Insufficient compton/photoefect ratio

Hi there. In my simulation I am looking for spectrum of optical photons in 1 inch NaITl crystal. Scoring is done by counting of optical photons arrived in photocatode of PMT. I have tried various physics lists from livermore, penelome, EMStandard opt3 and opt4. But I cannot get to the ratio of compton edge/photopeak heigth as is in measurement or simulation in MCNP. Even when I score pure energy deposit from secondaries, compton edge is not there with sufficient heigth. Backscattering peak should be higher too. Model corresponds to real crystal. Do you have any ideas how to resolve this?

Thanks Matus

Dear Matus, are you modelling the optical photons?

Yes, as I desribed. Do you need more info aboud the simulation? No prebuild physics model seems to work as expected. I suspect some missmatch in nuclear data.

Hello Matus,

Have you tried tuning the RESOLUTIONSCALE in the materials properties table for the NaI? This property varies the width of the yield distribution of the optical photons and should alter the ratio between the Compton edge and the photopeak.

Hello Michael,

yes I have Resolutionscale alterred to with 8% FWHM at Cesium peak. It effects ratio, but have a look what spectrum is in NaITl from pure energy deposit:

if(track->GetNextVolume() && particleDef != opticalphoton && particleDef != gamma){
if(track->GetStep()->GetPostStepPoint()->GetPhysicalVolume()->GetName() == “NaITl” ){
//G4cout<< track->GetStep()->GetTotalEnergyDeposit()<<" “<GetParticleName()<<” "<GetStep()->GetPostStepPoint()->GetProcessDefinedStep()->GetProcessName()<<G4endl;
fscintCountTotal += track->GetStep()->GetTotalEnergyDeposit()/keV;
//G4cout << fscintCountTotal/keV << G4endl;


Then why do you exclude them:

Thanks for response. This condition doesn§t apply to optical photon spectrum. This condition applies only to energy deposition and is there to primarily reduce computation time.


It’s not clear to me what you are doing. If it is the process G4Scintillation generating opticalphotons, as opposed to gammas, then you need to specify the spectrum yourself. What do you specify, and how does it compare to what you observe?

It is stated above. From each run I collect amount of collected photons at detector which creates spectrum (Figure 1 in this thread). However I collect other info as well, one of them is TotalEnergyDeposit (Fig. 2 in this thread). And the problem is that compton edge, nor the backscattering peak is proportional to what i see in measurement or in MCNP.