Discrepancy in photonNuclear yields between Geant4 11.2 and 11.3

Physics Processes, Models and Cross Sections
1

Geant4 Version: 11.2 (LCG 105) -11.3 (LCG 107)
Operating System: alma9 (lxplus)
Compiler/Version: gcc14
CMake Version: 3.26.5

Dear Geant4 developers,

I am studying photon-nuclear and electron/positron-nuclear processes in electromagnetic showers. My final goal is to investigate the production of leading neutral hadrons (neutrons, K0L, K0S) with energies above ~10 GeV leaking out of a target calorimeter.

When comparing Geant4 11.2 and 11.3, I observe a two-order-of-magnitude increase in the yield of events containing photonuclear processes in version 11.3. The results obtained with version 11.3 are significantly more consistent with our experimental data.

My setup (beam-dump configuration) is the following:

  • Beam: 100 GeV electrons

  • Target: Pb–scintillator sandwich calorimeter (~40 X₀)

  • Physics list: FTFP_BERT

My questions are:

  1. Are there changes in photon-nuclear or electron-nuclear processes between versions 11.2 and 11.3 that could explain the significant difference I observe?
  2. More broadly, which of the changes introduced in 11.3 could significantly affect EM-induced inelastic nuclear processes and the resulting hadronic activity?

Thank you very much in advance.

Kind regards,

Martina

1 Like
2

Hello,

I am not aware of any change happening in Geant4 version 11.3 which could change significantly the production of leading neutral hadrons with energies above ~10 GeV from gamma-nuclear or electron-nuclear reactions, with respect to Geant4 version 11.2.

The reason is that the main changes in hadronic physics happening between versions of 11.3 and 11.2 are the following:

  • Changes in Bertini (BERT) model, which affects the production of particles below 10 GeV.
  • Changes in nuclear datasets, which affect the production of particles below ~GeV.
  • Changes in Precompound/de-excitation, which affect the production of particles below ~GeV.

As far as I know, there are no relevant changes occurred for the hadronic string models (FTF, QGS) or for the gamma-nuclear and electron-nuclear cross sections - which could affect the production of particles above 10 GeV from gamma-nuclear and electron-nuclear reactions.

Said that, I cannot exclude that in gamma-nuclear and electron-nuclear reactions, there is a large increase of particle production, but only for energies below 10 GeV. But this is not something that I was expecting!

In the case you are interested/curious to find out what is causing the large increase of produced particles in Geant4 version 11.3 and 11.2, one thing that can be quickly check is the effect of the BERT model.
In fact, it is possible to roll back the developments of this model made in 11.3, and bring it as it was in 11.2, while using Geant4 11.3 for all the rest. This can be done by adding the following C++ line:

G4HadronicParameters::Instance()->SetBertiniAs11_2( true );

in the main program, before instantiating the physics list.

Best regards,

Alberto