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Question Cerenkov photons "stuck" in parameterised geometry using v9.6p02 

Forum: Processes Involving Optical Photons
Date: 22 Jul, 2013
From: Sam Blake <Sam Blake>

Hi everyone,

I have encountered a problem where optical photons frequently get stuck within my detector geometry, such that the stepping manager seems to get stuck in an infinite loop, trying to move the photon but not actually being able to. When I dump step information to the terminal, I can see that the photon's coordinates are not changing at all.

Interestingly, this only appears to be happening for optical photons created by Cerenkov events. By de-activating the Cerenkov process and only using scintillation to generate optical photons, this error disappears. I have recently upgraded from version 9.4p01 and am now (finally) using version 9.6p02.

I have registered Cerenkov, scintillation and optical processes in my PhysicsList class in the following way:

while( (*theParticleIterator)() )
{
    G4ParticleDefinition* particle = theParticleIterator->value();
    G4ProcessManager* pmanager = particle->GetProcessManager();
    G4String particleName = particle->GetParticleName();

    if (theCerenkovProcess->IsApplicable(*particle)) 
    {
      pmanager->AddProcess(theCerenkovProcess);
      pmanager->SetProcessOrdering(theCerenkovProcess,idxPostStep);
    }

    if (theScintillationProcess->IsApplicable(*particle)) 
    {	//scintillation is applicable for all except optical & short lived.
      pmanager->AddProcess(theScintillationProcess);
      pmanager->SetProcessOrderingToLast(theScintillationProcess, idxAtRest);
      pmanager->SetProcessOrderingToLast(theScintillationProcess, idxPostStep);
    }

    if (particleName == "opticalphoton") 
    {
      pmanager->AddDiscreteProcess(theAbsorptionProcess);
      pmanager->AddDiscreteProcess(theRayleighScatteringProcess);
      pmanager->AddDiscreteProcess(theBoundaryProcess);
    }
 }

My detector geometry consists of a square slab of scintillator with an incident pencil beam of x-rays. The x-rays lead to Cerenkov and scintillation events in the scintillator material, and I'm then interested in tracking the optical photons.

My scintillator geometry was previously a simple G4box slab, and this problem never occurred. However, I have now changed the scintillator to a pixelated design by creating a parameterised volume of G4boxes. I am therefore suspicious that part of the problem may be related to this parameterised geometry, although again - only the Cerenkov-generated photons appear to be affected.

I was wondering if anyone else may have encountered a similar problem, or may have any suggestions on what may be causing this issue.

Thank you in advance for your help. Sam

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1 Feedback: Re: Cerenkov photons   (Gumplinger Peter - 23 Jul, 2013)
(_ None: Re: Cerenkov photons   (Sam Blake - 24 Jul, 2013)
(_ Idea: Re: Cerenkov photons   (Gumplinger Peter - 24 Jul, 2013)
(_ None: Re: Cerenkov photons   (Sam Blake - 25 Jul, 2013)
(_ Ok: Re: Cerenkov photons   (Sam Blake - 05 Aug, 2013)
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