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Question Discrepancy in the number of optical photons produced by low energy charged particles in scintillator 

Forum: Processes Involving Optical Photons
Date: 07 Apr, 2017
From: John McFee <John McFee>

Hi,

I am simulating the response of an NE213 scintillator to various radiations and charged particles using Geant4 (10.3). The code is modified from OpNovice with modifications to allow particle-dependent scintillation yields. It seems to work well except for one thing which is puzzling me.

I have a 2.54 x 2.54 cm cylindrical scintillator with the ion source gun at its center emitting protons. At low proton energy (few hundred keV), I find that the ratio of the number of scintillation optical photons produced to the number expected from the corresponding yield vector entry for that energy is much less than 1. As the energy increases, the ratio climbs asymptotically to 1. The same low energy effect is seen for alphas, carbon nuclei and electrons. (I obtain the number of optical photons from OpNoviceStackingAction::NewStage() in OpNoviceStackingAction.cc.)

I have attached a plot for protons to show the effect. (To interpret the plot, for example, the first point is at E_proton = 130keV. The yield of optical photons for protons at that energy (see code below) is 114, so I would expect the simulation to produce roughly that many optical photons. It actually produces ~0.2*114 photons.

Relevant code:

In OpNovicePhysicsList.cc:

  // scintillation process for proton:
  fScintillationProcessProton = new G4Scintillation("Scintillation");
  fScintillationProcessProton->SetScintillationByParticleType(true);
  fScintillationProcessProton->SetTrackSecondariesFirst(true);
  fScintillationProcessProton->SetScintillationYieldFactor(1.0);
  fScintillationProcessProton->SetScintillationExcitationRatio(0.80);
  ...
  if (fScintillationProcessDef->IsApplicable(*particle))
      ...
      else if(particle->GetParticleName() == "proton")
      {
         pmanager->AddProcess(fScintillationProcessProton);
         pmanager->SetProcessOrderingToLast(fScintillationProcessProton,idxAtRest);
         pmanager->SetProcessOrderingToLast(fScintillationProcessProton,idxPostStep);
      }
      ...

and similar code for electrons, alphas, carbons.

In OpNoviceDetectorConstruction.cc I have these lines for protons:

     G4double energies_proton[NE213_NUMENTRIES_YIELD] = {0.10*MeV, 0.13*MeV, 0.17*MeV, 0.20*MeV, 0.24*MeV, 0.30*MeV, 0.34*MeV, 0.40*MeV, 0.48*MeV, 0.60*MeV, 0.72*MeV, 0.84*MeV, 1.00*MeV, 1.30*MeV, 1.70*MeV, 2.00*MeV, 2.40*MeV, 3.00*MeV, 3.40*MeV, 4.00*MeV, 4.80*MeV, 6.00*MeV, 7.20*MeV, 8.40*MeV, 10.00*MeV, 13.00*MeV, 17.00*MeV, 20.00*MeV, 24.00*MeV, 30.00*MeV, 34.00*MeV, 40.00*MeV};

     G4double yield_proton[NE213_NUMENTRIES_YIELD] = {87., 114., 156., 189., 237., 317., 374., 471., 623., 875., 1174., 1516., 2015., 3077., 4721., 6095., 8063., 11171., 13442., 17118., 22162., 29799., 38055., 46698., 58695., 82044., 113907., 139320., 174150., 228330., 264450., 319920.};

     NE213_mt->AddProperty("PROTONSCINTILLATIONYIELD", energies_proton, yield_proton, NE213_NUMENTRIES_YIELD); 

and similar lines for electrons, alphas and carbon ions.

I have set Birks constant by :
     NE213->GetIonisation()->SetBirksConstant(0.137*mm/MeV);

(but I've also used a zero Birks constant and it made no difference).

Am I forgetting some obvious effect which removes or does not count optical photons at low ionizing particle energy?

Thanks for any help.

John McFee

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/04/07/16.28-42773-o_Theory_vs_E_Protons.pdf

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1 Feedback: Re: Discrepancy in the number of optical photons produced by low energy charged particles in scintillator   (Gumplinger Peter - 13 Apr, 2017)
(_ More: Re: Discrepancy in the number of optical photons produced by low energy charged particles in scintillator   (John McFee - 14 Apr, 2017)
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