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Idea Reaction kinematics for neutrons and Lithium6: questions and a hard patch 

Forum: Hadronic Processes
Date: 21 Dec, 2009
From: Zach Hartwig <Zach Hartwig>

Hi Hypernews,

This post is an attempt to tie together the unresolved issues surrounding the incorrect G4NeutronHPInelastic nuclear reaction kinematics for the 6Li(n,t)4He reaction (Q=4.78 MeV). This issue has appeared previously in posts #1032(hadronic), #461(hadronic), #530(physics). As a reminder, G4NeutronHPInelastic is unable to correctly partition the kinetic energy to the triton and alpha in the 2 body final state. While it seems that the momentum is conserved (triton and alpha are emitted with equal and opposite momenta direction), Geant4 attempts to conserve energy by producing non-physical particles, usually gamma rays. Using geant4.9.3 + QGSP_HP_BERT (Fedora12, gcc 4.4.2) to model the 6Li(n,t)4He reaction, G4NeutronHPInelastic produces:

1) triton with 1.56 MeV; should be 2.730 MeV

2) alpha with 1.17 MeV; should be 2.050 MeV

3) 3 non-physical gammas (2 with 1.03 MeV, 1 with 0.00212 MeV )

It should be noted that similar problems exist for the 10B(n,a)7Li reaction, although the kinetic energies of the charged products are off by only ~30 keV in these cases.

Questions: 1) It has been suggested by Dr. Koi that this is due to ENDF.VI classification of these reactions, and Geant4's incorrect ability to simulate them correctly. Is there a patch forthcoming in the near future? 2) Will this issue be resolved when Geant4 moves to ENDF.VII or ENDL based libraries? Is there any progress on either of these fronts?

For those interested, I have developed a "hard" patch in the form of a new G4VDiscreteProcess that can be attached to the triton/alpha/gamma in the user's physics list that resolves the issue. The patch:

1) Detects triton/alpha/gamma from the 6Li(n,t)4He reaction

2) Correctly sets the KE energy of triton/alpha after a forced 0.1 nm step

3) Kills the gamma rays

4) Does not interfere with particle tracking after the first step

I have done some initial work to validate the patch for the case of thin-film based thermal neutron semiconductor detectors. Geant4 simulations have been validated against analytic calculations presented by McGregor et al, NIM A 500 (2003) 272-308. The initial results are excellent, matching calculated detector efficiencies quite well with no adverse effects in Geant4.

I am sure that this is not the best or "correct" way to patch this problem, but it seems to be a good working solution. For those interested, I have attached the header and implementation files, along with initial validation results. This is certainly a work in progress, so please use caution. Any comments on improving the patch are quite welcome.

Thanks everyone!



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