Neutron capture time constant in water is smaller than the experimental value

Physics Processes, Models and Cross Sections
1

Hello!
Recently, I have been simulating the process of neutron moderation and capture in water. I have set a sufficiently large water volume (a cylindrical tank with a radius of 5 m and a height of 10 m). However, I found that the fitted neutron capture time constant is about 90 μs, which is much smaller than the experimental value of 200 μs.

In my physics list, I have registered #include <G4HadronPhysicsQGSP_BERT_HP.hh>. In the output log, it only shows that the data file for hydrogen inelastic scattering is missing. As far as I know, this should not significantly affect neutron moderation.
The incident neutron energy is around 5 MeV.
And I want to know the output show that some file from database are missing,does this matter?
I would be very grateful if you could help me understand this issue.

/cvmfs/geant4.cern.ch/share/data/G4NDL4.6
@@@ G4ParticleHPInelastic instantiated for particle neutron data directory variable is G4NEUTRONHPDATA pointing to /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Inelastic
@@@ G4ParticleHPInelasticData instantiated for particle neutron data directory variable is G4NEUTRONHPDATA pointing to /cvmfs/geant4.cern.ch/share/data/G4NDL4.6
NeutronHP: /Inelastic file for Z = 1, A = 1 is not found and CrossSection set to 0.
NeutronHP: /Capture file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Capture/CrossSection/6_nat_Carbon
NeutronHP: /Elastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Elastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Inelastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 1, A = 1 is not found and CrossSection set to 0.
NeutronHP: /Capture file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Capture/CrossSection/6_nat_Carbon
NeutronHP: /Elastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Elastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Inelastic/CrossSection/6_nat_Carbon
NeutronHP: /Capture file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Capture/CrossSection/6_nat_Carbon
NeutronHP: /Elastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Elastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Inelastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 1, A = 1 is not found and CrossSection set to 0.
NeutronHP: /Capture file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Capture/CrossSection/6_nat_Carbon
NeutronHP: /Elastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Elastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 6, A = 12 is not found and NeutronHP will use /cvmfs/geant4.cern.ch/share/data/G4NDL4.6/Inelastic/CrossSection/6_nat_Carbon
NeutronHP: /Inelastic file for Z = 1, A = 1 is not found and CrossSection set to 0.

Geant4 Version:10.6.2
Operating System:
Compiler/Version:
CMake Version:

2

The warnings that are shown should not significantly affect the problem you are studying (neutron moderation and capture in water for ~5MeV incident neutrons). The warnings for Z=1 A=1 just say that the cross section for neutron inelastic scattering on 1H is being set to 0. That is what it actually is anyway (no bound excited states), so no problem. The warnings for Z=6 A=12 are because data is only available for natural carbon in the data base. The abundance of 12C accounts for ~99% of natural carbon. Also, neutron elastic cross sections greatly exceed capture or inelastic cross sections for few MeV neutrons, so the effect of any error due to capture or inelastic scattering should be small. (Capture can be significant at very low energies, e.g. ~eV, but again most of natural carbon is 12C so the error should be small.)

On another note, you are running a rather old verson of Geant4 (10.6.2). The latest relase version is 11.3.2. If you can, you might want to consider upgrading.

3

Here a macro for example Hadr04, its printout with Geant4 11.3 and the plot of time of flight.

On the printout you can read that the mean value of time of flight is ~197 us

deng.mac.txt (389 Bytes)

deng.out.txt (3.9 KB)

deng
deng698×475 23.9 KB

4

Thank you so much!I will consider updating my version.

5

Thank you so much!
I tried replacing the hadronPhysics of my simulation and get the correct time distribution.
But I placed stainless steel tubes in the water, which means neutrons can be captured by Fe56. When I plotted the time distribution of neutrons captured by 56^{56}56Fe, the capture time constant turned out to be around 140 microseconds, which is larger than the experimental value (about 65 microseconds).