Number of optical photons

Hi fluka experts,
I actually want to calculate the number of optical photons reaching pmt when the primary is photon with the eventbin card, but when the pmt thickness is very small (that is, in reality and experience), the number of optical photons is very, very small. It does not seem right and logical. Can you guide me where the problem comes from? Why does the change in pmt thickness affect the number of these photons so much?

Hi Hassan,

I’m new to FLUKA though, PMTs I’ve working a bit, so just curious about what you mean about thickness of PMT. When I do photon collection efficiency simulations, I consider the photons that collide with the face of scintillator that interfaces the PMT, to be collected (with some predefined probability, taking into account size of the photocathode and other losses though), when they hit the surface of the scintillator interfacing the PMT.

So, the problem usually boils down for me to basically calculate the probability of a photon generated anywhere in the bulk of the scintillator to hit the face interfacing the PMT.

So, for me it is the thickness of the scintillator that should matter. Pls elaborate.

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I mean the thickness of pmt is the length of a cylinder defined as pmt and placed in the mouth of another cylinder that is scintillator. Normally, the length of this pmt cylinder should not have much effect on the number of photons that reach its level, but unfortunately this happens when I use the eventbin card, can you Send me an example in this case? My goal is to calculate the number of opticalphotons that reach pmt level in the event.

How can I connect with you?
My telegram number is 09035622308 if you can

Well I have no experience in simulating PMT using FLUKA, so we may have to wait for an expert in these regards. But, I have a gut-feeling you may not actually need to simulate the photon interactions in PMT. One reason is the dynamics of secondary electrons in PMT is pretty well-understood so as long as you are using a standard PMT module with calibrated data, there should not be any reason for simulating the PMT at all. Probably all you might be interested in is simulating the fraction of total photons generated in the scintillator that the PMT receives. For me this figure is the fraction of total photons generated that hit the face of the scintillator to which the PMT is coupled.

For some standard shapes of scintillators, the fraction of photons hitting the PMT surface can be calculated using exact analytical expressions. I am doing the same for a cubic scintillator right now. Also, here what matters is the depth in scintillator at which the radiation is depositing most of its energy. Ex: when you are dealing with low energy electrons (~ 20-50keV) they would deposit all of their energy at a depth of a few hundred nanometers in an inorganic scintillator. So, for this case the photon collection efficiency would essentially reduce down to determining the fraction of photons that are emitted from one surface and end up in another. A crude way to compute that is by using the concept of radiation view-factor for radiating surfaces. A more generic approach would be to assume that photons are generated anywhere in the volume of the scintillator. For either of the approaches, calculation of closed form mathematical expression is possible.

Dear Hassan,

You can obtain the fluence of optical photons as a function of position in your PMT using a USRBIN score in the volume of your PMT. If you only care about the number of photons and not the spatial distribution you can use a USRBDX estimator between the scintillator and PMT regions. In any case I don’t think you require the details of each event by using the EVENTBIN card for this kind of problem.