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Forum: Processes Involving Optical Photons
Re: Question more photons counted than generated (Athreya Shankar)
Date: 27 Jun, 2011
From: Gumplinger Peter <Gumplinger Peter>

Hi Athreya,

You need to change your code to read:

if(aStep->GetPreStepPoint()->GetPhysicalVolume()->GetName()=="DummyDet")

A step is not actually "in" a volume until the PreStepPoint shows it to be in the volume. With:

if(aStep->GetPreStepPoint()->GetPhysicalVolume()->GetName()=="A"&& aStep->GetPostStepPoint()->GetPhysicalVolume()->GetName()=="DummyDet")

you get every photon 'arriving' at the volume boundary. Now, for most particles this also means that they will most likely then enter "DummyDet" but NOT so for optical photons. The reason is optical photon can be reflected at the boundary. This reflection is actually a little complicated for the G4Navigator because it involves a sudden complete change of direction with the direction 'suddenly' pointing in the opposite direction relative to the surface normal. The G4Navigator handles this by doing a very small 'reallocation step'. The step size for this extra step is small.

You should be conservative and besides changing the if-statement as above, you should also request that the step has:

aTrack.GetStepLength()>kCarTolerance/2

Please, see also:

http://geant4.web.cern.ch/geant4/support/faq.shtml#TRACK-1

Besides photons being refracted out of the scintillator you were also counting photons that reflected on your scintillator/vacuum interface.

Peter

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1 None: Re: more photons counted than generated   (Athreya Shankar - 27 Jun, 2011)
(_ Agree: Re: more photons counted than generated   (Gumplinger Peter - 28 Jun, 2011)
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