NeutronHP in QGSP_BERT_HP across Geant4 versions

Physics Processes, Models and Cross Sections
1

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Geant4 Version: 11.4.1
Operating System: macOS Tahoe 26.3.1
Compiler/Version: Apple clang 21.0.0
CMake Version: 4.3.3

Hi all,

I am comparing simulations performed with Geant4 v10.4.0, v11.2.2, and v11.4.1. In my study, I inject a 1 TeV π⁺ into a 100m ice cube and record the number of neutron captures over time.

I observed the following behaviours for the physics lists:

  • QGSP_BERT_HP:

    • The number of neutron captures is 10 times smaller in v11.2.2 than in other versions. More than 80% of neutron tracks terminate by “Transportation” at the detector boundary, which doesn’t occur for other physics lists and versions.

    • The number of captures increases in v11.4.2, but the capture lifetime (469 μs) is significantly longer than the expected 220 μs.

    • The neutron capture cross section differs between versions.

  • QGSP_BIC_HP:

    • The number of neutron captures decreases by about 25% across these versions.

    • The lifetimes are consistent with 220 us.

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I read this forum discussion from two years ago, and I was wondering whether there are further developments to the NeutronHP package that may explain these behaviours (the large reduction in captures in v11.2.2, the longer capture lifetime in v11.4.1, and a slight decrease in capture using QGSP_BIC_HP).

Many thanks,

Heiley

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I would suggest to forget about QGSP_BERT_HP : since 11.2, some developments were attempted, with little validation, and it is likely that there are bugs.

Notice that before Geant4 version 11.2, all _HP variants of the physics list were equivalent as far as the treatment of neutrons below 20 MeV is concerned.

So, we don’t recommend to use QGSP_BERT_HP starting with version 11.2.

Instead, QGSP_BIC_HP should be the best physics list that Geant4 can offer for those interested in neutronics.
The changes you can observe on this physics list as a function of the Geant4 versions, is part of the “normal” evolution of the code, with the hope that more recent versions should be better than older ones…

Eventually, to cross check the results with another set of models, you might try out the physics list FTFP_BERT_HP.

The simulation of neutrons below 20 MeV is the same between QGSP_BIC_HP and FTFP_BERT_HP, but the string model (FTF and QGS) and the intra-nuclear cascade model (BERT and BIC) are different, and these make significant differences on the particles produced by e.g. 1 TeV pi+ on 100 m of ice…