Thermal neutron capture time and spectra disagreement with NNDC data for 12C

Matthieu_Lecocq · February 3, 2026, 2:27pm

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Geant4 Version:11.4.0
Operating System: MacOS
Compiler/Version: Apple clang version 17.0.0
CMake Version: cmake version 3.30.5

Dear all,
I apologise if I am duplicating subjects but I haven’t found an answer to the behaviour I am seeing.
I am running a simulation where neutron are generated in a liquid scintillator (LAB) and I am looking at the energy spectrum of the emitted capture gammas as well as the neutron capture time.
For H, the energy of the gamma looks correct and in good agreement with NNDC data base. However, for capture on 12C the energy spectrum has peaks that should have very low intensity with respect to NNDC data base. Moreover, the capture time is ~400us where it should be around 200us for neutron capture in liquid scintillator.
I am using the QGSP_BERT_HP and HadronElasticPhysicsHP physics lists to describe the neutron.

Is this known behaviour and is there a way to correct it?

atolosad · February 4, 2026, 11:49am

Hi @Matthieu_Lecocq

Thank you for your detailed post. From the release notes of version 11.3 [1], the physics list QGSP_BERT_HP may not be suitable for physics studies.

Could you please try with other physics list like QGSP_BIC_HP? The NuDex model [2] can be enabled [3] with this physics list since version 11.4 [4]. Could please try it out?

thank you for your time.

Best,

Alvaro

[1] geant4/ReleaseNotes/ReleaseNotes.11.3.html at b4a16de652ec244f7a0ecc0a5cacfee21930bf75 · Geant4/geant4 · GitHub
[2] NuDEX — Physics Reference Manual 11.4 documentation
[3] geant4/examples/extended/hadronic/Hadr01/Hadr01.cc at b4a16de652ec244f7a0ecc0a5cacfee21930bf75 · Geant4/geant4 · GitHub

[4] geant4/ReleaseNotes/ReleaseNotes.11.4.html at b4a16de652ec244f7a0ecc0a5cacfee21930bf75 · Geant4/geant4 · GitHub

Matthieu_Lecocq · February 4, 2026, 2:31pm

Hi @atolosad

Thank you for your reply! I tried using QGSP_BIC_HP (without NuDEX), and the neutron time is now correct but the energy spectrum for 12C is not.
I am now enabling NuDEX, but an error is raised because it cannot find the correct file in LevelDensities. What version of G4NUDEXLIB is required for Geant4 v11.4.0? I am using v.1.0 that I downloaded here: Download Geant4-11.4.0 - Geant4

Matthieu

atolosad · February 4, 2026, 4:25pm

Hi @Matthieu_Lecocq

It might be a missing environmental variable. I installed the library using the CMake option, and I can see it in the list of datasets:

geant4-config --datasets
G4NDL G4NEUTRONHPDATA /usr/local/g4data/G4NDL4.7.1
G4EMLOW G4LEDATA /usr/local/g4data/G4EMLOW8.8
PhotonEvaporation G4LEVELGAMMADATA /usr/local/g4data/PhotonEvaporation6.1.2
RadioactiveDecay G4RADIOACTIVEDATA /usr/local/g4data/RadioactiveDecay6.1.2
G4PARTICLEXS G4PARTICLEXSDATA /usr/local/g4data/G4PARTICLEXS4.2
G4PII G4PIIDATA /usr/local/g4data/G4PII1.3
RealSurface G4REALSURFACEDATA /usr/local/g4data/RealSurface2.2
G4SAIDDATA G4SAIDXSDATA /usr/local/g4data/G4SAIDDATA2.0
G4ABLA G4ABLADATA /usr/local/g4data/G4ABLA3.3
G4INCL G4INCLDATA /usr/local/g4data/G4INCL1.3
G4ENSDFSTATE G4ENSDFSTATEDATA /usr/local/g4data/G4ENSDFSTATE3.0
G4CHANNELING G4CHANNELINGDATA /usr/local/g4data/G4CHANNELING2.0
G4NUDEXLIB G4NUDEXLIBDATA /usr/local/g4data/G4NUDEXLIB1.0

Could you please try to define the environmental variable that points to the data library, or simply enabling Nudex option in CMake?

Thank you for your time.

Best,

Alvaro

Matthieu_Lecocq · February 4, 2026, 5:26pm

Hi @atolosad

The library is correctly listed in the datasets. However, I get this error:

If you have any idea on how to solve it, I’d greatly appreciate it.

Thank you for your help!

Matthieu

Matthieu_Lecocq · February 13, 2026, 2:22pm

Hello @atolosad,

I have made several tests to correct the wrong gamma rays from the neutron capture on C and found a new interesting behaviour. Using G4HadronPhysicsQGSP_BIC_HP, I changed the NeutronRadCaptureHP builder to NeutronHPCapture, and I get correct emitted gamma rays for neutron capture on C12:

However, if I am looking at the energy spectrum of the emitted gamma from the neutron capture on H with the same setup, I get a continuous spectrum instead of a ray at 2.223 MeV: