at the same time, since the �ssion products are not identi�ed on an event-by-event basis, but their production is integrated over several milliseconds before the yieldis measured.

1.3.2 Towards neutron-induced �ssion yield measurements

The upgrade of the IGISOL facility and in particular the installation of the new MCC30/15 cyclotron, opened the way to new possible measurements at IGISOL.

The relatively high output current of the MCC30/154, will allow the use of protons toproduce neutrons that will then induce �ssion in an actinide target. This intermediatestep, even with an e�ciency of approximately 1%, will produce particles on target at levelsthat are still suitable for measurements of FY with the IGISOL-JYFLTRAP technique.

The installation of a neutron source will not only bene�t application-focused measurementsof n-induced FY, but will also be interesting for fundamental research. In �ssion inducedby fast neutrons, in fact, very short-lived nuclides far from the line of stability will beproduced. These exotic nuclei can then be separated with the IGISOL technique andstudied in the Penning trap, or by other techniques used at JYFL. Such measurements areof interest, for example, in nuclear astrophysics for the understanding of the nucleosynthesisprocess [17].

For the production of a neutron beam suitable for FY measurements, a proton-neutronconverter was designed and characterized. This will be the subject of the next chapters.

4For comparison, the K130 cyclotron used up until now at IGISOL-JYFLTRAP for FY measurementshas a maximum output current for protons of Imax = 60 µA [35].

13

Chapter 2

The Neutron Source

In order to perform measurements of neutron-induced �ssion yields at the IGISOL-JYFLTRAPfacility, a neutron source has been designed. The source takes advantage of the new high-current MCC30/15 cyclotron available in the new upgraded setup IGISOL-4 [36].

Several constraints in�uenced the design of the neutron source, up to the �nal prototypethat was tested at The Svedberg Laboratory in Uppsala before the installation at JYFL(see Chapter 3). These will be discussed in the opening of this chapter. The varioussteps to reach the �nal prototype of a water-cooled 5 mm-thick beryllium converter will bepresented.

The �nal prototype was evaluated in terms of neutron yield as well as heat-removal ca-pabilities with Monte Carlo and deterministic codes, respectively. Some results of thesestudies will be discussed.

2.1 A neutron source for IGISOL-JYFLTRAP

The neutron source was designed to be placed inside the �ssion chamber (cfr section 1.3),as close as possible to the �ssion target, in order to maximize the neutron �ux. However,IGISOL-4 will not be only dedicated to studies involving neutron-induced �ssion. The sameexperimental line and �ssion chamber will be used in experimental campaigns requiringdi�erent incoming particles (protons or heavy ions). For this reason, the setup was keptas simple as possible, with a proton-neutron converter that could easily be installed andremoved without major intervention on the beam-line.

Besides practical considerations on the simplicity of the target installation, several otherrequirements were set for the �nal proton-neutron converter:

� Total neutron yield: the �rst requirement, in terms of number of neutrons ontarget, was to have a rate of �ssions comparable to the one achieved in the p-induced�ssion experiments.

� Fast neutron fraction: once the source is installed, it is intended for a wider usethan measurements of independent �ssion yield curves. One further application willbe the production and characterization of n-rich nuclei far from the stability line.

15

These will be produced from �ssion events and their yield increases with increasingexcitation energy. For this reason a goal of ≈ 1012 fast (En > 1 MeV) neutrons persecond on the �ssion target was set.

� Neutron Spectra: for studies of n-induced �ssion yields for applications, the neu-tron spectrum should ideally resemble that of a typical fast reactor (FR). For thesake of comparison with existing data and validation of the technique, a Light WaterReactor (LWR) spectrum would also be desirable. This latter would also be usefulfor measurements of �ssion products with very low yield in the tails of the FY dis-tributions of well-studied �ssile targets such as 235U. In order to extract the energydependence of FYs, (quasi)mono-energetic neutron spectra would also be useful.

� Heat removal: at the maximum nominal current and energy of the MCC30/15, theproton beam will carry a power of 6 kW that will be partly deposited in the targetin the form of heat. This heat will have to be removed, preferably with a simplewater-cooling system, able to keep the target and all other elements of the proton-neutron converter well below their thermal failure temperatures (melting or boilingpoints). One additional complication in this regard is the fact that, in order to limitthe energy loss of the protons impinging on the target, the target must also act as avacuum window between the beam-line to the cyclotron and the �ssion chamber.

� Activation: the reaction with protons will transmute nuclei of the target. In somecases, this will only be an issue during, or right after, the proton irradiation, butsome longer-lived nuclides can also be produced. These issues should be consideredin the choice of the materials for the converter and the mechanical support.

Trying to ful�l all the aforementioned requirements, a �rst selection was done on thematerials suitable for a proton-neutron converter target. Looking in particular at the heatdissipation issue and the desire to avoid too tight margins on the operation of the neutronsource, it was chosen to investigate tungsten and beryllium as two of the most promisingmaterials, given their high melting points (3422 ◦C and 1287 ◦C, respectively) and thermalconductivities (173 W m−1K−1 and 200 W m−1K−1, respectively1).

Despite being toxic [37], beryllium was still thought to be suitable as a material for theproton-neutron converter thanks to its low chemical reactivity with air and water and sinceno handling will be required once the source is in place.

2.2 The early design: a full-stop target

The �rst designs of the neutron source aimed at maximizing the neutron yield, and a full-stop target was studied. The idea was developed starting from the design of the ANITAwhite n-source at The Svedberg Laboratory (TSL) [38].

1thermal conductivity at 20 ◦C

16

30 MeV-protons have a range of 5.8 mm in beryllium and 1.1 mm in tungsten [39]. Cylin-drical targets were then considered with a thickness that would completely stop the protonbeam.

Monte Carlo (MC) simulations were performed using both FLUKA [40] and MCNPX [41].The result of the simulations for a 20 mm-thick tungsten target is shown in blue in Fig.2.1. A version of this source with an additional moderator was obtained adding a layer of10 cm of water after the tungsten converter. The latter setup was investigated in order toproduce di�erent neutron energy spectra for studies of the energy dependence of FY. Theresults are shown in �gure 2.2.

Also full-stop beryllium targets were simulated: the results are shown in red in Fig. 2.1and 2.2.

From this �rst investigation on a thick-target design, it was possible to conclude thatspectra resembling - in the low energy region - that of a LWR are easy to obtain with bothmaterials considered. This can be achieved with a layer of water or of another equivalentlight material placed after the proton-neutron converter. Fast Reactor (FR) spectra areinstead harder to reproduce with this kind of setup.

2.2.1 Heat removal

The heat-transfer inside the metal target and the possibility to remove it with �owingwater was investigated using the COMSOL Multiphysics �nite element code [42].

For both materials it was found that it is possible to cool the target with a layer of �owingwater. The maximum temperatures reached by the water and the target, however, was ofseveral tens and hundreds of ◦C, respectively. The result from a calculation where watersurrounded a beryllium target is shown in Fig. 2.3.

2.3 A thin beryllium target

From the results obtained in the previous investigation, beryllium was chosen as a targetmaterial due to its higher yield of high-energy neutrons (cfr. �gure 2.1). This is a goodcompromise for FY measurements induced by neutrons of energies that were not exploredbefore and constitutes a very desirable quality for the production of n-rich nuclei far fromthe line of stability.

Besides the heat-removal, however, another problem can compromise the mechanical in-tegrity of a full-stop target: namely, hydrogen build-up (i.e. the accumulation of protonsinside the target). This was not addressed in the COMSOL calculations.

Hydrogen build-up was shown to be a major problem causing breakdown and disruption ofservice in the case of the LENS target, at Indiana University [43,44]. Long-term mechanical

17

Energy (MeV)­410

­310 ­210 ­110 1 10

s))

2 (

n/(

cm

dEΦ

d E

710

810

910

1010

thick Be ­ FLUKA

thick Be ­ MCNP

thick W ­ FLUKA

thick W ­ MCNP

Simulated Neutron Energy Spectrum

Fig. 2.1: Comparison of the neutron spectra from 30 MeV protons on thick Beryllium (red)and Tungsten (blue) targets simulated with FLUKA (solid) and MCNPX (dashed). Thespectra are rescaled to an incoming proton current of 100 µA and a distance of 15 cm fromthe source.

Energy (MeV)

­910

­810 ­710

­610

­510 ­410

­310 ­210 ­110 1 10

s))

2 (

n/(

cm

dEΦ

d E

610

710

810

910

1010

Simulated Neutron Energy Spectrum

thick Be + MOD ­ FLUKA

thick Be + MOD ­ MCNP

thick W + MOD ­ FLUKA

thick W + MOD ­ MCNP

Simulated Neutron Energy Spectrum

Fig. 2.2: Comparison of the neutron spectra from 30 MeV protons on thick Beryllium (red)and Tungsten (blue) converter, with a 10 cm layer of water used as moderator, simulatedwith FLUKA (solid) and MCNPX (dashed). The spectra are rescaled to an incomingproton current of 100 µA and a distance of 15 cm from the source.

18

(a) (b)

Fig. 2.3: Example of a COMSOL simulation of the heat removal in a thick berylliumtarget. In this design, cooling is achieved by means of a layer of cooling water surroundingthe sides of the cylinder. The maximum temperature reached in the target is ≈ 495 ◦C(a), while water reaches ≈ 88 ◦C (b), below the melting point of beryllium and the boilingpoint of water, respectively.

stability has to be guaranteed since the beryllium will be the only layer of material betweenthe vacuum of the cyclotron beam-line and the cooling water loop. For this reason, a newgeometry was considered with a target thinner than the range of protons.

In this case, the proton beam would still deposit the greater part of its energy in beryllium,but would eventually stop in the layer of water used to cool the target. This solution solvesthe problem of hydrogen accumulation in the target and simpli�es signi�cantly the heatremoval. A sketch of this design is shown in Fig. 2.4.

2.3.1 n-production reactions

In the new con�guration, a reduction of the neutron yield can be expected compared to afull-stop solution. An overview of the reactions contributing to the neutron production inberyllium by E ≤ 30 MeV protons is shown in table 2.1.

The energy that the 30 MeV protons will have after a 5 mm layer of Be is ≈ 10 MeV [45].From table 2.1 it is possible to see how the reactions with a lower threshold could contributeto the production of neutrons in the last millimetres of the proton path. These neutronswill be lost in the current design.

Production of neutrons in water is not expected to make up for this loss, since the thresholdsfor 16O(p,nX) reactions is & 16.7 MeV. On the down side, some reaction products (e.g.18F and 13N) are expected to contribute to the activation of the cooling water [46].

Further activation in the water loop will be induced by the neutrons produced in theberyllium converter.

19

Fig. 2.4: Exploded view of the target assembly. In the �nal design, the 5 mm-thick beryl-lium converter (orange) is placed inside the aluminium target holder (white hatched). Theprotons impinging on the beryllium (red arrow from the bottom) will stop in the �owingwater (blue) used to cool the target assembly. In this design, the beryllium is used as awindow between the �ssion chamber and the beam-line to the cyclotron (green).

2.3.2 Existing data on Be sources

A number of 9Be(p,xn) converters have been designed and characterized in the past, be-tween the 1960s and today. Some of the more relevant measurements, where the incomingbeam energy and the target thickness are close to the case proposed for the IGISOL p-nconverter, are summarized in table 2.2.

None of the listed examples could be directly referred to the IGISOL-JYFTRAP source.Some of the early studies would focus on thin targets for mono-energetic neutron sources,while - more recently - the interest for neutron sources was aimed at BNCT uses with anepithermal beam. In this latter case, the maximum proton energy impinging on the setupdid not reach 30 MeV or the neutrons were to some extent moderated before the spectrawere measured.

Studying previous experiment gave however the chance to learn what to expect in termsof total neutron yield. Brede and co-authors, in their paper from 1989 [47], extract anempirical relationship between incoming proton energy and neutron yield per unit beamcharge (Y/Q) at 0◦:

Y/Q = C · (Ep)D , (2.1)

with C = 2.62× 1012 sr−1C−1, D = 2.73 and Ep expressed in MeV. The values refer to theproduction of neutrons above 2 MeV by a thick beryllium target.

With these assumptions, a neutron yield of ≈ 5.6× 1012 neutrons sr−1s−1 above 2 MeVcould be expected for a 200 µA proton current on target, which would satisfy the require-ments on the fast neutron fraction for the neutron source.

20

Reaction ProductsQ-value(MeV)

Threshold(MeV)

n + p + 2 α -1.57 1.758Be + n + p -1.66 1.859B + n -1.85 2.055Li + n + α -3.54 3.937Be + n + d -18.34 20.386Li + n + 3He -18.45 20.527Li + n + 2p -18.92 21.03n + d + 3He -19.93 22.158B + 2n -20.43 22.717Be + 2n + p -20.56 22.866Be + n + t -22.76 25.305He + n + p + 3He -22.89 25.445Li + n + p + t -23.35 25.966Li + n + p + d -23.95 26.325Li + 2n + 3He -24.11 26.816Li + 2n + 2p -26.17 29.10

Table 2.1: Open channels of the reaction p + 9Be for proton energies below 30 MeV [46]

2.3.3 Monte Carlo calculations of the neutron yield

A complete characterization with MC calculations of the neutron yield from the source hasbeen performed also for this design. The result (Fig. 2.5) shows a reduction of the totalneutron �ux compared to a full-stop beryllium target of approximately 5%, but con�rmsthat a �uence on target of about 8.6× 1011 neutrons sr−1s−1 above 1 MeV can be reached,that is su�cient for the intended use of the neutron source.

The result of the simulations for the two codes do not agree within the quoted uncertainties.In particular, the MCNPX simulation shows an enhanced high energy (> 5 MeV) yieldcompared to the FLUKA neutron spectrum.

A measurement was planned and performed at The Svedberg Laboratory to characterizethe neutron source output before its installation at IGISOL-JYFLTRAP. The measure-ment, used to benchmark the neutron energy spectrum, was also used to try to settle thedisagreement between the two codes and thus validate the MC simulations.

An extended-range Bonner Sphere Spectrometer (BSS) was used to validate the simulationsup to a few MeV, while a Time of Flight system was used to characterize the high-energypart of the spectra, where the BSS response is not optimal.

The measurement setup and the results will be discussed in Chapter 3.

21

Ref. YearTarget thickness Proton Energy

Notes[mm] [MeV]

[48] 1958 Thick 32 Full-stop target[49] 1977 9.5-31.7 22.5-65.5[50] 1977 4.6, 30 25 - 55 Full-stop target[51] 1977 11.6 - 24.1 35, 65 Full-stop target[52] 1978 12.7 30, 40 Full-stop target[53] 1979 14 35, 46 Full-stop target (at 0, 15 and 45◦)[54] 1981 11.6 35 Full-stop target[55] 1981 ≈ 1.3 - 3.5 9 - 23[56] 1987 Thick 35 Full-stop target[57] 1988 1 - 2 20 - 40[47] 1989 5 17.2 - 22 Full-stop target[58] 1990 10 45 Full-stop target[59] 1992 2 31.9 - 71.2[60] 1995 10 35 Full-stop target[61]

2005 13Low Energy n-Source moderated

[62] to obtain cold neutrons (meV)[63] 2009 5.5 30 Moderated source for BNCT

Table 2.2: Summary of previous measurements of Be(p,xn) sources around 30 MeV.

Energy (MeV)

­810 ­710

­610

­510 ­410

­310 ­210 ­110 1 10

s))

2 (

n/(

cm

dEΦ

d E

610

710

810

910

1010

5mm Be ­ FLUKA

5mm Be ­ MCNPX

Simulated Neutron Energy Spectrum

Fig. 2.5: Comparison of the neutron spectra from 30 MeV protons on a 5 mm Be con-verter simulated with FLUKA (black) and MCNPX (red). The spectra are rescaled to anincoming proton current of 100 µA and a distance of 15 cm from the source.

22

Chapter 3

Validation measurement

In this chapter the measurement performed to characterize the neutron source will bepresented. The �rst measurement on a prototype of the proton-neutron converter wasperformed in June 2012 at the The Svedberg Laboratory (TSL).

The measurement took place at the Paula beam-line of the TSL facility in Uppsala, Sweden[64]. Two di�erent techniques were used to characterize the neutron source, a BonnerSphere Spectrometer (BSS) and a Time of Flight (TOF) system with a liquid scintillator.

It was decided to perform the characterization with two independent methods that couldcomplement each-other: the BSS spectrometer could provide a good overall picture of theenergy distribution of the neutrons. The TOF was instead used to measure the high-energypart of the neutron spectrum, where the BSS is less accurate due to the lower response ofBonner Spheres to fast neutrons.

This work will focus on the TOF measurement. The results of the BSS measurement willbe shown for comparison.

Targets of three di�erent thickness were measured: 1 mm, 5 mm and 6 mm. The resultswill concentrate on the 5 mm-target, a mock-up of the one that is planned to be used inthe �rst experimental campaigns at IGISOL-JYFLTRAP.

3.1 Experimental Setup

The 5-mm thick beryllium converter was installed in a 20 × 20 cm2 aluminium support.In order to reproduce as accurately as possible the conditions in which the source wouldbe used at IGISOL-JYFLTRAP, a 1 cm-thick layer of water was included in the beamdirection to simulate the �owing water that is needed to cool the p-n converter. A pictureof the target in the target holder is shown in Fig. 3.1.

The energy of the proton beam available at the Paula facility, as measured by the sta� atTSL, was 37.3± 0.5 MeV [65]. In order to obtain an energy comparable to the one availableat IGISOL-JYFLTRAP, a 1.015 mm aluminium degrader was added on the proton path.The �nal energy of the protons was estimated to be 29.6 MeV at the so called Actual UserPosition (AUP) [65]. Graphite collimators were used for a �rst shaping of the proton beamin the x- and y-directions (z is the direction of the beam). The size of the beam-spot

23

Fig. 3.1: Picture of the target holder at AUP right after the graphite cylindrical collimator.The aluminium cylinder taped to the graphite collimator is the ionization chamber usedfor monitoring the proton current on target.

was �nally determined by an additional collimator with a cylindrical aperture 15 mm indiameter. The distance of the elements on the beam-line up to the AUP is shown in �gure3.2.

3.1.1 Measurement positions

The detector was placed at an angle of 10◦ with respect to the beam axis (Fig. 3.3). Thepurpose was to allow a background measurement, with target assembly removed, withoutthe proton beam hitting the detector.

As far as the TOF measurement is concerned, the energy resolution improves with increas-ing �ight path and with decreasing energy of the neutrons.

The parameters that are contributing to the uncertainty in the determination of the energyfrom TOF are the uncertainty in the measurement of the �ight path and of the �ight timeof the neutrons. The time uncertainty is the combination of the detector and DAQ timeresolution and the time spread of the proton bunch hitting the proton-neutron converterand it was estimated from the width of the peaks in Fig. 3.4. They correspond to the burstof γ-rays produced when protons hit elements on the beam-line upstream of the neutronsource.

The uncertainty in the distance was evaluated assuming a uniform distribution of the

24

AB

CD

E

Fig. 3.2: Sketch of the elements along the beam-line preceding the proton-neutron converterpositioned at the AUP. Distances are in cm if not otherwise speci�ed [65]. The circled letters( AO - EO) refer to the peaks in �gure 3.4.

Fig. 3.3: Measurement positions for the Time of Flight (TOF) system and the BonnerSphere Spectrometer (BSS) at the Paula beam-line at TSL. The beam enters from thetop left (red arrow). The neutron source at AUP is shown in yellow. Concrete walls andshielding are shown in dark grey.

25

energy deposition along the whole scintillator's length as ∆L = 16 cm√12≈ 4.6 cm, while the

average interaction point was found from simulation to be 7.4 cm from the front face ofthe detector (details of this calculation will be discussed in Appendix A).

With these assumptions, it was possible to calculate the uncertainty in the Energy, ∆Efrom:

∆E = 2E ·

√(∆T

T

)2

+

(∆L

L

)2

, (3.1)

where L is the source-to-detector distance and T is the �ight time of a neutron of energy E.A graph of the distance-dependent energy resolution as a function of the neutron energyis shown in Fig. 3.5.

Since the main goal of the TOF measurement was to estimate the high-energy end ofthe neutron spectrum, it was important to obtain good energy resolution at energies upto 30 MeV with a measurement at the furthest possible distance. However, another e�ecttends to make shorter distances preferable: this is the frame-overlap, or wrap-around (WA).The WA is the superposition in the TOF spectrum of neutrons with di�erent energies due tothe repetition rate of a pulsed neutron source. This causes ambiguities in the determinationof the energy of neutrons arriving at the detector position when the �ight time of the slowestneutrons is longer than that of the fastest neutrons of the subsequent bunch. In this case,slow neutrons can be misidenti�ed as fast neutrons.

It was decided to perform the TOF measurement at three di�erent distances, to optimizethe energy resolution and minimize the e�ect of the WA. The typical energy resolution andthe WA energies of the 3 source-to-detector distances that were chosen for the measurementare summarized in Table 3.1.

Wrap-Around ∆E/E at ∆E/E atEnergy WA-energy 30 MeV

Distance1.25 m 2.2 MeV 7.6 % 10.5 %2.07 m 4.3 MeV 4.8 % 6.4 %4.89 m 10.5 MeV 2.2 % 2.7 %

Table 3.1: Wrap-around energy and typical energy resolution at the three measurementdistances used in the Time of Flight experiment.

3.1.2 TOF detector

The detector used for the TOF measurement is a 3.3 l liquid scintillator that was used inthe Nordball array during the 1980s [66]. The detector is shown in �gure 3.6 and its maincharacteristics are summarized in table 3.2.

26

ToF (ns)

80 85 90 95 100 105 110 115

Nr.

eve

nts

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

ToF

EA

B

C

D

X-YCOLLIMATORS

DEGRADER

CYLINDRICALCOLLIMATOR

TARGET

Fig. 3.4: TOF spectrum of the gamma rays reaching the liquid scintillator. The peakscorrespond to the gamma �ashes produced when protons hit elements on the beam-lineupstream of the neutron source. From the FWHM of the peaks, the uncertainty in theTOF determination was evaluated to ∆T = 0.64 ns. Time increases from right to left. Thecircled letters ( AO - EO) refer to the beam-line elements in Fig. 3.2.

Neutron Energy (MeV)0 5 10 15 20 25 30 35

E)

∆R

esolu

tion (

0

0.5

1

1.5

2

2.5

3

3.5

4 L = 1.26 m

L = 2.07 m

L = 4.89 m

Energy Resolution

Fig. 3.5: Energy resolution of the TOF measurement as a function of the neutron energy.The longer distances are preferable especially at higher energies.

27

Fig. 3.6: The Nordball detector used for the TOF measurement.

ScintillatingBC501 [67]

MaterialPhoto-Multiplier

PHILIPS XP2041Tube (PMT)PMT voltage −1.75 kVDetector Volume 3.3 lDetector Thickness 16 cm

Table 3.2: Characteristics of the NORDBALL scintillator detector used for the TOF mea-surement.

28

Functioning Principle of the liquid scintillator

Scintillation is the property of certain organic or inorganic compounds to emit visible lightwhen ionizing radiation deposits energy in the material. The amount of light is, to a largeextent, proportional to the absorbed energy, making spectroscopy possible.

Scintillation has been used for detection of radiation since the beginning of the 20th century,but it was not until the second half of the 1940s that this kind of detector became popular,when scintillators were �rst coupled to a PhotoMultiplier Tube (PMT).

Liquid scintillators are an example of organic scintillators, where hydrocarbon compoundsare dissolved in an organic solvent. They generally exhibit a fast response, with decay timesof the prompt �uorescence of the order of 3− 4 ns [68]. An important feature of organicscintillators is the presence of a slow component of the scintillation light corresponding todelayed �uorescence. This slow component appears as a tail in the light pulse and - inmost cases - depends on the type of particle causing the excitation [69]. In particular, theslow component is more pronounced for particles with larger energy loss per unit lengthdE/dx. This property can be easily observed in liquid scintillators (Fig. 3.7).

With appropriate analysis of the pulses, it is then possible to identify the particles causingthe ionization, in particular to discriminate between gamma rays and ions.

In a n-γ mixed �eld, such as the one produced by the neutron source in the present work, agood particle discrimination is important to reduce the unwanted γ background and makeit possible to study the neutron energy spectrum.

(a)

Time (ns)20 40 60 80 100 120 140

PH

(AD

C)

-400

-350

-300

-250

-200

-150

-100

-50

0

50

100

Pulse Shape

photon

neutron

(b)

Fig. 3.7: Time dependence of scintillation intensity for stilbene (�gure from Ref. [70]) (a),and for the Nordball BC501 liquid scintillator (b). In (b), the pulse is negative like theoutput of the PM-tube from the scintillator.

29

Data AcQuisition systems

The signal from the Photomultiplier coupled to the Nordball liquid scintillator was splitand connected to two Data AcQuisition (DAQ) systems that were used in parallel:

� a NIM-based analog DAQ with an on-line Pulse Shape Discrimination (PSD) modulefor neutron-γ discrimination [71], connected to a Time-to-Digital Converter (TDC)card. Information on the PSD as well as the arrival time of the neutron with respect tothe Radio Frequency (RF) pulse from the cyclotron were saved on an event-by-eventbasis. During the measurement, the threshold was varied between three di�erentvalues, in order to be able to reduce the e�ect of the frame overlap in the dataanalysis phase.

� A digital system with an ADQ412 High-speed Multi-Channel Analyser1 (MCA) tosave the entire pulse collected by the PMT. Another channel of the MCA was con-nected to the RF signal from the cyclotron in order to extract the TOF. The pulseswere sampled at 1 GHz, with an amplitude resolution of 12 bit.

The RF signal provided the reference to which TOF was calculated, since it is synchronouswith the arrival time of the protons on the beryllium target, except for a constant unknowndelay. In the DAQ con�guration used, the signal from the scintillator provides the start ofthe TOF, while the RF gives the stop. For this reason, in the raw TOF spectra (such asthe one shown in Fig 3.4) the time increases from right to left.

Because of its superior PSD capabilities [5] (due to the possibility to perform pulse-height-dependent n-γ selection), the digital DAQ system was the one used to collect the datapresented in the following. The analog DAQ, thanks to the online PSD, could be used toextract TOF spectra already during the experimental run and was used in a �rst phase ofthe data analysis [1].

A check for the compatibility of the two datasets has been performed and, for the samedistance and similar threshold settings, they show very good agreement (an example ofthis comparison is shown in Fig. 3.8).

Evaluation of the background

To evaluate the contribution from the background of radiation scattered from the room,for each source-to-detector distance, a run was performed where a Shadow Cone (SC) wasused to block the primary beam [72]. The shadow cones were manufactured with di�erentsizes to be used at di�erent source-to-detector distances. They consist of a 50 cm longtruncated cone. The �rst 20 cm from the small base are made of iron, while the rest ismade of polyethylene, see Fig. 3.9.

1SP Devices (http://spdevices.com/index.php/adq412).

30

Energy (MeV)0 5 10 15 20 25 30 35 40

Nr.

Counts

0

500

1000

1500

2000

2500 Digital DAQ ­ th = 100 ph­units

Digital DAQ ­ th = 250 ph­units

Digital DAQ ­ th = 350 ph­units

Digital DAQ ­ th = 500 ph­units

Analog DAQ ­ th = 500 mV

DAQ comparison

Fig. 3.8: Comparison of the raw results obtained with the NIM-based analog DAQ withonline PSD and a 500 mV-threshold (blue line) and the digital MCA (red line). In the datacollected with the digital DAQ is possible to set the threshold arbitrarily above ≈ 200 mVor 100 ph− units.

Fig. 3.9: Picture of a typical Shadow Cone (SC) used in the measurement. Five cones weremanufactured with di�erent sizes of the major and minor radii in order to accommodatemeasurements performed at di�erent distances and with di�erent detector sizes.

31

The ability of SCs with such a design to stop the direct beam was veri�ed with MCcalculations before the experiment.

3.2 Data Analysis

In this section, the various steps of the data analysis, from the raw TOF spectrum to arelative neutron �ux (E dΦ

dE), will be described.

3.2.1 Selection of the neutron events

The pre-treatment of the raw data was performed in order to obtain a value for the PSDparameter and the maximum Pulse Height (PH) of each pulse. In particular, the parameterfor the PSD was obtained using the charge integration method, as the ratio of the slowcomponent of the light pulse and the total pulse integral [73, 74]. The integration limitswere optimized to maximize the separation between the two peaks in the PSD distribution(Fig. 3.10(a)), representing γ and neutron events. Some details of the procedure can foundin Ref. [2, 5].

PSD (A.U.)0.15 0.2 0.25 0.3 0.35 0.4

Nr.

events

0

500

1000

1500

2000

2500

3000

3500

4000

Pulse Shape DiscriminationPulse Shape Discrimination

(a)

Pulse Height (A.U.)0 200 400 600 800 1000 1200

PS

D (

A.U

.)

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

2D PSD vs PH

(b)

Fig. 3.10: 1-dimensional PSD distribution (a) and 2-dimensional PH vs PSD distribution(b). The red line represents the 2D-cut performed to select neutron events.

In the data-analysis, the PSD selection was performed introducing a cut on a 2D plot of thePSD parameter versus the PH (�gure 3.10(b)). The selection performed with this 2D-cut,with respect to a selection based on the sole PSD parameter (e.g. as shown in Fig. 3.10(a))shows an improvement in the gamma-rejection especially for low pulse height thresholds.

The parameters for the PSD selection were kept constant in the analysis of data collectedat the 3 distances.

32

3.2.2 Choice of the PH threshold(s)

The threshold on the event PH was chosen according to the distance, in order to minimizethe contribution of events with an energy lower than the wrap around energy. This selectionwas performed on a 2D plot of the Energy (obtained from TOF) versus the PH (Fig. 3.11).The e�ect of the threshold is evident on the resulting 1D energy histograms (Figure 3.12).

Pulse Height (A.U.)100 200 300 400 500 600 700 800 900 1000

Energ

y (

MeV

)

0

5

10

15

20

25

30

35

40

1

10

210

EnergyPH

Fig. 3.11: Pulse Height vs Energy distribution. The red, blue and black boxes representthe events selection in Energy and PH for the measurement performed at 1, 2 and 5 m,respectively.

For sets of data collected at longer distances, the chosen PH threshold had to be higher, dueto a higher wrap around energy (see Table 3.1). This was in line with what was plannedin the design of the experiment, where the longer distances would be used to obtain the�ux of the higher-energy part of the neutron energy spectrum.

The total energy spectrum is obtained by merging the measurement acquired at the threedi�erent distances. After choosing the threshold, it was possible to set the energy limitsat which each measurement would be used. The data collected at a longer distance werepreferred, whenever available, due to their superior energy resolution.

The energy limits of the histograms and the threshold (in PH and corresponding neutronenergy) used to perform the cut are shown in Table 3.3.

3.2.3 Correction for the e�ciency of the detector

The probability to detect a neutron of energy En, or the neutron detection e�ciency,depends on many factors, among which are the neutron energy itself, the geometry andmaterial of the detector, as well as the chosen PH threshold.

33

Energy (MeV)0 5 10 15 20 25 30 35 40

Nr.

Counts

0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200 Th = 100 ph

Th = 200 ph

Th = 250 ph

Th = 300 ph

Th = 350 ph

Th = 400 ph

Th = 450 ph

Th = 500 ph

Threshold Comparison

Fig. 3.12: Comparison of the uncorrected energy spectra to which di�erent PH thresholdare applied. The energy spectrum is obtained from the conversion of the TOF data collectedat 1.26 m.

DistanceThreshold Histogram Energy

PH En Interval1.26 m 100 ≈ 4 MeV 6 7→ 33 MeV2.07 m 250 ≈ 7 MeV 10 7→ 33 MeV4.89 m 500 ≈ 12 MeV 16 7→ 33 MeV

Table 3.3: Choice of threshold and histogram lower and upper limits for data acquiredat the di�erent distances. The overlapping energy region of the histograms was used fornormalization.

34

In order to correct the shape of the neutron energy spectrum for the e�ect of the detectore�ciency, a MC calculation was performed in Geant4 [75]. Mono-energetic neutrons weregenerated at the neutron source position and impinged on the detector.

The energy deposited for each particle type was scored on an event-by-event basis andtransformed to light output (LO) using Energy-to-LO functions for similar scintillators[60,76].

The model used in the MC simulation was validated against measurements of the neu-tron detection e�ciency found in the literature. The choice of the physics libraries, thevalidation and the estimation of the uncertainties are discussed in Appendix A.

The thresholds reported in Table 3.3 were used to the extract e�ciency curves shown in Fig.3.13. The e�ect of the e�ciency correction has been veri�ed applying the proper thresholdcorrection to the curves in Fig. 3.12. The result is quite satisfactory (with curves thatagree within the estimated uncertainty) and is shown in Fig. 3.14.

Neutron Energy (MeV)5 10 15 20 25 30

Effic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

L = 1.26 m ­ Threshold = 4.00 MeV

L = 2.07 m ­ Threshold = 7.00 MeV

L = 4.89 m ­ Threshold = 12.00 MeV

Efficiency curve

Fig. 3.13: Detector e�ciency curves for the thresholds used in the TOF measurement at1.26 (black), 2.07 (red) and 4.89 m (green).

3.2.4 Proton current correction and subtraction of the background

Information on the relative proton current on target was collected with an ionization cham-ber placed after the last collimator (Fig. 3.1). The current was integrated for a periodof 1 s and saved with an absolute time stamp. The number of events was then re-scaledtaking into account the actual integrated current of each experimental run.

35

Energy (MeV)5 10 15 20 25 30 35 40

Nr.

Counts

0

1000

2000

3000

4000

5000

6000

7000

8000

9000 Th = 100 ph

Th = 200 ph

Th = 250 ph

Th = 300 ph

Th = 350 ph

Th = 400 ph

Th = 450 ph

Th = 500 ph

Threshold Comparison ­ corrected

Fig. 3.14: Comparison of energy spectra with PH thresholds from Fig. 3.12 once thecorrection for the detector e�ciency is applied. The energy spectrum is obtained from theconversion of the TOF data collected at 1.26 m.

After the correction for the proton current was included, it was possible to subtract thebackground spectra collected with the Shadow Cone runs. The uncertainty was propagatedfor each bin.

The histograms corresponding to the SC-subtracted neutron spectra collected at the threedistances were renormalized to each-other using the integral of the counts in the overlappingregion. They were then merged to obtain a single neutron energy spectrum that wouldcover the energy range from 6 MeV up to the maximum neutron energy.

3.3 Results and Uncertainties

The results of the TOF measurement is shown in �gure 3.15, compared to MC calculationsperformed with FLUKA and MCNPX. The histograms were renormalized to the integralof the counts above 6 MeV.

The curve representing the FLUKA calculation has been obtained subtracting simulationsperformed with and without SC, just like in the case of the TOF measurement [1]. TheMCNPX simulation was performed without including the e�ect of the scattering of thematerial surrounding the TOF detector (e.g. concrete walls, ceiling, �oor).

The uncertainty in each data-point includes both systematic and statistical uncertainties.The uncertainty contributions can be summed up as follows:

36

� Statistical uncertainty due to counting statistics: due to the large number of eventsacquired this is a negligible contribution to the total uncertainty for the measure-ments performed at the two closest distances. This becomes more important at thefurthest measurement distance, where it accounts for approximately half of the totaluncertainty.

� Statistical uncertainty in the determination of the energy from TOF: it is due to theuncertainty of the position in the detector where the interaction took place and tothe time spread of the proton bunch. It can be estimated analytically (cfr. equation3.1 and Fig. 3.5). The binning on the x-axis of the histogram has been chosen torepresent this uncertainty and the width of each bin is approximately 2∆E.

� Systematic uncertainty due to the rescaling of the histograms: it is due to the un-certainty on the integral of the histograms in the overlapping region. It accounts foralmost half of the uncertainty in the points at energy < 16 MeV, that correspond tothe data collected at 1 and 2 m distances.

� Systematic uncertainty in the determination of the energy from TOF: it is due to theuncertainty with which the positions of the detector, the source and the elements onthe beam-line are known. These latter were used to obtain the time at which pro-tons reach the beryllium target. The systematic uncertainty (± 0.1√

12cm) is negligible

compared to the statistical uncertainty (± 16√12

cm) and was therefore neglected.

� Systematic uncertainty from the e�ciency correction: it is mainly due to the uncer-tainty in the Energy-to-LO conversion models and the physics interaction models.It has been evaluated using the approach described in Appendix A and is slightlyenergy- and distance-dependent. It accounts for approximately half of the total un-certainty.

3.4 Discussion

The result of the characterization of the high energy part of the neutron spectrum fromthe mock-up of the IGISOL target shows a better agreement with the MCNPX simulation.The FLUKA simulation underestimates the production at energies close to the maximumneutron energy.

It should be pointed out, however, that in Fig. 3.15, all the histograms have been re-normalized to match the integral of the overlapping region. At this stage, the absolutecurrent of protons on target is not yet available. This is expected to be delivered during2015 by the sta� at TSL, so that an absolute estimation of the neutron yield of the neutronsource and a comparison in absolute terms with the MC calculations will be possible.

During the same experimental campaign at TSL, also Bonner Sphere Spectrometers (BSS)have been used to characterize the neutron source. The measured energy spectra were

37

Energy (MeV)5 10 15 20 25 30

/dE

(A

.U.)

ΦN

eu

tro

n F

lux E

d

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

­910×

MCNP simulation

FLUKA simulation

TOF measurement

Fig. 3.15: Result of the TOF measurement compared with MC calculations in FLUKAand MCNPX.

38

not compared to the TOF measurement, since they show the neutron spectrum from thesource without subtraction of the background of scattered radiation from the room.

Some details of the procedure to extract the BSS spectra can be found in reference [2]. InFig. 3.16, the BSS results are compared to the FLUKA and MCNPX simulations of theexperimental hall that include both the direct and the scattered fraction of the neutron�ux. In the BSS analysis, some input spectrum must be provided as a guess to startthe deconvolution procedure. In Fig. 3.16 are shown the results obtained with both theFLUKA and the MCNPX simulated input spectra (green and red markers, respectively).

The big advantage of the BSS measurement is the possibility to cover the entire energyrange from thermal neutron energies up to 30 MeV. However, the low energy peak isdominated by scattering of low energy neutrons from the concrete �oor and walls, so -without a proper estimation of the background - it gives little information on the actualoutput of the neutron source in conditions that are di�erent than the ones where themeasurement was performed.

At this stage, with the �uxes normalized to unit integral, the BSS measurement betterfollows the FLUKA simulated neutron spectrum.

Energy (MeV)

­910

­810 ­710

­610

­510 ­410

­310 ­210 ­110 1 10

/dE

(A

.U.)

ΦN

eu

tro

n F

lux E

d

0

0.05

0.1

0.15

0.2

0.25

MCNP simulation

FLUKA simulation

BSS measurement ­ MCNPX input

BSS measurement ­ FLUKA input

Fig. 3.16: Result of the BSS measurement compared with MC calculations in FLUKA andMCNPX. The histograms have been rescaled to unit integral.

39

Chapter 4

Current & Future Work

After the experiment at TSL (chapter 3), work has been performed on the beam-line atIGISOL-JYFLTRAP and on the MCC30/15 cyclotron in order to prepare for the �rstruns with neutrons. The JYFLTRAP Penning trap has been tested successfully with acampaign for the measurement of isomeric yield ratios in proton-induced �ssion of 238U inJune and August 2013 [32].

In March 2014, the latest prototype of the proton-neutron converter has been installed inthe �ssion chamber at IGISOL. The output of the neutron source has been measured withThin Film Breakdown Counters (TFBC) and neutron activation analysis. In this chaptersome selected results of this measurement will be presented, at least for what concerns thedata collected with TFBCs.

Some conclusions from the preliminary work on the data of other target con�gurationsmeasured during the run at TSL will also be presented. These will be useful for a secondphase of measurements at IGISOL, aimed at investigating the energy dependence of the�ssion yields.

4.1 Installation and �rst characterization of the con-

verter at IGISOL

The newly installed MCC30/15 cyclotron was used to deliver 30 MeV-protons up to acurrent of 2 µA. The value of the current was chosen to obtain a suitable count rate in thedetectors used for the characterization. The neutron energy spectrum from the prototypewas measured in order to benchmark the Monte Carlo calculations of the neutron �eldinside the �ssion chamber and to estimate the ratio of neutrons in various energy ranges.

The measurement was performed with two techniques: Thin Fission Breakdown Counters(TFBC) were used to monitor online the neutron �ux, while an integral activation mea-surement was performed to evaluate the spectral distribution based on di�erent reactionthresholds of the irradiated materials. This section will focus on the TFBC measurement.The analysis of the activation data is currently ongoing at JYFL. Some details of the func-tioning principle of TFBCs and of the procedure followed in the data analysis can be foundin Appendix B.

41

Fig. 4.1: Experimental setup for the measurement of the output from the neutron sourceinstalled in the �ssion chamber at the IGISOL facility. The proton beam enters fromthe bottom-right corner of the picture and the proton-neutron source is attached to anextension of the cyclotron beam-line. TFBC detectors are placed in the position where the�ssion target would be, during the experiments of �ssion yields measurement.

The experimental setup with the neutron source in place is shown in Fig. 4.1. The twoTFBCs were positioned at a distance of 26 cm from the neutron source, close to where the�ssion target would be placed during experiments from the measurement of �ssion yields.

The �rst results of the analysis show that a total neutron �ux between 2 and 5× 1012 n/(sr s)at a proton-incoming current of 100 µA can be reached with this setup. Of these, between2 and 3× 1012 n/(sr s) are fast neutrons (En > 1 MeV). Both these numbers ful�l the re-quirements in terms of total and fast neutron �ux that were set in the initial phases of theneutron source design (see section 2.1).

An attempt has been made also to derive the energy spectrum from a TOF measurement.The preliminary results show that - also in this case - the MCNPX simulation seems to bet-ter reproduce the neutron output at the higher neutron energies. The assumptions behindthe numbers presented here and the limitations of the TOF measurement are discussed inmore detail in appendix B.

A longer and more thorough measurement of both the total neutron yield and the neutronenergy spectrum will be performed in conjunction with the �rst experiment of neutron-induced �ssion yield measurements planned for the �rst half of 2015.

42

4.2 Other targets

The experimental campaign at TSL was focused on the characterization of the mock-upof the proton-neutron converter with a design close to the one that has been installedat IGISOL and consisting of a 5 mm-thick water-cooled Beryllium disc embedded in analuminium case. This was, however, not the only con�guration that was measured: alsoa 6 mm-thick target in the aluminium case, a stand-alone 1 mm-thick Beryllium, anda version of the IGISOL target with an additional 10 cm polyethylene moderator werestudied.

This was done in view of a second phase of the �ssion yields measurements that would bedirected towards the study of their energy dependence. In particular, a 1 mm target wouldbe used, despite its lower neutron yield, as a source more peaked at higher energies, whilethe moderated target could simulate LWR-like thermal spectra.

4.2.1 Thin target

The 1 mm-thick beryllium disc was placed in the same position as the 5 mm version de-scribed in chapter 3. However, the thin proton-neutron converter was not embedded in thealuminium case, but a metal clam was used to hold it in place, so that no material otherthan beryllium was hit by the direct proton beam.

The analysis procedure to extract the neutron energy spectrum followed the same steps asdiscussed in chapter 3. The neutron spectrum is shown in �gure 4.2. The data from theTSL measurement are compared with data from a similar target measured by Uwamino etal. [57]. The histograms have been rescaled to match Uwamino's data at 7 MeV.

The results should still be considered preliminary: the background subtraction used in factthe same SC measurement performed for the complete target assembly (including waterand aluminium case), even if the geometry changed in the measurement of the thin target.

It is however evident how the data from the TOF measurement do not match the result ob-tained by Uwamino and colleagues, and it severely underestimates the quasi-monoenergeticpeak at about 28 MeV. Nevertheless, also the MCNPX simulation seems to underestimatethe measurement by Uwamino. The discrepancy could be only partly explained by thefact that Uwamino measures the neutron spectrum at 0◦, while the measurement setup atTSL was placed at a 10◦ angle with respect to the incoming beam direction. More work isplanned to estimate the background and understand the reason for this discrepancy.

4.2.2 Moderated neutron source

Also a version of the neutron source with an additional moderation step was considered.Such a prototype, consisting of a 5 mm-thick beryllium in the aluminium case, and anadditional 10 cm polyethylene moderator was measured during the campaign at TSL.

43

Energy (MeV)5 10 15 20 25 30 35

/dE

(A

.U.)

ΦN

eutr

on F

lux E

d

0

2

4

6

8

10

12

14

16

18

20

22

1510×

Uwamino et al.FLUKA simulation

MCNP (0 deg) simulation

MCNP (10 deg) simulation

TOF measurement

Fig. 4.2: Comparison of the energy spectrum from a 1 mm target from the TSL measure-ment (black) and the data from Uwamino et al. [57]. Also the simulation performed inFLUKA (green) and MCNPX (blue) are shown. In the case of the MCNPX simulation,the spectrum was scored both at 0 and 10◦.

The analysis of the TOF data is not �nalized, but some results could be obtained from theBSS spectrometer. The spectrum in �gure 4.3 shows the result of the BSS measurement,where the moderated target (red) is compared to the un-moderated one [2]. Also in thiscase, the plot shown for the BSS is not background-subtracted, so that the low energy peakincludes both contributions from the direct and the scattered radiation.

What can be learned from this preliminary result is, however, that the high-energy peak isto some extent attenuated, but there is not a major boost in the epithermal and thermalneutrons. More work is probably required to design a neutron source with the desiredneutron spectrum (e.g. resembling the one of a LWR).

44

Fig. 4.3: Neutron energy spectrum of the moderated neutron source (5 mm-thick berylliumwith an additional 10 cm polyethylene moderator) measured with the BSS spectrometerduring the campaign at TSL. The �ux from the moderated source (red) is compared to theunmoderated case (black) [2].

45

Conclusion

In this work, the design and characterization of a neutron source for �ssion yields studieshave been presented.

The neutron �eld will be used for the measurement of neutron-induced �ssion yields ofvarious actinides. These are of interest for fundamental physics, in order to understandthe complex phenomenon of �ssion or for a better modelling of nucleosynthesis throughthe r-process in nuclear astrophysics.

Requests for better nuclear data are, however, also coming from applications: for the newgeneration of �ssion reactors, including fast reactors and Accelerator Driven Systems, abetter knowledge of �ssion yields at various incoming neutron energy will be valuable.This will improve predictions of the reactor kinetics, where a precise knowledge of delayedneutron precursors and neutron poisons plays an important role; but also the �ssion prod-ucts inventory at various stages of the fuel cycle to evaluate, e.g., the requirements for thestorage or the recycling of spent nuclear fuel.

Neutron sources are a valuable tool to provide application-oriented nuclear data. In thisthesis, the design of a proton-neutron converter has been presented.

The neutron source consists of a 5 mm-thick Be proton-neutron converter that will beinstalled at the IGISOL-JYFLTRAP facility, Jyväskylä, Finland and will take advantageof the new MCC30/15 cyclotron that delivers 30 MeV protons up to a current of 200 µA.

The neutron energy spectrum has been simulated with two Monte Carlo codes (FLUKAand MCNPX) and it has been validated with a measurement performed at The SvedbergLaboratory, Uppsala, Sweden. The measurement constitutes the larger part of the originalmaterial presented in this thesis and was based on the data collected with a Time of Flightsystem able to detect neutrons above 5 MeV.

The results of the comparison show that the Monte Carlo simulations are not alwaysaccurate in the prediction of the neutron yields. In particular, the Monte Carlo codeFLUKA seems to underestimate the high-energy end of the neutron energy spectrum.MCNPX shows, instead, a neutron energy spectrum that is more consistent with the TOFmeasurement.

The Monte Carlo simulations have also been compared with the preliminary results of ameasurement with a Bonner Sphere Spectrometer, able to provide the neutron �ux overthe entire energy range of the neutron source. What is possible to conclude from thesepreliminary results is that the low energy part of the neutron spectrum is heavily sensitiveto the scattered radiation from the experimental hall and has to be estimated for theparticular environment where the neutron source will be used.

47

The work on the neutron source is still ongoing: a prototype of the proton-neutron convertercharacterized at The Svedberg Laboratory has been installed for a test-run in the �ssionchamber at IGISOL, where it is intended to be used. During this test-run, a measurementwith Thin Film Breakdown Counters estimated the absolute neutron yield of the source,concluding that - at the nominal currents of the MCC30/15 - it will be possible to reachthe goal of 1012 fast neutrons per second on the �ssion target.

A better characterization of the neutron energy spectrum and of the total neutron yieldis planned in conjunction with the �rst experimental run for the measurement of neutron-induced �ssion yields, planned for the �rst half of 2015.

At the same time, other possibilities for the neutron source are being explored. New designswith a quasi-monoenergetic neutron source and one where neutrons are thermalized arebeing considered, that could open the way to studies of the energy dependence of �ssionyields.

48

Appendix A

Monte Carlo simulation of the

scintillator detector

In this appendix, the main steps of the Monte Carlo simulation of the liquid scintillatorused in the TOF measurement at TSL will be described.

The MC simulation has been used to extract, among other parameters, the neutron detec-tion e�ciency.

A.1 Geometry

The detector was reproduced using the class G4Polyhedra in Geant4. The scintillatormaterial (BC501) was de�ned according to reference [67], whereas the material for the case(Stainless Steel) was selected from the G4Nist material library available in Geant4. Thedetector was placed at 2.08 m from the source and the room was �lled with air.

A.2 Conversion from Energy to Light Output

The energy released in the scintillator material was scored according to the particle re-sponsible for the ionization. Besides the energy, also the Light Output (LO) was scored onan event-by-event basis. The Energy-to-LO conversion functions were obtained from thereferences [60,76].

In Fig. A.1 the Energy-to-LO curves used for the di�erent particles are shown. No LO wasscored for Carbon and other heavy ions. Their contribution to the e�ciency at the neutronenergies considered in this work (up to 30 MeV) was not signi�cant, given the low LO ofthese particles and the relatively high Pulse Height thresholds set in the data analysis.

A.3 Extraction of the detector e�ciency

24 runs with mono-energetic neutrons between 100 keV to 30 MeV were simulated. Theoutput �les were then processed to include the e�ect of the energy resolution (as discussed

49

Energy (MeV)0 5 10 15 20 25 30 35 40

Lig

ht

Ou

tpu

t (M

eV

ee

)

0

5

10

15

20

25

30

Proton

Deuteron

Alpha

Light Output Curve

Fig. A.1: Energy-to-LO conversion curves for protons (blue), deuterons (green) and alphaparticles (red).

in section A.3.1) and sum the LO event-by-event to construct a response function to mono-energetic neutrons for each simulated neutron energy.

These response functions were the starting point to calculate the number of events in thehistogram above a selected threshold.

A.3.1 Light Output resolution

The light output resolution of the scintillator was evaluated with the help of a charac-terization measurement. Four gamma sources (22Na, 207Bi, 60Co and 137Cs) were used toirradiate the scintillator. A DAQ system similar to the one used for the TOF measurement,based on a digital MCA, was used to collect the data.

Once the maximum PH was extracted, the energy spectra of the di�erent sources werecompared with a Geant4 [75] calculation of the same setup.

The spread in the Light Output (∆L) was de�ned from the parametrization described inRef. [77] as:

∆L = L

√α2 +

β2

L+(γL

)2

, (A.1)

that describes the detector resolution in terms of the light transmission from the scin-tillator to the photocatode (α parameter), the statistical e�ects of the light production,photon-to-electron conversion and ampli�cation (β parameter) and all noise contributions(γ parameter) [78].

50

Energy (keV)200 400 600 800 1000 1200 1400 1600 1800

Rate

(ev/s

ec)

0

1000

2000

3000

4000

5000

6000

7000

Na22

Nordball Detector Calibration

(a)

Energy (keV)200 400 600 800 1000 1200 1400 1600 1800

Rate

(ev/s

ec)

0

1000

2000

3000

4000

5000

6000

Bi207

Nordball Detector Calibration

(b)

Energy (keV)200 400 600 800 1000 1200 1400 1600 1800

Rate

(ev/s

ec)

0

500

1000

1500

2000

2500

3000

3500

Co60

Nordball Detector Calibration

(c)

Energy (keV)200 400 600 800 1000 1200 1400 1600 1800

Rate

(ev/s

ec)

0

1000

2000

3000

4000

5000

6000

7000

8000

Cs137

Nordball Detector Calibration

(d)

Fig. A.2: Fit (red) of the measured γ-ray Compton edge (black) for 22Na (a), 207Bi (b),60Co (c) and 137Cs (d).

51

The spectra from the MC simulation have been smeared using equation A.1. A �t has thenbeen performed for each measured γ-source spectrum where the α, β and γ parametershave been varied to minimize the χ2. Two more parameters were left free to allow theconversion from ADC to MeVee of the x-axis of the spectra.

The result is shown in Fig. A.2. The parameters obtained from the �t (α = 7.2%, β =17.2% and γ = 4.3%) are comparable with values for similar detectors .

A.4 Validation and selection of the libraries

It was observed that the result of the simulation was in part dependent on the selectedphysics library. In particular, due to the wide energy range extending above the typicalenergies where the Evaluated Nuclear Data libraries provide accurate estimation for crosssections (20 MeV) some settings in Geant4 gave rise to sharp jumps in the neutron detectione�ciency due to an abrupt change in the model used to simulate the nuclear reactions.

Furthermore, the High Precision (-HP) libraries, that are intended for use with low-energyneutrons, did not include all the reactions important for the LO production (especially the12C(n,n)3α, which starts to contribute signi�cantly at energies above 10 MeV).

In order to �nd the best physics library and validate the LO conversion model, the resultsof the MC simulation were compared to experiments where the neutron detection e�ciencyof liquid scintillators had been measured directly.

A.4.1 Comparison with experimental data (Drosg [79])

The work by Drosg was performed at Los Alamos National Laboratory in the 1970s withthe main purpose of determining the neutron detection e�ciency of a liquid scintillator forneutron energies between 2 and 26 MeV. Details of the experimental setup and the dataanalysis procedure can be found in Ref. [79].

For the purposes of this work, the BC501A liquid scintillator detector, with a sensitivelength of 5.7 cm, was reproduced in Geant4. The same LO-conversion steps described insection A.2 were used and the e�ciency was compared with the measured data for threedi�erent thresholds.

The scintillating material used by Drosg (BC501A) is not of the same kind as the one used inour experiment (BC501) [67], but was shown to exhibit similar scintillation properties oncethe absorbed energy and the interaction probability were corrected for the di�erent density[80]. This experiment represented a good case to study the uncertainty and the validity ofthe simulation given the neutron energies at which the experiment was performed.

The plots in Fig. A.3 show the results of the e�ciency calculation for six di�erent PhysicsLibraries available in Geant4. In Fig. A.3(b) and A.3(d), corresponding to the e�ciency

52

curve obtained using the HP Physics Libraries, it is evident how the physics abruptlychanges above 20 MeV

The library that better reproduces the e�ciency is the CHIPS-TPT library [81]. The libraryfollows well (within ± 10%) the shape of the e�ciency curves measured by Drosg up toabout 18 and 21 MeV at thresholds of 1.9 and 4.1 MeVn, respectively. Above these energy,the discrepancy is due to the a non-perfect modelling of the 12C(n,d) reaction. This e�ectis less and less important as the detection threshold increases and does not constitute amajor problem for the settings used in the TSL measurement.

Determination of the systematic uncertainty

The systematic uncertainty in the e�ciency due to the models used to convert depositedenergy to LO and the energy resolution in the detector has been estimated for seven selectedincoming neutron energies.

In particular, the energy-to-LO curves were varied within the approximate uncertaintiesprovided by the authors (± 2% for protons and± 10% for α particles and deuterons [60,76]).The same was done for the energy resolution parameters (α, β and γ in equation A.1),that were varied within 15%. The uncertainty in the light output resolution proved not tocontribute signi�cantly to the total uncertainty in the neutron detection e�ciency and willbe disregarded in the following discussion.

In total, approximately 3000 runs were performed with random variation of the inputparameters to obtain a probability distribution of the resulting neutron detection e�cien-cies. In each run, a new energy-to-LO curve was obtained by multiplying the referencecurve with a random number sampled from a Gaussian distribution centred in 1, and withthe width speci�ed above. The light output curves of the three particles were assumeduncorrelated.

The result in �gure A.4 shows the percentage uncertainty in the neutron detection e�ciencycalculated as the variance of the probability distributions obtained for all the seven neutronenergies considered.

As expected, the uncertainty in the e�ciency is more a�ected by modi�cations in theEnergy-to-LO curve the closer the incoming neutron energy is to the threshold.

A.4.2 Comparison with previously calculated detector e�cienciesfor the Nordball detector (Arnell et al.)

The results of the e�ciency have been compared with previous calculations performed forthe Nordball detector using Stanton's MC code [82] (Fig. A.5). The e�ciencies in Ref. [66]were calculated for lower thresholds and up to a neutron energy of 20 MeV.

No uncertainty has been provided in the work of Arnell et al. and few details were giventhat could show the procedure followed to extract the e�ciency. The uncertainty in the

53

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency ­ BERT

PhyLib = BERT

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(a)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency ­ BERT ­ HP

PhyLib = BERT ­ HP

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(b)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency ­ BIC

PhyLib = BIC

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(c)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency BIC ­ HP

PhyLib = BIC ­ HP

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(d)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency ­ INCLXX

PhyLib = INCLXX

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(e)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Neutron Detection Efficiency ­ TPT

PhyLib = TPT

Drosg (Data­1972)

th = 1.9 MeVn

th = 1.9 MeVn ­ proton

th = 3.1 MeVn

th = 3.1 MeVn ­ proton

th = 4.1 MeVn

th = 4.1 MeVn ­ proton

(f)

Fig. A.3: Comparison of the e�ciency curves for the Drosg experimental setup simulatedusing di�erent Physics Libraries available in Geant4 (BERT (a), BERT-HP (b), BIC (c),BIC-HP (d), INCLXX (e), CHIPS-TPT (f)).

54

Neutron Energy (MeV)5 10 15 20 25 30 35

Un

ce

rta

inty

(%

)

0

5

10

15

20

25

Percentage Uncertainty

th = 1.9 MeVn

th = 3.1 MeVn

th = 4.1 MeVn

Percentage Uncertainty

Fig. A.4: Systematic uncertainty in the neutron detection e�ciency (in %) due to theuncertainty in the energy-to-LO model in the simulation of the detector used in the Drosgexperiment.

Geant4 simulation has been extracted with the procedure described in the previous section.The two calculated e�ciencies, even if not compatible within uncertainties in absolutevalue, show the same relative variation, at least for the preferred physics library TPT-CHIPS. Since no absolute values for the neutron �ux will be measured, the e�ciency willbe su�cient to correct the TOF spectra at this stage of the data-analysis. The TPT-CHIPSlibrary was also the one used to extract the e�ciency curves and the uncertainty shown inFig. 3.13, A.4 and A.6.

55

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ BERT

PhyLib = BERT ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(a)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ BERT­HP

PhyLib = BERT ­ HP ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(b)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ BIC

PhyLib = BIC ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(c)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ BIC­HP

PhyLib = BIC ­ HP ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(d)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ INCLXX

PhyLib = INCLXX ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(e)

Neutron energy (MeV)5 10 15 20 25 30

Dete

ction E

ffic

iency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Neutron Detection Efficiency ­ TPT

PhyLib = TPT ­ BC501

Arnell et al. (CECIL)

th = 0.7 MeVn

th = 0.7 MeVn ­ protons

th = 1.0 MeVn

th = 1.0 MeVn ­ protons

th = 13.0 MeVn

th = 13.0 MeVn ­ protons

(f)

Fig. A.5: Comparison of the e�ciency curves for the Nordball detector simulated usingdi�erent Physics Libraries available in Geant4 (BERT (a), BERT-HP (b), BIC (c), BIC-HP (d), INCLXX (e), CHIPS-TPT (f)). The curves in red are the simulated detectione�ciency taken from [66].

56

Neutron Energy (MeV)5 10 15 20 25 30 35

Un

ce

rta

inty

(%

)

0

5

10

15

20

25

th = 4 MeVn

th = 7 MeVn

th = 12 MeVn

Percentage Uncertainty

Fig. A.6: Systematic uncertainty in the neutron detection e�ciency (in %) due to theuncertainty in the energy-to-LO model in the simulation of the Nordball detector. Theincrease of the uncertainty around En ≈ 19, 21 and 28 MeV for the threshold set at 4, 7and 13 MeV, respectively are due to the alpha particles from the 12C break up approachingthe threshold level.

57

A.5 Extraction of the interaction depth

The quite large depth of the Nordball liquid scintillators brings about the problem ofde�ning the exact source-to-detector distance at which the energy from TOF should becalculated. The Geant4 simulation was then used to extract the average position in thescintillator detector where the neutron interactions occur.

For all steps of each event, the energy deposited inside the liquid scintillator was talliedtogether with the position at which this energy was released. The average of the positionsin the z-direction, weighted with the deposited energy for each step, was assumed as theinteraction position of that event and scored. The average of these depths was extractedfor every run and the result is shown in Fig. A.7.

The average interaction depth is indeed energy dependent, as it appears evident fromFig. A.7, however - after a quite steep change at lower energies - this quantity stabilizesfor En > 5 MeV. Since the TOF measurement performed at TSL only detected neutronenergies above this threshold, an average value of d = 7.4 cm from the front face of thedetector was assumed as the source-to-detector distance for all neutron energies.

Neutron Energy (MeV)0 5 10 15 20 25 30

Depth

(cm

)

1

2

3

4

5

6

7

8

Average interaction depth

Fig. A.7: Average interaction depth inside the liquid scintillator as a function of the in-coming neutron energy.

58

Appendix B

Measurement of the neutron �ux with

TFBC detectors

In this chapter, some details of the measurement with Thin Film Breakdown Counters(TFBCs) of the neutron �ux from the proton-neutron converter prototype performed atthe IGISOL facility in March, 2014 will be presented. The principle behind the detection ofneutrons with TFBCs will be introduced in the beginning and some details of the analysisprocedure and the assumptions necessary to extract the absolute neutron �ux will bediscussed.

In the end of this chapter, some results of an attempt to measure the neutron energyspectrum with Time of Flight will be shown and compared with simulations.

B.1 Functioning principle of TFBCs

Thin Film Breakdown Counters are detectors sensitive to heavy charged particles. As thename suggests, TFBCs exploit the phenomenon of electric breakdown in a semiconductorcaused by heavy ions. They are a sandwich of di�erent layers: two electrodes are interleavedwith a dielectric layer. If a �ssionable material is placed on top of one of the electrodes,heavy ions can be produced in a �ssion reaction as �ssion products, making it possible forTFBCs to detect neutrons, as in the recent measurements described in Ref. [83] and [84].

An electric �eld is applied between the two electrodes: this will be su�cient to induce abreakdown once a heavy ion from the �ssion target creates free charges in the dielectriclayer. The high current generated in the process will generate a detectable signal and - atthe same time - evaporate a portion of the electrode, interrupting the short-circuit. Theportion of the electrode that evaporates will not be able to detect any further ions, slightlyreducing the e�ciency of the detector [85, 86].

The count rate (R) of the TFBC will thus depend on the detector e�ciency ε̃ (i.e. its activesurface and how many events it has already detected) and the number of �ssion events perunit time. This latter depends on the target applied to the TFBC detector (the �ssionablematerial used and its thickness ρ), as well as on the neutron �ux j and energy spectrum.These factors are summarized in equation B.1:

59

R = ε̃︸︷︷︸cm2

1/cm2

︷ ︸︸ ︷ρNav

Aj︸︷︷︸

1/(cm2 s)

cm2

︷︸︸︷σF , (B.1)

where Nav is Avogadro's number, A is the mass number of the target, and σF is thespectrum-averaged cross section that depends on the target's �ssion cross section and theneutron energy spectrum (see also section B.3).

The neutron �ux can be extracted from equation B.1:

j =R ·A

ε̃ · ρ ·Nav ·σF(B.2)

B.2 Experimental Setup

The experimental setup consisted of two TFBCs equipped with 238U targets. The TFBCswere positioned inside the IGISOL �ssion chamber. The outputs of the TFBCs were con-nected to a scaler for integral counting. Along with the counts of the TFBCs, informationon the proton current impinging on the neutron converter was also recorded for each run.In addition to the integral information, one of the TFBC provided the start for a timingsignal used for time of �ight measurement. The stop signal was provided by the cyclotronRF with a frequency of 40.6 MHz.

The TOF data were used to obtain more information on the energy spectrum of the pro-duced neutrons. Although a straight TOF-to-Energy conversion was not possible becauseof too low energy resolution, the TOF spectrum was compared with the spectra simu-lated using Monte Carlo codes to judge how well the simulations can reproduce the energyspectrum.

B.3 Data analysis

The total neutron �uence measured with the TFBCs relies to a large extent on the knowl-edge of the �ssion cross section for the material selected. On top of that, if the cross sectionis not constant over the neutron energy range, the energy spectrum that impinges on theneutron detector has to be assumed or measured.

Equation B.2 expresses the dependence of the total neutron �ux on the incoming energyspectrum in the term σF . This can be made more explicit by expanding the spectrum-averaged cross section σF as:

60

Energy (MeV)0 5 10 15 20 25 30

pro

t))

⋅2

/dE

(n

/(cm

Φ E

d

0

1

2

3

4

5

6

­610×

MCNP

FLUKA

/dEΦ d⋅E

Fig. B.1: Energy spectra from the Monte Carlo calculations performed with MCNPX (red)and FLUKA (black) of the neutron �ux at the experimental position (TFBC2).

σF =

∫ E2

E1φ(E)σf (E)dE∫ E2

E1φ(E)dE

, (B.3)

where the integral is carried out between the minimum and the maximum neutron energies(E1 and E2 respectively).

In this work, the �ux φ(E) in equation B.3 is obtained from Monte Carlo calculations.

In a similar fashion as for the measurement at TSL, two independent codes have been used:FLUKA [40] and MCNPX [41]. These codes, as it happened before, give di�erent energyspectra as well as total neutron yield (�gure B.1).

Data from the Monte Carlo simulations have been used to calculate the average cross sectionσF (adapting equation B.3). The integration was performed between 0.1 and 30 MeV, sincethe contribution at lower energies is strongly suppressed by the low �ssion cross section of238U. The data for the �ssion cross section were retrieved from ENDF/B-VII.1 [24]. Theresults of this calculation are shown in table B.1.

61

MCNPX FLUKAtotal neutron �ux n/(cm2 proton) 9.4 10−6 6.4 10−6

σF cm2 8.7 10−25 5.6 10−25

neutrons > 1 MeV % 94 74neutrons > 10 MeV % 42 23

Table B.1: Average cross section (σF ), total neutron �ux and fraction of neutrons above 1and 10 MeV calculated for the MCNPX and FLUKA input spectra. The integration wasperformed between 0.1 and 30 MeV.

B.4 Results

In table B.2 the neutron �ux from the counts in the TFBCs are shown. The counts havebeen rescaled to n/(sr s) to remove the dependence on the TFBCs' positions. The �ux isnormalized to an incoming proton current I = 1 µA, in order to be able to compare thedi�erent runs. A complete uncertainty analysis has not been performed at this stage, andthe uncertainty on the average is given as the standard deviation of the neutron �uxesmeasured in each run1.

MC code for TOTAL En > 1 MeV En > 10 MeVσF n/(sr s) n/(sr s) n/(sr s)

FLUKA (4.03 ± 0.91) 1010 (2.96 ± 0.67) 1010 (0.92 ± 0.21) 1010

MCNPX (2.60 ± 0.59) 1010 (2.45 ± 0.56) 1010 (1.10 ± 0.25) 1010

Table B.2: Neutron �ux based on the counts registered in the two TFBCs using the inputspectrum from the FLUKA simulation (σF = 0.56 b). The �ux was rescaled to a protoncurrent I = 1 µA.

Time of Flight

The TOF spectrum obtained with the TFBC, has been compared with the time of �ightspectra obtained from the Monte Carlo calculations. The �ight time of neutrons has beencalculated from the energy spectra in �gure B.1. The results from the simulations havebeen weighted with the 238U �ssion cross section [24] to obtain the number of counts thatwould be detected in a �ssion target like the one installed on the TFBCs. The comparisonhas been done both for MCNPX and for FLUKA and the results are shown in �gure B.2.The results, even if preliminary, lie in between the predictions of the two codes, but seemto better agree with the MCNPX simulation in the high-energy part of the spectrum (lowTOF). Also in absolute value, the MCNPX neutron �ux better agrees with the result ofthe measurement.

A new measurement will be performed in conjunction with the �rst experiment of neutroninduced �ssion yields measurement that is planned for the �rst half of 2015.

1six runs with di�erent settings of the incoming proton currents were performed.

62

TOF (ns)2 4 6 8 10 12 14 16 18 20 22 24

Nr.

Eve

nts

0

50

100

150

200

250

300

350

400

Data ­ TOF

MCNP (sim) ­ (res fwhm = 1.9 ns)

FLUKA (sim) ­ (res fwhm = 1.9 ns)

U counts238

TFBC 30 10 5 2 1 MeV

Fig. B.2: Comparison of the measured TOF distribution (red hatched histogram) with theone from the Monte Carlo calculation in MCNPX (blue line) and FLUKA (black line).The energy distributions obtained from the simulation have been converted to TOF to becompared with the data. A time spread of 1.9 ns (FWHM) was added to the simulateddata to reproduce the �nite time resolution of the TOF measurement system. The countsin the measured spectrum at TOF larger than 17 ns are probably due to the backgroundof the detector.

63

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Acknowledgements

I would like to acknowledge the support from SKB (Svensk Kärnbränslehantering Ak-tiebolag), SSM (StrålSäkerhetsMyndigheten) and from the European Commission throughFission-2010-ERINDA (project no.269499).

I would also like to thank my supervisors for their support and help: I've been learning alot during these years in Uppsala.

Stephan Pomp, for always being available to share your knowledge and experience in nu-clear physics and for giving comments straight-to-the-point: they are always very appre-ciated. Mattias Lantz, for your continous and tireless support: your enthusiasm is muchinspiring. I am very grateful for the many discussions (day and night, week and holidays)and your encouragement. Andreas Solders, for your comments, the smart questions andthe good advice whenever I come with a �can I ask you something?�.

My deep gratitude to Vasilis (aka Bill) Rakopoulos, for your constant help (in the exper-iments, in the analysis and during the writing of the thesis) and, most of all, for yourfriendship.

It is a real pleasure to work with you all.

Thanks also to:

Alexander Proko�ev, for your help during the several periods of work together, at TSL,TK and IGISOL. For being ready to give suggestions and prompt in providing good criticalfeedback.

Dmitry Gorelov, Heikki Penttilä, Veli Kolhinen, Sami Rinta-Antila and the entire IGISOLgroup. Elke Passoth and the rest of the sta� at TSL, for your valuable help before, duringand after the experiments.

Erik Andersson Sundén for your comments and for some guidance through the wild �eldof detector e�ciency. Petter Helgesson and Kaj Jansson for the interesting discussions andyour help.

To the rest of the reaction group and of the whole TK, for a nice environment to work in:Erwin, Diego, Ali, Cecilia, Michael, Henrik, Tomas, Vasudha, Peter, Matilda, and everyoneelse...

Finally, thanks to all those people who - even if less involved into my work - helped mekeep my balance during these weeks, months (...years?):

Leif Nilsson and Gunilla Gunnarsson, for being so kind and helpful during my �rst periodhere in Uppsala.

73

Giovanni, per la tua amicizia e il tuo umorismo. È stato un piacere lavorare e passare imiei primi mesi ad Uppsala in tua compagnia.

Vasily Simutkin: for the good conversations, your comments and suggestions and for yourtireless e�ort to drag me into all kinds of competitions...

Katerina, who shares in these days the exciting experience of writing a thesis (and to Billagain): it's a real pleasure to spend time with you, relaxing and easygoing. Federico andCristina, for your true friendship. It's so good to know you and to feel that there's a smallpiece of Italy in Uppsala.

And to the small piece of Italy in Italy: Andrea, Alessandro, Alessandro e Maria. Anchese a volte scompaio per settimane, sapere che ci siete sempre mi fa sentire bene...

Finally, to my families: Mamma, Ste e Patti - per il vostro supporto e amore, continuo eincondizionato. Carola, Lennart, Aili, Mille, Fredrik, Magda, Stefan, Mia, Janne, and thesweetest William, Astrid and Sigrid: tack för att ni får mig att känna mig välkommen.And Henrik, who �nally gave me a good reason to go home early from work, bears withme when I'm annoyed and can make me forget a bad day.

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