For nearly a half a century, neutron sources have been of high interest in both basic and applied area; extending from the very low energies primarily relevant to condensed matter studies , to the very high energies associated with elementary particle interactions .A diversity of neutron source types are involved ranging,for example,from small sources implanted in human tissue to the very largest of the accelerator based research facilities.Neutron source are widely applied in other fields; for example astronomy and biomedical.The requirements for neutron data and neutron source properties generally,have shifted from a qualitative , to the present,very quantitative status;often reflecting very large cost and or safety impact.
VIETNAM NATIONAL UNIVERSITY, HANOI VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS NGUYEN THI HUYEN TRANG SIMULATION OF THE PRODUCTION OF MONO-ENERGETIC NEUTRONS FROM LI(P,N)7BE REACTION Submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Nuclear Technology ( Advanced Program ) Hanoi - 2017 VIETNAM NATIONAL UNIVERSITY, HANOI VNU UNIVERSITY OF SCIENCE FACULTY OF PHYSICS NGUYEN THI HUYEN TRANG SIMULATION OF THE PRODUCTION OF MONO-ENERGETIC NEUTRONS FROM 7LI(P,N)7BE REACTION Submitted in partial fulfillment of the requirements for the degree of Bachelor of Science in Nuclear Technology ( Advanced Program ) Supervisors: Dr PHAN VIET CUONG, INSTITUTE OF PHYSICS Hanoi - 2017 ACKNOWLEDGMENT I would like to express my special thanks of gratitude to my teacher Doctor Phan Viet Cuong who gave me the golden opportunity to this wonderful project on this thesis, which also helped me in doing a lot of research and I came to know about so many new things Besides, I would like to thank all teachers, lecturers, researchers and other seniors in Faculty of Physics, particularly Department of Nuclear Technology, Vietnam National University, VNU University of Science, who always create good conditions for students to work and experience I would like to give special thanks to my family and all my friends who have supported and encouraged me in studying and researching Student, Nguyen Thi Huyen Trang LIST OF ABBREVIATION LET: Linear energy transfer GEANT4 : Geometry and Tracking IFIN-HH : Horia Hulubei National Institute of Physics and Nuclear Engineering CERN : European Organization for Nuclear Research RTNS : Rotating Target Neutron Source TABLE OF CONTENTS CHAPTER 1: MONO-ENERGETIC NEUTRON SOURCES 1.1 The T(p,n)3He reaction 1.2 The T(d,n)4He reaction 1.3 The D(d,n)3He reaction 1.4 The 7Li(p,n)7Be reaction 1.4.1 Theoretical basic 1.4.1 Neutron production yield CHAPTER 2: SIMULATION METHOD 10 2.1 Geant4 toolkit 10 2.2 Simulation processes 13 CHAPTER 3: SIMULATION RESULTS 18 3.1 Neutron energy spectra 18 3.2 Neutron production yield 20 REFERENCES 26 LIST OF FIGURES Figure 1: 3He particles from the T(p,n) reaction at a proton energy of 2.25 MeV Figure 2: (d,np)/(d,n) neutron yields as function of ED Figure 3: Breakup fraction observed using 7Li(p,n)7Be target Figure 4: Correlation between energy and emission angle of neutrons produced by 7Li(p,n)7Be with different primary proton energies Figure 5: Some basic classes in Geant4 Toolkit 12 Figure 6: The structural diagram of Geant4 program 14 Figure 7: Geometric simulation with world, source and target 15 Figure : Some codes for detector construction 15 Figure : Some codes for physics processing 16 Figure 10: The neutron energy spectra simulated by Geant4 (left) in comparison to the experimental data (right) See text for more detail….17 Figure 11: Geant4 simulation results (red line) in comparison to 19 Figure 12: Geant4 simulated neutron spectra at different angles in comparison to measured data The data sets are normalized to the same integral 20 Figure 13: Total neutron yield vs proton energies for 1m LiF target 21 Figure 14: Energy spectrum (left) and angular distribution of the neutron produced by the 2297±1.5 keV proton bombarding on 70g/cm2 LiF target 22 Figure 15: Produced neutron energy vs emission angle 23 Figure 16: Differential neutron yields at degree 24 INTRODUCTION For nearly half a century, neutron sources have been of high interest in both basic and applied area ; extending from the very low energies primarily relevant to condensed matter studies, to the very high energies associated with elementary particle interactions A diversity of neutron source types are involved ranging, for example,from small sources implanted in human tissue to the very largest of accelerator based research facilities Neutron source are widely applied in other fields ; for example astronomy and bio-medical The requirements for neutron data and neutron source properties generally ,have shifted from a qualitative, to the present, very quantitative status ; often reflecting very large cost and/or safety impact In many cases, very small parameter variations can be decisive The interest in and utilization of neutron sources is world wide though the character, the application, and the magnitudes may be very different [1] In medical field, a therapy using neutron source application is boron neutron capture therapy (BNCT) which is a cancer treatment based on capturing thermal neutron reaction of isotope 10B irradiated with low energy thermal neutrons to yield high linear energy transfer alpha particles and recoiling lithium-7 nuclei Boron neutron capture therapy (BNCT) a binary cancer treatment modality The treatment consists of two parts : A boron-10 (10B)-labeled compound is administered that delivers high concentrations of 10B to the target tumor relative to surrounding normal tissues This is followed by irradiation with thermal neutrons or epithermal neutrons that become thermalized at depth in tissues The short range (5–9 micrometers) of high LET alpha and 7Li particles released from the 10B(n, alpha) 7Li neutron capture reaction realizes tumor-selective killing without damage to adjacent normal brain tissue A popular reaction considered for accelerator BNCT neutron sources is 7Li(p,n)7Be because the rapid rise of the cross section near threshold provides large quantities of relatively low energy neutrons, which will be described more detail in chapter Beside that, the 7Li(p,n)7Be reaction has been used to produce quasiMaxwellian neutrons in order to measure Maxwellian-Averaged Cross-Sections in the relevant temperatures for stellar nucleosynthesis As we know, nucleosynthesis of heavy elements in stellar environments is governed in major part by neutron capture processes Among these, the slow neutron capture s-process is the only one directly amenable to experiments because it involves nuclei in the valley of stability or close to it A large amount of experimental data has been collected in the last 20 years on Maxwellian Averaged Cross Sections (MACS) of neutron capture reaction , either derived from time of flight measurements or by the activation method using neutron sources mimicking stellar conditions by their energy distribution Neutrons emitted by the 7Li(p,n)7Be reaction for protons bombarding a thick solid target at an energy of 30 keV above the reaction threshold have been extensively used for the activation method A large effort is presently deployed to replace conventional Van de Graaff accelerators used for production of proton beams with larger intensity machines (mA to tens of mA) , on particularly radio frequency linear accelerators, and simultaneously to develope lithium targets capable of sustaining the high beam power The increased neutron intensity achievable from such setups will be used for the study of neutron induced reactions by activation of unstable or rare isotope targets limited to small mass, to short lived reaction product or to cases of small cross sections [3] The purpose of this thesis is to describe a computational tool ( GEANT4 ) to simulate energy spectra and angular distribution as well as total yield of neutron produced by 7Li(p,n)7Be reaction to proton energy up to 10 MeV In Chapter 2, Geant4 toolkit will be described and the results of simulation processes will be shown and compared with several other calculation codes on chapter CHAPTER : MONOENERGETIC NEUTRON SOURCE Since the advent of the electrostatic accelerator and the discovery of the neutron nearly half a century ago, the light particle monoenergetic neutron sources have played a major part in experimental fast neutron physics The attributes of these sources remain great They provide control of all canonical variables (energy ,time, space and momentum) and, in certain classes of experiments, this characteristic places these sources in a uniquely favorable position The neutron yields are prolific, and in optimum configurations at modest facilities, comparable with those of the very large white source facilities Most of the reactions are truly monoenergetic over only limited energy ranges Characteristically, there are secondary and/or minority neutron groups and the curve of binding energy dictates troublesome multi particle breakup a few MeV above the primary threshold The challenges are the selection of the source in the context of the problem and facility, the innovative ability of the experimenter and his mastery of practical technology [1] There are three monoenergetic neutron source categories 1) The primary “big-4” T(p,n)3He T(d,n) He Q= - 0.763 MeV Q= 17.590 MeV D(d,n)3He Q= 3.270 MeV Li(p,n)7Be Q= -1.644 MeV 2) The secondary group Be(d,n)10B Q= 4.361 MeV Li(d,n) Be Q= 15.031 MeV 51 51 V(p,n) Cr Q= -0.331 MeV 3) Advanced or developmental sources exemplified by, for example, those induced by tritium beams Our attention is primarily on the “big-4” as they constitute 90% of the sources used in programs at, particularly , smaller facilities The secondary group is of interest in specialized field The “advanced and developmental” sources can be very powerful but often involve serious facility considerations that make them difficult [1] 1.1 The T(p, n)3He Reactions The inherent advantages of this source are: a relatively low energy threshold ,a very wide monoenergetic range (0.1-8.0 MeV), and a simple reaction mechanism relatively free of background effects (for example no associated gamma-ray) Multi-particle breakup is a problem only at relatively high energies There are inherent and practical limitations The very light system makes the reaction sensitive to kinematic effects There is some uncertainty associated with the absolute yields but the relative angular distributions and polarizations are reasonably known Best yields are obtained with gas targets— and with associated health hazards Power handling capabilities are approximately 10 w/mm The limitation is the gas-cell window usually of nickel, molybdenum or tungsten The power capability can be somewhat improved through the use of multiple and cooled windows or supporting and cooled grids Windowless cryogenetic or hypersonic gas cells have been proposed and even tested However, they remain complex developmental devices that have not found wide application There are backgrounds associated with contaminants and the beam stop The latter is subject to hydrogen inclusion resulting in the well known blistering phenomena even in heavy metals Use of special low background stops (for example Si-28) is limited by material availability and fabrication problems.[1] Yields from gas-absorbed metal foils (for example titanium) are inferior, target life is limited and target thickness difficult to control However, the foils are easier to use and the health hazards reduced The kinetics of the reaction make it attractive from the point of view of the associated- particle calibration technique despite the low energies frequently involved This capability is illustrated by the low energy results of Mier et al shown in Figure 1.These were obtained using gas loaded titanium foils and form a part of an extensive program for the provision of precisely known neutron fields [1] Figure 5: Some basic classes in Geant4 Toolkit Users may construct stand-alone applications or applications built upon another object-oriented framework In either case the toolkit will support them from the initial problem definition to the production of results and graphics for publication To this end, the toolkit includes: user interfaces, built-in steering routines, and command interpreters which operate at every level of the simulation 12 At the heart of Geant4 is an abundant set of physics models to handle the interactions of particles with matter across a very wide energy range Data and expertise have been drawn from many sources around the world and in this respect, Geant4 acts as a repository which incorporates a large part of all that is known about particle interactions [5] Geant4 is written in C++ and exploits advanced software-engineering techniques and object-oriented technology to achieve transparency For example, the way in which cross sections are input or computed is separated from the way in which they are used or accessed The user can overload both of these features Similarly, the computation of the final state can be divided into alternative or complementary models, according to the energy range, the particle type, and the material To build a specific application the user-physicist chooses from among these options and implements code in user action classes supplied by the toolkit A serious problem with previous simulation codes was the difficulty of adding new or variant physics models; development was difficult due to the increased size, complexity and interdependency of the procedure-based code In contrast, objectoriented methods help manage complexity and limit dependencies by defining a uniform interface and common organizational principles for all physics models Within this framework the functionality of models can be more easily recognized and understood, and the creation and addition of new models is a well-defined procedure that entails little or no modification to the existing code [5] 13 Figure : The structural diagram of Geant4 program 2.2 SIMULATION PROCESSES In this thesis, in order to simulate 7Li(p,n)7Be, we have to define geometry of this reaction first There are parameters which to be declared : World: The largest space surrounds the detector , include everything declared, is a cube with size 22 x 22 x mm3 and full of air Detector ( Target) : a cube with size 10 x 10 x 0.001 mm3 ,made of LiF , density =2mg/cm2 on the 1mm Cu backing Source : A proton source with energy 1.912 MeV Physics processes : there are all physics processes occurs include interaction of proton with matter, interaction of neutron with matter, interaction of electron/gamma with matter as mentioned above in Chapter 14 Figure : Geometric simulation with world, source and target Figure 8: Some codes for detector construction 15 Figure 9: Some code for physics processing In this thesis, data processing program after simulation is Origin To process the data according to the thesis requirements, some of the following features are used: Graph Drawing: Use the Plot menu to choose how you want to draw Matching functions: Origin's match function is located in the Analysis menu Library of Origin provides some popular functions (Fit Linear, Fit Polynomial, Fit Exponential, Fit Gaussian, Fit Multi-peaks, ) to non-Linear function (non-Linear Curve Fit …) Note that Geant4 theoretical models not work well for charged particle (p, d, t, He3,α, γ) inelastic interactions at E < 200 MeV Therefore ,in order to simulate the neutron source generated by the 7Li(p,n)7Be reaction, a code derived from Geant4, in which the particle_hp package was used for inelastic interaction of proton, has been developed We first validated the code for 16 incident proton with energy Ep of 1.912 MeV, which is close to the reaction threshold Eth = 1.88 MeV and LiF target of 2mg/cm2 on the 1mm Cu backing was used These parameters are to match with the experimental conditions in [3] The calculation results in comparison to the data ones are shown in figure Figure 10: The neutron energy spectra simulated by Geant4 (left) in comparison to the experimental data (right) See text for more detail.[2] From the figure 8, we can see a big difference between the data and simulation result The calculated neutron kinetic energy is too high than that of expectation It implies that the calculation for kinematic of two body reaction in the particle_hp package is not correct To solve that problem, in our work, particle_hp package was revised then correction relating to kinematic of two - body reaction in classes G4ParticleHPCompFC and G4ParticleHPDiscreteTwoBody was performed The results will be shown in next chapter.[2] 17 CHAPTER 3: SIMULATION RESULTS In this chapter, the results obtained from Geant4 simulations program with the model which was developed as in Chapter will be presented Matching data, spectral and graphical representation, as discussed at the end of Chapter 2, is done using Origin 9.0 software The results include the following content: neutron energy spectra after reaction, angular distribution of these neutron, yield distribution of neutron source obtained after reaction 3.1 Neutron energy spectra Calculation was carried out with the parameters of proton beam and target as used in the experiments, and produced neutrons flying out of the target was analyzed and compared to the data, by that the interaction of created neutrons with target was considered to mimic the experiment set-up In Fig 9, we show the Geant4 simulated integral neutron spectrum compared to the recent experiment of G Feinberg et al [5] and to the Maxwellian flux, a general good agreement between experimental data and simulated result are observed In the high energy cut-off region, simulated spectrum is quite different from Maxwellian behavior due to nearly mono-energetic proton Ep = 1912 ±1.5 keV used, which is also expected in other simulations [7, 8] 18 Figure 11 Geant4 simulation results (red line) in comparison to experimental integral neutron spectra of the 7Li(p,n)7Be reaction from [10] (blue line) The black line represent energy dependence of a Maxwellian flux (E.exp(-E/kT)) The Geant4 simulated results and data from [10] are normalized to the same integral In Figure 10, we compare the energy spectra at different angles simulated by Geant4 and data measured by G Feinberg et al [10] A general good agreement is obtained As expected from kinematic calculation [1] as well as by our calculation (see next section), the neutron yield at o is very small then we omitted the comparison for energy spectra at this angles 19 Figure 12 Geant4 simulated neutron spectra at different angles in comparison to measured data from [10] The data sets are normalized to the same integral 4.2 Neutron production yield Fig 11 shows total neutron yield as function proton energy for 1mm LiF target obtained by Geant4 for energy range of 1.890 MeV to 2.378 MeV covering broad resonance centered at 2.25 MeV, in comparison with ones calculated by C L Lee and X.-L Zhou using kinematic non-relativistic calculation [1] for energy varying from 1.890 MeV to 2.0 MeV The trend of the yield curve obtained by Geant4 is as expected and reflecting shape of the 7Li(p,n)7Be reaction excitation 20 function in this energy range (see inlet in Fig 11) and being consistent to results from [6] Discrepancy of around 0.6 % between our result and one of C L Lee and X.-L Zhou can be explained by the effect of the hadron elastic scattering of the proton with the target considered in Geant4 which implies to the added path length of the proton in the target which was not fully taken into account in the analytical calculation Figure 13 Total neutron yield vs proton energies for 1m LiF target In the field of the neutron metrology, production of the mono-energetic neutron field is required For the reaction 7Li(p, n)7Be by using proton beam having energies in the range of 1920 keV to 2378 keV bombarding very thin lithium target, it can produce the mono-energetic neutrons with energy from 120 keV to 652 keV which was specified in the ISO standard 8529 [11] The validation of the code for simulation of this neutron group is done for the proton energies Ep = 1940 ±1.5 keV, 2020±1.5 keV and 2297±1.5 keV A very thin LiF target with the thickness of 70 mg/cm2 is used 21 In the Fig 12, we show the Geant4 simulated neutron energy spectrum and angular distribution for the case of Ep = 2297±1.5 keV while Fig 13 shows the neutron energy vs emission angles and and Fig 14 presents the differential neutron yield at 00 At the 00, the neutrons with mean energies 𝐸´ = 144 keV, 250 keV and 565 keV are observed as predicted by non-relativistic kinematic calculation [9, 12] and as presented in [13] The energy spread of neutron due to the energy loss of the proton in target is also presented The mean energy loss of 1940 keV and 2020 keV, 2297 keV protons are 8.87 keV, 8.61 and 7.87 keV respectively Figure 14 Energy spectrum (left) and angular distribution of the neutron produced by the 2297±1.5 keV proton bombarding on 70g/cm2 LiF target 22 Figure 15 Produced neutron energy vs emission angle 23 Figure 16 Differential neutron yields at degree Summary: The Geant4-based code toolkit for simulation the generating and transporting of neutron by 7Li(p,n)7Be reaction is implemented for the first time The validation is done by comparison between the results obtained by this code and the experimental data including 30 keV neutron energy regions which is numerically very sensitive to the cross-section near threshold, as well as by comparison to the other codes shown the excellent agreement Due to the fact that code simulates both production and transportation of neutron then it would be used to characterize neutron field as well as for the design experiment It will also help to improving the data analysis capability 24 Conclusion: With the goal of simulating the near threshold 7Li(p,n)7Be using Geant4 simulation program for researching neutron source,this thesis has achieved the following results: Builds the geometry of 7Li(p,n)7Be, gives the relevant physical parameters and settings requirements to initialize a simulation program Survey the characteristics of neutron energy spectra, angular distribution, kinematic relation between emission angle and energy of neutron and calculate yield of neutron produced after reaction Comparison to some other calculation methods and the experiment results have shown the excellent agreement 25 REFERENCES K Okamoto (1980), “Proceedings of tie IAEA consultants' meeting on neutron source properties”, pp 24-27 Phan Viet Cuong - Production of mono-energetic neutron from 7Li(p,n)7Be reaction for testing neutron detectors K H BECKURTS and K WIRTZ (1964) , “Neutron physics” M Friedman , D Cohen , M Paul , D Berkovits , Y Eisen , G Feinberg , G Giorginis, S Halfon , A Krása , A.J.M Plompen , A Shor, (2012) “Simulation of the neutron spectrum from the Li(p,n) reaction with a liquid-lithium target at Soreq Applied Research Accelerator Facility”,p1 URL C.L.Lee, X.-L.Zou, Nucl Instr Meth B 152 (1999) 1-11 M Friedman, D Cohen, M Paul, D Berkovits, Y Eisen, G Feinberg, G Giorginis, S Halfon, A Krása, A.J.M Plompen, A Shor, Nucl Instr Meth A 698 (2013) 117-126 R Reifarth, M Heil, F Käppeler, R Plag, Nucl Instr Meth A 608, 139 (2009) María S Herrera, Gustavo A Moreno, Andrés J Kreiner, Nucl Instr Meth B 349 (2015) 64-71 10 G Feinberg et al, Phys Rev C 85, 055810 (2012) 11 ISO 8529-1 2001, Reference Neutron Radiation: I Characteristics and Methods of Production (Geneva: International Organization for Standardization) 12 K H Beckurrts and K Wirtz, Neutron Physics, Springer Verlag-1964 13 Ralf Notle et al, Metrologia 48 (2011) S263-S273, IOP Publishing 26