THE simulation of the production, interaction and transportion

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1 28 IEEE Nuclear Science Symposium Conference Record N4-6 Validation of Neutrons in Geant4 Using TARC Data - production, interaction and transportation Alexander Howard, Gunter Folger, Jose Manuel Quesada, Vladimir Ivanchenko Abstract The TARC (Transumatation by Adiabatic Resonance Crossing) experiment has been simulated with the Geant4 toolkit. The experiment measured neutron production from 2.5 GeV/c and 3.5 GeV/c protons incident on high purity lead. The Geant4 simulation utilised either the Bertini or Binary nuclear cascade models together with the low energy Neutron HP extension. The neutron time-energy correlation, absolute fluence spectrum, and radial fluences are compared between data and simulation. Recent improvements to the PreCompound and De-excitation modules within Geant4 are presented and their influence on the TARC simulation demonstrated. Overall the Geant4 simulation reproduces the TARC data. Index Terms neutron, TARC, Geant4, PreCompound/Deexcitation I. INTRODUCTION THE simulation of the production, interaction and transportion of neutrons is important for a number of applications including: background radiation studies; radiation effects (singe event upsets in electronics); background and spill-over (LHC experiments). The Geant4 toolkit offers the possibility to simulate particle interactions and transportation from a very high energy right down to thermal neutron energies [], [2]. The high quality of data make TARC a good benchmark for validation of neutron physics in Geant4 over a broad energy and process range, specifically: Neutron production from GeV protons Secondary neutron production Thermalisation and capture Absolute fluence measurement TARC had the unusual capability of determining neutron energies from higher energy evaporation ( 2 MeV)downto thermal energies. This is the first time that neutron validation of Geant4 across this energy range has been made. A. The TARC Experiment The TARC experiment took data in in the CERN PS beam in order to study neutron driven nuclear Transmutation by Adiabatic Resonance Crossing [3], [4]. Protons of Manuscript sent Novermber 3th, 28. This was supported in part by the Commission of hte European Commmunities under the 6th Framework Programme Structuring the European Research Area contract number RII3- CT A. Howard is with CERN and ETH Zurich alexander.howard@cern.ch correspondence: CERN, CH-2 Genève 23, Switzerland G. Folger is with CERN J.M. Quesada is with the Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla V. Ivanchenko is with CERN Fig.. The TARC geometry implemented within Geant4. The colours are geometrically different blocks, high-purity lead is used throughout. momentum 2.5 GeV/c and 3.5 GeV/c were incident on a large lead target. Fig. shows the geometry as implemented within Geant4. The target is approximately cylindrical in crosssection with a diameter of 3.3m and length 3m, comprising 334 tons of natural lead. The colours correspond to different geometries of blocks, with pure lead used throughout. High purity (99.99%) lead was used in order to maximise the neutron elastic scattering, subsequent thermalisation and to reduce capture from impurities. The target volume was large enough to stop the protons and contain 7% of the produced neutrons. The beam entered through a blind hole.2 m long which resulted in a secondary shower profile approximately centred in the volume. Twelve sample holes were located in the experiment to allow the placement of detectors and materials in order to measure the neutron fluence as a function of energy and to determine the capture cross-section on a selection of isotopes of interest for nuclear waste management. B. Geant4 Physics Modelling To simulate the hadron interaction and production the Geant4 Bertini and Binary cascade physics models were chosen [5]. Both of these models include independent low energy nuclear de-excitation models. The low energy (<2 MeV) Neutron HP package was used for neutron interaction, transporation, elastic scattering and capture. In addition, other standard Geant4 processes were included for hadron elastic, electromagnetic and stopping physics. The QGSP BERT HP and QGSP BIC HP physics lists were utilised /8/$ IEEE 2885

2 (a) A single proton interaction viewed side-on (b) A single proton interaction viewed end-on Fig. 2. A single 3.5 GeV/c proton interacting in the lead volume from the side (a) and end-on (b) where the resulting neutron shower can be seen to be well contained within the lead volume. using the Neutron HP model can thus be tested. A plot of the time (μs) vs. energy (ev) for neutrons produced from 2.5 GeV/c protons using the Bertini cascade is shown in Fig. 3. Between μs and ms a clear correlation can be seen with quite narrow distribution. The simulation results are plotted for all neutrons crossing a fixed radius in the TARC volume. This results in some neutrons being sampled many times as they re-cross the boundary, thus some excess off-correlation background is visible in the plot. The resulting distribution is very close to that produced from the original TARC study using FLUKA for hadronic interactions and custom-written neutron transport below 2 MeV [3], [4]. The neutron energy-time is correlated according to [6], [7]: Fig. 3. The neutron energy vs. time correlation as simulated using the Bertini nuclear cascade model within Geant4. The simulation of the interaction of a single 3.5 GeV/c proton is shown in Fig. 2. The proton track is shown in blue entering from the left, the neutrons are black and gammas are shown in red. II. GEANT4 COMPARISON WITH DATA The Geant4 simulation was run with the two different nuclear cascade models and compared against the TARC data. Thin target comparison was also carried out across the relevant energy range in order to further understand the physics modelling in a more discrete manner. A. Neutron Energy-Time Correlation Due to the highly elastic nature of neutron scattering in lead it is possible to correlate the energy of a neutron with the time of measurement. The energy time correlation assumes that neutrons start with very high energy and only interact elastically. Below 2 MeV, the neutron transport in Geant4 E(t)(t + t ) 2 = K () where E(t) corresponds to the energy of the neutron at a given time t. The off-set t is introduced in order to compensate for the fact that the neutrons do not begin with infinite energy. This correlation function is plotted for simulated neutrons of energy.- ev in Fig. 4 for the two cascades considered - Bertini (4(a)) and Binary (4(b)). A quasi-gaussian distribution is produced which can be fitted over a reduced range to give the mean values of 67.2 and 68.6 for the Bertini and Binary models respectively. The TARC experiment measured the correlation to be 73±2 using resonance capture with eight different isotopes across the energy range ev. The original TARC simulation confirmed this value. The smaller values for Geant4 indicate an under-production of higher energy neutrons from both cascade models. The slightly higher value with the Binary cascade can be attributed to a harder secondary neutron spectrum. B. Absolute Neutron Fluence The TARC experiment measured the neutron fluence at a number of positions in the target using a selection of complementary techniques. The energy-time correlation on lithium and uranium detectors gave a natural way to assess 2886

3 dn/d K / 22 Constant.27E Mean E- Sigma E- dn/d K / 22 Constant Mean E- Sigma E Correlation, K (a) Bertini Cascade Correlation, K (b) Binary Cascade Fig. 4. The correlation function (see text) fitted to a Gaussian distribution for Bertini (a) and Binary (b) cascades. The red line at 73 ± 2 corresponds to the TARC experimental value. precisely the neutron energy. Also direct measurements were made via a fixed final-state 3 He neutron capture and ionisation detector. For monoenergetic neutrons of velocity, V, and density, n, the neutron flux is defined as φ = Vn and is a quantity that upon multiplying by the macroscopic cross-section (Σ), one obtains the neutron reaction rate per unit volume. This quantity should not be confused with the rate of particles crossing a surface element, which is a current and depends on the orientation of the direction of the particles Three procedures were used to determine the fluence: ) dn/ds perp is the number of neutrons crossing a surface element ds, with ds perp = ds cos θ wher θ is the neutron angle to the normal; 2) the average fluence in a volume element dv as dl/dv, where dl is the total track length of neutrons in dv ; 3) Number of interactions in a detector and computing fluence as (/Σ)dN/dV, where dn is the number of interactions ndv. The first two were used in simulation and found to be in good agreement with each other. A plot of the simulation output is shown Fig. 5. The neutron fluence is plotted as a function of neutron energy from. ev up to 2 MeV. The energy bins were chosen to match those of the experimental data which are plotted in blue. The green data points correspond to the +/ combined statistical and systematic error of the experiment. The simulation data are plotted for the Bertini (magenta) and Binary (black) cascades. The experimental data lie in between the two cascade models, with Bertini over-producing and Binary underproducing the neutron fluence. However, both are within the systematic errors of the experiment. C. Radial Neutron Fluence TARC measured the radial dependence of the neutron fluence in order to measure the slowing down within the lead Fig. 5. The TARC absolute fluence compared to the Geant4 simulation with Bertini (magenta) and Binary (black) cascades. The experimental data are plotted in blue with green for the +/- errors. volume with protons at 3.5GeV/c (slightly higher than fluence data). The detectors were moved along the holes in the set-up and measured at a number of different energies. In the Geant4 simulation, a sequence of parallel shell volumes were created at different radii according to the experimental measurements to increase CPU efficiency. Fermi Age Theory predicts that the flux should be isotropic and TARC experimental data (+/- z-values) supports this hypothesis over the energy range investigated. The agreement between experimental data and simulation is very good over all the measured energies with the Binary 2887

4 events binary 3.5 GeV/c proton df/de n/cm 2 /ev/ 9 p 7 6. ev.5 ev 5 5 ev 8 ev 4 3 ev 48 ev kev kev 5 kev 2 Fig. 8. As previous figure with the improvements to the PreCompound model (BIC and PRECO). The unchanged Bertini model (BERT is included for reference. events binary 2.5 GeV/c proton Radial Distance/cm Fig. 6. The absolute fluence measured at different energies and different radial positions within the TARC volume (blue) compared to the Binary cascade model (red). EdF/dE n/cm 2 / 9 p Fig. 7. Previous situation: double differential cross sections for 27 Al(p,xn) at 22 MeV incident energy. BIC curve corresponds to Binary cascade calculation; it includes as a later stage preequilibrium, calculated with G4PreCompound model. PRECO is the calculation with G4PreCompoundModel directly accessed to. BERT is the calculation with Bertini cascade, which includes its own preequilibrium and equilibrium processes cascade as displayed in Fig. 6. The Bertini cascade gave slightly higher fluences in all cases. III. IMPROVEMENTS TO THE PRECOMPOUND/DE-EXCITATION MODULES WITHIN GEANT4 The precompound stage a nuclear reaction, described in in the framework of the exciton model [8], is considered until nuclear system is at equilibrium. Further emission of nuclear fragments or photons from excited nucleus is simulated using Fig. 9. The absolute fluence as simulated with the improved PreCompound/De-excitation model from Geant4 (black) compared to data (blue). the Weisskopf-Ewing [9] equilibrium theory, which by itsef is not able to describe the high energy tails of emission spectra. The inverse reaction cross section is a key ingredient in the calculation of particle emission probabilities. Initial formulation [] was previous to the wealth of experimental data since the sixties. Therefore, several parameterizations, either of experimental cross sections [] or of calculated cross sections from optical potentials in turn fitted to the available nuclear reaction data sets [2]-[3], have been included as options. For light systems and low emission energies, near the Coulomb barrier, where no experimental data are usually available, additional refitting has been done in order to improve the production of secondaries. These new cross sections carry by themselves the Coulomb barrier; therefore no additional 2888

5 G4/Data 2.8 Binary-v9.-Ref G4 Shell 45.6cm G4/Data 2.8 Binary G4 Shell 45.6cm (a) The previous PreCompound/De-excitation model (b) The new revised PreCompound/De-excitation model Fig.. The ratio of simulation over data for the absolute neutron fluence simulated using the Binary cascade comparing the old and new PreCompound/Deexcitation models. explicit parameterization of this quantity is needed, as it was formerly. The emission probabilities, either for single nucleons or for complex fragments, have been calculated including combinatorial factors R j (which take into account the availability among the excitons of enough particles to form the one to be emitted); these factors were not included previously. We have found that attention must be paid to the transition from preequilibrium to equilibrium. In our MonteCarlo simulation a physically consistent condition has been directly implemented by means of the apropriate algorithm. Moreover a smooth transition from preequilibrium to equilibrium stage, which initially was set into the model (soft cutoff criterium), according to the proposal in [5] has proven not to be necessary in our case to enhance evaporation at the expense of preequilibrium. The improvement in thin target double differential crosssections can be seen in Fig. 7 and Fig. 8 for the old and new models, respectively. The binary cascade is coupled to the preequilibrium and de-excitation models, whilst the preco values are directly using the pre-equilibrium/de-excitation models. A more detailed explanation of these preequilibrium and equilibrium related aspects can be found in [6]. The affect of these improvements on the TARC absolute fluence is shown in Fig. 9 and Fig.. IV. CONCLUSION The Geant4 simulation using the Neutron HP module gives good agreement with the TARC experiment for neutron transportation, in particular the energy-time correlation and number of neutrons exiting the set-up. The simulated neutron fluence disagrees by 5% between the two nuclear cascade models, Bertini and Binary. With the improvements to the PreCompound/De-excitation modules, absolute neutron fluence both radially and spectrally agree very weel between Geant4 and the experimental data. ACKNOWLEDGMENT Thanks go to John Apostolakis for many helpful discussions on the Geant4 simulation and Jean-Pierre Revol for assisting in understanding the TARC experiment. REFERENCES [] S. Agostinelli et al., Nuclear Instruments & Methods A 56 (23) [2] J. Allison et al., IEEE Trans. on Nucl. Sci. 53 No. (26) [3] A. Abánades et al. NIM A 478 (22) [4] CERN Yellow report: The TARC Experiment (PS2):Neutron-Driven Nuclear Transmutation by Adiabatic Resonance Crossing Editor J.-P. Revol (CERN 99-) [5] More information on the Geant4 physics models can found in the Physics Reference Manual: PhysicsReferenceManual/html/PhysicsReferenceManual.html [6] A.A. Bergman et al., Proc. Geneva Conf. IV (955) [7] M. Sawan and R.W. Conn, Nucl. Sci. Eng. 54 (974) 27. [8] J. J. Griffin,Phys. Rev. Lett. 7(966) 478. [9] V. E. Weisskopf and D. H. Ewing, Phys. Rev. 57(94)472. [] I. Dostrovsky, Z. Fraenkel and G. Friedlander, Phys. Rev. 6,vol.3(959)683. [] H. P. Wellisch and D. Axen, Phys. Rev. 54 (996)329. [2] A. Chaterjee, K. H. N. Murthy and S. K. Gupta, Pramana, vol. 6, No 5, May 98, p [3] Kalbach, PRECO-2 Exciton Model Preequilibrium Code with Direct Reactions [4] S. Gupta,Z. Phys. A33(98)329. [5] S. G. Mashnick, A. J. Sierk, K. K. Gudima and M. I. Baznat, Journal of Physics, Conference Series 4 (26) [6] J.M. Quesada, M. A. Cortés, A. Howard, G. Folger and V. Ivantchenko, Improvements of Preequilibrium and Evaporation Models in Geant4, contribution sent to the Proceedings of 28 Nuclear Science Symposium, 9-2 October 28, Dresden, Germany. 2889