1. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Benchmark for Geant4 using TARC – initial status 1)TARC – experimental set-up and aims 2)Geant4 Simulation 3)Initial output 4)Energy-time correlation 5)Comparison between physics lists 6)Absolute fluence 7)Exiting neutrons 8)Future plans 9)Conclusions Alex Howard, PH-SFT

2. Alex Howard PH-SFT LCG-PV 10 th May 2006 TARC – neutron validation ● TARC – Neutron-Driven Nuclear Transmutation by Adiabatic Resonance Crossing ● Validates spallation neutron production from 2.5 and 3.5 GeV/c protons on pure lead ● Validates energy-time relationship and thermalisation of neutrons ● Absolute Neutron fluence spectrum from spallation production ● Measures capture cross-sections on a number of specific isotopes -

3. Alex Howard PH-SFT LCG-PV 10 th May 2006 TARC – aims and setup ● Aims of experiment: – neutron production by GeV protons hitting a large lead volume; – neutron transport properties, on the distance scale relevant to industrial applications (reactor size); – efficiency of transmutation of 99 Tc and 129 I. ● Two types of measurements performed: – neutron fluence measurements with several complementary techniques over a broad neutron energy range from thermal up to a few MeV; – neutron capture rate measurements on 99 Tc (both differential and integral measurements) and on 129 I and 127 I (integral measurements). For 99 Tc a high statistics measurement of the apparent capture cross-section was obtained up to ~1 keV. Below this energy 85% of all captures occur in a typical TARC neutron spectrum. ● The set-up is 334 tonnes of pure lead with approximate cylindrical symmetry about the beam axis. Diameter is 3.3m and length 3m. Lead volume is 29.3 m 3. The beam is introduced through a 77.2mm diameter blind hole, 1.2m long. This leads to the neutron shower being approximately centred in the middle of the 3m lead length. The lead is 99.99% pure.

4. Alex Howard PH-SFT LCG-PV 10 th May 2006 Data taken 1996-1997, final publication 2002

5. Alex Howard PH-SFT LCG-PV 10 th May 2006 TARC – experimental set-up

6. Alex Howard PH-SFT LCG-PV 10 th May 2006 Experimental Goal1: 99 Tc capture cross-section

7. Alex Howard PH-SFT LCG-PV 10 th May 2006 Experimental Set-up ● TARC comprises 334 tonnes of lead in a 3.3m x 3.3m x 3m cylindrical block ● 12 sample holes are located inside the volume ● Primary particle beam is either 2.5 or 3.5 GeV/c protons

8. Alex Howard PH-SFT LCG-PV 10 th May 2006 Experimental Set-up ● TARC comprises 334 tonnes of lead in a 3.3m x 3.3m x 3m cylindrical block ● 12 sample holes are located inside the volume ● Primary particle beam is either 2.5 or 3.5 GeV/c protons TARC simulation Geant4

9. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation ● Due to the highly elastic nature of neutron scattering in lead and the high nuclear mass, there is a correlation between the neutron energy and its time of observation. This is particularly true at low energy, approximately independent of the initial neutron (and proton) energy ● This has been verified experimentally and by TARC's own simulation (FLUKA + EA custom) ● TARC measurements were carried out by measuring the time of capture on different resonant isotopes and by measuring the prompt signal in 3 He gas counters ● Geant4-v8.0-ref4 also agrees well, but with some excess of neutrons produced at low energy ● Some differences between physics list

10. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation ● Neutron energy and time are stored for the flux through a given radial shell ● Reasonable agreement with expectation, although the low energy population is quite different between physics list (as expected) Geant4

11. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation ● Neutron energy and time are stored for the flux through a given radial shell ● Reasonable agreement with expectation, although the low energy population is quite different between physics list (as expected) TARC simulation Geant4 TARC Paper

12. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation NB: At this energy QGSP and LHEP have different cross-sections but same LEP model QGSP_HP LHEP_BIC_HP LHEP_BERT_HP LHEP_HP ● Some difference between physics lists is observed ● Mostly due to 1keV production from the low energy parametrised models ● The two cascade models remove this effect ● It is possible to fit the correlation according to: E(t) (t+t 0 ) 2 = √K where t 0 is a correction for non-infinite initial energy

13. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation ● The slope of the correlation can approximate a Gaussian distribution LHEP_HP LHEP_BIC_HP QGSP_HP LHEP_BERT_HP ● The correlation distribution can be fitted across a restricted energy range of between 0.1 eV and 10 keV ● t 0 is fixed to 37  s throughout

14. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron Energy-Time Correlation ● The slope of the correlation can approximate a Gaussian distribution Minuit errors on the mean are between 0.03 and 0.08 LHEP_HP LHEP_BIC_HP QGSP_HP LHEP_BERT_HP 160.2 158.9 167 168.5 experiment and TARC simulation give 173±2 ● Bertini and Binary cascades are close to agreement with experiment

15. Alex Howard PH-SFT LCG-PV 10 th May 2006 Mono-energetic Neutron Correlation ● Fixed incident neutron energy and record energy-time relationship – similar to protons LHEP_BIC_HP 0.1 MeV 10 MeV 1 MeV 100 MeV 0.1 MeV 10 MeV 1 MeV 100 MeV

16. Alex Howard PH-SFT LCG-PV 10 th May 2006 Mono-energetic Neutron Correlation ● Distribution fits LHEP_BIC_HP 0.1 MeV 10 MeV 1 MeV 100 MeV 0.01 MeV K=145.6 K=168.9 K=167.0 K=161.0 K=168.2 lower energy neutrons give lower values for the correlation slope

17. Alex Howard PH-SFT LCG-PV 10 th May 2006 Secondary Neutron Energy Spectrum ● Energy of neutron is recorded at first step ● The hard process and evaporation spectrum are clearly visible for the different physics lists ● Cascades are not applied to all particles so some 1 keV LEP component is still present LHEP_HP LHEP_BIC_HP QGSP_HP LHEP_BERT_HP

18. Alex Howard PH-SFT LCG-PV 10 th May 2006 Secondary Neutron Spectrum

19. Alex Howard PH-SFT LCG-PV 10 th May 2006 Secondary Neutron Spectrum ● 1/p spectrum

20. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron parent particles ● Significant fraction are produced from neutrons 1 = gamma 2 = neutrons 3= electron 4 = pi- 5 = pi+ 6 = pi0 7 = positron 8 = proton

21. Alex Howard PH-SFT LCG-PV 10 th May 2006 Neutron parent particles ● Significant fraction are produced from neutrons ● 247000 from neutrons, 21700 from protons for BERTINI ● 173240 from neutrons, 1116 from protons for BINARY 1 = gamma 2 = neutrons 3= electron 4 = pi- 5 = pi+ 6 = pi0 7 = positron 8 = proton

22. Alex Howard PH-SFT LCG-PV 10 th May 2006 Fluence Calculation ● In the TARC analysis they use a definition of fluence as follows: ● For monoenergetic neutrons of velocity V and density n, the neutron flux is defined as  = Vn and is a quantity that upon multiplying by the macroscopic cross-section (  ), one obtains the neutron reaction rate per unit volume ● 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: 1) dN/dS perp is the number of neutrons crossing a surface element dS, with dS perp = dScos  where  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 (1/  dN/dV, where dN is the number of interactions in dV ● The first two were used in simulation

23. Alex Howard PH-SFT LCG-PV 10 th May 2006 Fluence Red = QBBC Blue = BERTINI Green = BINARY ● Spectral fluence is determined from the energy-time correlation with cross-checks (lithium activation and He3 ionisation detectors) ● The simulated fluence is still below measurement ● The bertini cascade gets closest to the data ● The spectral shape looks reasonable

24. Alex Howard PH-SFT LCG-PV 10 th May 2006 ● TARC determined the number of neutrons exiting their set-up as 30% of those produced. This is primarily based on the FLUKA neutron production code with their own transportation ● Geant4 can be used to compare the difference energy spectra and multiplicities of the cascade models ● So far initial numbers are: – BERTINI gives 32.3% exiting (22464 tracks) – BINARY gives 56.7% exiting (27175 tracks) – QBBC gives 32.7% exiting (15810 tracks) Number of neutrons exiting

25. Alex Howard PH-SFT LCG-PV 10 th May 2006 What next? ● Activation calibration for energy-time relationship ● Absolute Neutron fluence comparison ● Activation cross-section vs. energy ● Detector simulation?

26. Alex Howard PH-SFT LCG-PV 10 th May 2006 Conclusions 1)Initial simulation of the TARC set-up agrees with measurements and their own simulation 2)Energy-time correlation differs between physics list, but with the cascades is consistent with measurement although with slightly less slope 3)Secondary neutron spectra show the distinct evaporation and harder production of neutrons 4) BERTINI gives the same number of exiting neutrons as TARC (FLUKA) 5)Absolute fluence and detailed capture cross-section studies will give precise testing of Geant4 neutron physics in the low energy regime (<3.5 GeV) – presently fluence values are below expectations