Globaltime distribution of thermal and fast neutron

Particles, Track, Event, Run and Biasing
1

Hello everyone, I am simulating the luminescence process of the scintillator after the Li-6(n,α)T reaction, I set two time constants, as shown in the following code, when I count the Globaltime distribution of all scintillation light reaching the PMT in each Event , there is a big difference between thermal neutrons and fast neutrons. I would like to ask what is the reason for these huge gaps.

      Scint->AddProperty("SCINTILLATIONCOMPONENT1", 
                    scintResponseWavelengths, scintFastResponseSpectrum, NPHOTONENERGIES);
      Scint->AddProperty("SCINTILLATIONCOMPONENT2", 
                   scintResponseWavelengths, scintSlowResponseSpectrum, NPHOTONENERGIES);
          G4double energy[6]={1.*keV,10.*keV,100*keV,1.*MeV,10*MeV,100.*MeV};
          G4double alpha[6]={40.,400.,4000.,40000.,400000.,4000000.};
          G4double triton[6]={40.,400.,4000.,40000.,400000.,4000000.};      
         G4double zero_yield[6]={0,0,0,0,0,0};         
       Scint->AddProperty("ALPHASCINTILLATIONYIELD",energy,alpha,6);
       Scint->AddProperty("TRITONSCINTILLATIONYIELD",energy,triton,6);
       Scint->AddConstProperty("RESOLUTIONSCALE", 1.); 
       Scint->AddConstProperty("SCINTILLATIONRISETIME1",15*ns);
       Scint->AddConstProperty("SCINTILLATIONRISETIME2",15*ns);
       Scint->AddConstProperty("SCINTILLATIONTIMECONSTANT1",180*ns);
       Scint->AddConstProperty("SCINTILLATIONTIMECONSTANT2",1080*ns);
       Scint->AddConstProperty("TRITONSCINTILLATIONYIELD1",0.61);
       Scint->AddConstProperty("TRITONSCINTILLATIONYIELD2",0.39);
       Scint->AddConstProperty("ALPHASCINTILLATIONYIELD1",0.61);
       Scint->AddConstProperty("ALPHASCINTILLATIONYIELD2",0.39);

Here is the physics list in the code

  auto physicsList = new FTFP_BERT_HP;
  physicsList->ReplacePhysics(new G4EmStandardPhysics_option4());
  G4OpticalPhysics* opticalPhysics = new G4OpticalPhysics();
  auto *opticalParams= G4OpticalParameters::Instance();
  opticalParams->SetVerboseLevel(0);
  opticalParams->SetScintTrackInfo(false);
  opticalParams->SetScintTrackSecondariesFirst(true);
  opticalParams->SetScintStackPhotons(true);
  opticalParams->SetScintFiniteRiseTime(true);
  opticalParams->SetScintByParticleType(true);
  physicsList->RegisterPhysics(opticalPhysics);
  runManager->SetUserInitialization(physicsList);

Thermal neutron GlobalTime distribution
Thermal_neutron.pdf (33.6 KB)
Fast neutron GlobalTime distribution
fast_neutron_pulse.pdf (27.6 KB)

2

Hi,
Loosely speaking, thermal neutrons are very slow compared to fast ones. A 0.025eV neutron has a speed of ~2.2E-04 cm/ns. So a 400ns to 1000ns time span translates to ~0.10cm to 0.22cm. A 1MeV neutron will require ~0.14ns to 0.31ns to cover that same distance. So the difference in global time profiles is likely in large part due to the difference in neutron speeds. The fast neutron response follows what I’d expect from the time response as dictated by the scintillation time constants that you provided.The thermal neutron response appears to be a convolution of the neutron time profile and the scintillator time response.

Without details of the detector geometry and materials and the source details (position, angular distribution, energy distribution, etc.), it’s difficult to say more.

1 Like
3

Hello John_McFee，
Thank you very much for your reply, I understand what you mean, I have a big wrong understanding of the GetGlobaltime function, by the GetGlobaltime function I can’t get the result I want.
I want to simulate the light pulse of the scintillator through Geant4, mainly the decay curve. I also tried to get the result through the GetLocalTime function, but I failed, I want to ask if there is a good way to simulate the light pulse of the scintillator.

4

Hi.
I’m not sure that I understand what you want. The globalTime distribution IS the distribution of scintillator light pulses (assuming that “Counts” refers to number of scintillation photons). It includes all the physics of the neutron transport, subsequent charged particle recoil, ionization, etc. If you want the time distribution for a monoenergetic neutron incident at a single location, modify your neutron source accordingly. If you want the light profile due to the moving charged particle that the neutron produces, put that charged particle in your source and put the source inside the scintillator.

Perhaps you could clarify what you wish to do.