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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?
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 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
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:
Comparing NNDC and G4 I notice the following regarding gamma transition (in keV):
4095 (into G.S emitting 4095 full) <-- seems OK
1856 (into 3089 which decay emitting 3089) <-- both too intense in G4
1261 (into 3684 which may decay emitting 595+3089 or 3684 full) <-- not enough of both
Looking at the corresponding C13 entry from the Geant4 datafile. The 12C(n,g) level `(4946.33 5)` (parenthesis from ENSDF) is missing. So the entire population and emission is handled somewhere else. I have to see in more detail
One solution is downgrade. I see issues with neutron capture gammas in both 11.3 and 11.4 (even though the bug report is marked as fixed in 11.4). Things look correct in 10.7. The bug report suggests that anything <=11.2 would work.
I suppose you could also try just replacing the data files from an older version but I have no idea if that’s likely to work.
Thank you for your suggestion!
For now, I bypassed the issue by “hardcoding” the value of the hydrogen capture gamma ray in the code. I hope a quick fix can be done soon…
Hi @Matthieu_Lecocq , how do you fixed the n-H gamma emission? I’m having the same issue using 11.4 and nudex, however it seems also related to the physics list QGSP_BIC_HP. Using another physics list, for example Shielding_HPT produce the “expected” output for neutron captures on hydrogen. The problem is that nudex is not working for this physics list.
Unfortunately, I never managed to make it work using Nudex. Actually, I have some issues running my simulations with Nudex and ended up using NeutronHPCapture and reading a .txt file for the n-H gamma energy.
Looking at the code, the work around I found is editing the: G4NUDEXLIB1.0/KnownLevels/levels-param.data
Specifically the line for 2H and just set a value higher that 3.22 for Umax. This removes the warning: “No level density and level scheme not complete … Ecrit = 0 MeV … MaxExcEnergy = 3.22457 MeV“ and just produce a single gamma with the correct energy. Now there is new warning: “############## NuDEX: WARNING, there are no thermal capture for ZA=1001 , with Sn = 2.22457 ##############“ but it seems to work.
