G4 vs. Fluka comparison for single neutron

Transcription

1 From SguazzWiki NeuCal: NeuCalG4vsFlukaReport G4 vs. Fluka comparison for single neutron Sguazzoni & Sorichetti On this page... (hide) 1.!Prototype geometry 2.!Simulated samples 2.1!G4 details 2.2!Fluka details 3.!Scintillator comparison plots 3.1!Step E DEP vs. Time 3.2!Energy deposited in the Scintillators 3.3!Time development of energy deposited in the scintillators 3.4!Scintillator efficiency 4.! 3 He counter comparison plots 4.1!Energy deposited in the 3 He counters 4.2! 3 He efficiencies The MC simulated response of a NeuCAL prototype for single incident neutrons has been studied with for GEANT4 (ver ) and Fluka (ver ). Here some results are compared and discussed. 1.! Prototype geometry The NeuCAL prototype under study in GEANT4 (G4) is represented in figure. There is one 3He counter for each of te nine scintillator drawers. The geometry used in G4 and Fluka is the same. Only few not relevant details differ. Either in G4 and in Fluka the aluminum box top cover is missing. NeuCAL, cross section NeuCAL (*) Page 1 of 17

2 NeuCAL with 20 events, single n, 1MeV (*) NeuCAL with 20 events, single n, 10MeV (*) Color code: red 3He counters; blue scintillator slabs; gray scintillator drawer Al support plates; green drawer service volume with light guides and PMs, for the moment just filled with the scintillator material; brown external Al box and support. (*) The external Al box is not shown. In both simulations the neutrons travel downward and impinge NeuCAL in the middle of the top central drawer just above the 3 He counter wall. 2.! Simulated samples The table summarizes the samples produced for the comparison: Generator Notes Energy Size Fluka - 1 MeV Fluka - 10 MeV Geant4 LHEP_PRECO_HP 1 MeV Geant4 LHEP_PRECO_HP 10 MeV Geant4 QGSP_BERT_HP 1 MeV Geant4 QGSP_BERT_HP 10 MeV Geant4 QGSP_BIC_HP 1 MeV Geant4 QGSP_BIC_HP 10 MeV Each sample consists of events. This number of events is necessary to have a statistically significant estimate of the 3 He counters efficiency, especially in case of 10MeV impinging neutrons. Both G4 and Fluka tracks the particles in steps (hits in Fluka terminology). Each step/hit is characterised by the deposited energy, the time from the begin of event (that starts when the neutron is fired from the particle gun) and the position. Care has been taken to have both simulation frameworks synchronous: this is achieved by throwing the primary neutrons exactly from the same spatial position. Geant4 code implements a root based class for the production of relevant histograms. The HBook Fluka output containing all hits is converted into a root file by using the same ROOT based class used in G4; doing so the converted files has the same format of the Geant4 output root file. 2.1! G4 details The G4 samples have been produced with three different physics lists (a physics list represents the Page 2 of 17

3 activated interaction processes and the corresponding simulation models): LHEP_PRECO_HP (LHEP). The LHEP Physics lists combine the high energy parameterised (HEP) and low energy parameterised (LEP) models describing inelastic interactions for all hadrons. The modeling of elastic scattering off a nucleus and of capture of negative stopped particles and neutrons proceeds via parameterised models. Cross sections used are based on Gheisha parameterizations. The precompound model (PRECO) generates the final state for hadron inelastic scattering. The data driven high precision neutron package (HP) is used to transport neutrons below 20 MeV down to thermal energies. QGSP_BERT_HP (QBERT). QGSP is the basic physics list applying the quark gluon string model for high energy interactions of protons, neutrons, pions, and Kaons and nuclei. The high energy interaction creates an excited nucleus, which is passed to the precompound model modeling the nuclear de-excitation. The G4 Bertini cascade (BERT) is used for primary protons, neutrons, pions and Kaons below ~10GeV. The data driven high precision neutron package (NeutronHP) is used to transport neutrons below 20 MeV down to thermal energies. QGSP_BIC_HP (QBIC). As above but using the G4 Binary cascade (BIN) for primary protons and neutrons with energies below ~10GeV. QGSP_BERT_HP is thought to be more adequate than QGSP_BIN_HP at lower energy and is chosen as the baseline for our comparison. LHEP_PRECO_HP is considered somehow obsolete. Nevertheless in this study no difference have been observed between the above physics lists, as the different models mainly apply in case of cascades following high energy interactions; all of them use the high-precision low-energy neutron transportation model. 2.2! Fluka details The Fluka samples have been produced with the following options activated: PHOTONUC to activate photonuclear interactions in each material; PRECISIO within the DEFAULTS datacard that activates the tracking until 100keV in kinetic energy for all particles and until 10-5 ev for neutrons. 3.! Scintillator comparison plots 3.1! Step E DEP vs. Time The scatter plot E DEP vs. time for all steps/hits that have non zero energy deposition in the active scintillator volumes is below. Each plot in the 9x9 matrix represent a scintillator drawer, from top to bottom, from left to right, looking NeuCAL from the photomultiplier side. The same convention applies to all the plots shown here. Page 3 of 17

4 Scintillator, step/hit log 10 (E DEP /MeV) vs. log 10 (t/s), 1MeV Scintillator, step/hit log 10 (E DEP /MeV) vs. log 10 (t/s), 10MeV For the reader convenience, the full-resolution plot for the top-central drawer, neutrons at 1MeV, is below: Page 4 of 17

5 Scintillator, top-central drawer, step/hit log 10 (E DEP /MeV) vs. log 10 (t/s), 1MeV This kind of plot may reveal subtle differences in the details of the simulation. In fact: Fluka is characterized by a much higher number of steps/hits; these have about one order of magnitude smaller E DEP in the time domain from a fraction to hundreds of ns; Fluka has structure (0.1 ev < E DEP < 1eV, 0.01ms < t < 1ms) that is completely missing in G4. 3.2! Energy deposited in the Scintillators The drawer event energy is just the sum of all energy deposits in any of the three scintillator slabs that are readout by the same photomultiplier within a given event. The drawer event energy is plotted below for 1 and 10MeV impinging neutrons: Page 5 of 17

6 Scintillator, E DEP event, 1MeV Scintillator, E DEP event, 10MeV For the reader convenience, the full-resolution plot for the top-central drawer, neutrons at 10MeV, is below: Page 6 of 17

7 Scintillator, top-central drawer, E DEP event, 10MeV In general the agreement between G4 and Fluka is pretty good. The Fluka spectra are in general more populated for E DEP >E 0, being E 0 the neutron initial kinetic energy, although this region is statistically poorly significant. The Fluka spectra for 10MeV neutrons show a peak at ~1.2MeV. It is completely absent in Geant4 spectra that, on the other hand, are more populated for 0<E DEP <1MeV. The effect is present only for the central column of drawers. On 1MeV samples a peak is visible, less pronounced, at ~0.12MeV. This is thought to be an artifact due to the Kerma approximation, i.e. the way Fluka handles the interaction between the neutrons and most of the nuclei of the material. In an elastic interaction of the type n+a->n+a, A being a nucleus of atomic number A, the energy of the outcoming neutron E OUT is comprised between E 0, the initial neutron energy, and E 0 *(A-1) 2 /(A+1) 2. The recoil energy of the nucleus A in the final state is clearly E 0 -E OUT. Kerma approximation roughly corresponds to giving to the outcoming nucleus a fixed energy value corresponding to some kind of average over the interval of the allowed values, i.e. 0.5*E 0 *(1-(A-1) 2 /(A+1) 2 ). Doing the math one discover that for Carbon (A=12) the average recoil energy of the nucleus then deposited by ionization is 1.4MeV and 0.14MeV for 10MeV and 1MeV impinging neutrons respectively, not too far from the observed 1.2MeV and 0.12MeV. On the other hand G4 extracts the recoil energy on random basis and this origins the more populated region below 1MeV, visible only in 10MeV spectra. The attentive reader will notice that another peak is visible in Fluka 10MeV samples, central column drawers, around 2.5MeV. Its source is not clear but there is the suspect that its origin is similar to the "Carbon" peaks. The lower part of the scintillator energy spectrum can be put in evidence by using the logarithmic scale. The entire series of plots Page 7 of 17

8 Scintillator, log 10 (E DEP event /MeV), 1MeV Scintillator, log 10 (E DEP event /MeV), 10MeV shows an overall good agreement. Only the lower part of the spectrum, i.e. the sub ev region, shows some differences between Fluka and G4. This energy range is meaningless with respect to the quantities that are measured in the real life (see next paragraph). The bumps of G4 with respect to Fluka in the region from ~10keV to ~1MeV are due to the Carbon peak issue discussed above. Page 8 of 17

9 3.3! Time development of energy deposited in the scintillators The profile of the energy deposition in the scintillator drawers vs. time is important to estimate the response of the device. Also in this respect Fluka and G4 agree reasonably well, as demonstrated by the following series of plots where <de DEP (t)> is plotted against t. <de DEP (t)> is the average of the energy deposited between t-dt/2 and t+dt/2. In the following plots dt=5ns. The linear time scale allows the response within the first 100ns to be compared: Scintillator, <de DEP (t)> vs. t, 1MeV Page 9 of 17

10 Scintillator, <de DEP (t)> vs. t, 10MeV The subdivision of the time domain in logarithmic intervals allows a more wide time domain to be compared: Scintillator, <de DEP (t)> vs. log 10 (t/s), 1MeV Scintillator, <de DEP (t)> vs. log 10 (t/s), 10MeV Page 10 of 17

11 Differences between G4 and Fluka are visible in the early and in the late part of the time domain. They can be well explained with the differences seen in the step/hit E DEP vs. time scatter plots discussed above. 3.4! Scintillator efficiency The event energy above is obtained just integrating all the energy depositions within a given drawer scintillators regardless of the time delay, that can extend up to the ms range, and regardless of the minimum detectable energy. According to the experts, the detection threshold could be around ~100keV (order-of-magnitude) and the gate could be extended up to ~1ms (order-of-magnitude). We assume a neutron has been recorded by a given drawer if the energy deposited in its scintillators within 1ms exceeds 100keV. Given the drawer numbering convention represented in the following table (again NeuCAL seen from the PMs side) Column 1 Column 2 Column 3 Row Row Row the plots below summarize the resulting scintillator efficiencies. Scintillator efficiency, 1MeV Scintillator efficiency, 10MeV The agreement between G4 and Fluka is good. Note that error bars are masked by the markers. 4.! 3 He counter comparison plots 4.1! Energy deposited in the 3 He counters The neutron detection principle of the 3 He counters is based on the inelastic reaction n+ 3 He-- >p+ 3 H+765keV, the 765keV to be shared between the proton and the tritium. The n on 3 He inelastic cross section is shown below: Page 11 of 17

12 3 He inelastic cross section (Knoll) The comparative study of the 3 He counters response is jeopardized by two issues, one for each generator: Fluka does not track the products of the reactions: 765keV are just deposited into the 3 He counter active volume for each single inelastic interaction events; G4 does track the products of the reaction so taking into account the possibility that part of the 765keV is not detected if the proton of the tritium does escape from the 3 He counter active volume; unfortunately the Q-value of the reaction is not 765keV but 1273keV as a consequence of a known bug in G4 version that will be fixed in the next version. Nevertheless some interesting comments can be done by analysing the scatter plot E DEP vs. time for all steps/hits that have non zero energy deposition in the 3 He counter active volumes. 3 He counter, step/hit log 10 (E DEP /MeV) vs. log 10 (t/s), 1MeV Page 12 of 17

13 3 He counter, step/hit log 10 (E DEP /MeV) vs. log 10 (t/s), 10MeV The Fluka depositions are 'monochromatic', as expected given the above discussed feature. In some of the plots, in particular the ones referring to the central column counters, two different time domain are visible: one below ~100ns, one well above ~100ns. For a better understanding of the phenomenology two event-wise quantities can be studied; the energy collected in the first 100ns, E t<100ns DEP, and the energy collected afterwards, E DEP t>100ns. The plots relative to these two quantities are: E DEP t<100ns, 1MeV Page 13 of 17

14 E DEP t<100ns, 10MeV E DEP t>100ns, 1MeV Page 14 of 17

15 E DEP t>100ns, 10MeV NB In all plots the entries below 100keV are ignored to remove accidental energy depositions not related to processes originated in the 3 He counters active volumes. The late depositions (t>100ns) are reasonably due to genuine n+ 3 He-->p+ 3 H+765keV reactions, for the following reasons: a considerable amount of time is needed to bring the neutron to thermal energies where the 3 He inelastic cross section is huge; entries for t>100ns are present for all counters, even the side ones; the deposited energy of the corresponding Fluka entries is 765keV. The origin of the 'prompt' component (t<100ns) is less clear. Two hypotheses can be considered: 1. prompt 3 He inelastic reactions: the cross section plot above shows that the 3 He inelastic cross section is ~1barn at 1MeV and <1barn at 10MeV; but it is easy to estimate that no more than 1-2 neutrons are expected to interact promptly (i.e. without thermalizing) in the top central counter, the most favourably placed; moreover the Fluka deposits in this time range are clearly at an energy greater than ~765keV (the Fluka line is at ~3.2MeV); 2. 3 He elastic reactions: the elastic cross section is slightly bigger than the inelastic one (see picture below) and the structure of the prompt deposits is very similar to the 'carbon peak' discussed above (effect more pronounced for central column counters, monocromatic Fluka energies due to the Kerma approximation), where the Carbon has to be replaced with the 3 He. Page 15 of 17

16 Hypothesis number 2 is more likely. 4.2! 3 He efficiencies 3 He elastic cross section (Knoll) The problems of either Fluka and G4 do not prevents to give an efficiency estimate. The efficiency is derived just assuming that any neutron interaction with 3 He that results in a entry within the above histograms is detectable. With the position numbering convention given above these are the efficiency summary plot. Also the overall efficiency obtained integrating over the entire event the deposited energy is given. 3 He prompt efficiency (E DEP t<100ns ), 1MeV 3 He prompt efficiency (E DEP t<100ns ), 10MeV 3 He late efficiency (E DEP t>100ns ), 1MeV 3 He late efficiency (E DEP t>100ns ), 10MeV Page 16 of 17

17 3 He efficiency (E DEP event ), 1MeV 3 He efficiency (E DEP event ), 10MeV Despite in some case the statistic is really not sufficient, the agreement between G4 and Fluka is good, either overall either separating the 'prompt' and the 'late' components. It is interesting that for some counter the 10MeV efficiency, suffering in general figures in the per mil region, profits of a ~50% increase thanks to the 'prompt' component. Retrieved from Page last modified on August 15, 2009, at 08:17 PM Page 17 of 17