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_Geant4 Version:_11.2.2
_Operating System:_Alma Linux 9.4
_Compiler/Version:_gcc 11.4.1
_CMake Version:_3.26.5
Dear all,
I have been conducting simulations of low energy (1 keV) neutron events in gadolinium-doped liquid scintillator. However, I observed significant descripancies in the nCapture cross section between QGSP_BERT_HP and other models, such as FTFP_BERT_HP.
Following figure shows the proportions of nCapture targets using the two different models. In QGSP_BERT_HP, most neutrons are captured by H, instead of Gd, which is unexpected. While the FTFP_BERT_HP gives more reasonable results.
I also extracted the nCapture cross section on Gd and H using Hadr00 example, they are shown below. The two different models give quite different nCapture cross sections at low energy region.
Thank you for reporting this issue. After a quick inspection of both physics lists, the neutron processes at low energy seem to be different in FTFP_BERT_HP compared to QGSP_BERT_HP [1]. From the last plot you attached, it seems FTFP_BERT_HP gives sensible values, do they match with evaluated data?
It seems in the last year some changes happened to low energy neutron processes activated by QGSP_BERT_HP physics lists, maybe @ribon@civanch can comment further.
Thank you for your reply! Please find below the nCapture cross sections on H-1 and Gd-157 obtained from Geant4 built-in database ($G4PARTICLEXSDATA/neutron/cap1_1) and JEFF-3.3 database, they are almost the same (not sure if they are from the same source).
Compared with the above plots, it seems the cross section obtained from FTFP_BERT_HP model is correct, while the QGSP_BERT_HP model is problematic.
The difference may be caused by the recent updates in the QGSP_BERT_HP model as you have mentioned. It will be great if other experts can give further comments on it. Thank you!
yes, FTFP_BERT_HP and QGSP_BERT_HP physics lists, in Geant4 11.2 (and patches) do not have the same “HP” (i.e. treatment of neutrons with kinetic energy below 20 MeV). This unexpected behaviour is due to the fact that in 11.2 we have included a first round of revisions of the HP package: all the “technical” changes - that are not supposed to affect any physics observable - are common to all the HP-based physics lists; some changes that might affect physics observables have been included only on the QGSP_BERT_HP physics list. The choice of this physics list was motivated by the fact that those changes in HP are preliminary and still need extensive validation, therefore we decided to include them only in one HP-based physics list, the one that is less used by users (to the best of our knowledge). Given the preliminary nature of these changes, we did not put much emphasis in the release notes of G4 11.2, although there is the following statement:
“G4NeutronHPCaptureXS, G4NeutronHPElasticXS, G4NeutronHPInelasticXS, G4NeutronHPFissionXS, G4CrossSectionHP, G4NeutronFissionVI, G4NeutronRadCaptureHP: updated models and cross-sections which are included only in the physics list QGSP_BERT_HP”.
The result reported result seems to show that there is some bug or issue in the HP version included in QGSP_BERT_HP.
We will investigate it - but in the meantime, please use another HP-based physics list (e.g. FTFP_BERT_HP) for your simulations.
Thanks a lot for the feedback, and apologies for the unexpected feature of QGSP_BERT_HP, certainly not documented very clearly in our release notes!
