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None Photons skipping volumes? 

Forum: Processes Involving Optical Photons
Date: 29 Jun, 2016
From: Lew Classen <Lew Classen>

Hi all,

I used Geant to build an "optical module" of a neutrino telescope. It consists of a number of PMTs coupled with silicon gel to a near-spherical glass vessel. In addition to the PMTs the gel volume also includes reflectors and a fully absorbing structure to the hold the PMTs in real life. The PMT is modeled in a simplified way: It is a massive glass solid with a photocathode ellipsoid and an absorbing disc (to prevent cathode hits from the rear) placed inside (no vacuum sub-volumes simulated). Photon counting is done by stopping photons which entered the cathode volume. The module is surrounded by air. The volumes are nested inside each other like:

cathode & disc -> PMT

24 PMTs & 24 reflectors & holder -> gel -> glass -> world

The geometry test did not find any overlaps or protrusions.

Now comes the strange part: When shooting optical photons at the module I found that for certain positions and directions a photon would enter the vessel glass, then enter the gel but instead of entering the PMT glass it would just continue propagating like it was still in the gel volume (see transparent_pmt.png, interaction points marked yellow). Interestingly, this feature seems to be trigged by volumes behind the PMT such as a reflector (see internal_photons1.png) or the absorbing disc inside another PMT (shield_bug1.png - shield_bug3.png)

Any suggestions what causes this behaviour and how to get rid of it?

Cheers, Lew

P.S.: If you wonder why there are lines on the cathode in some pictures: It is a union of a sphere and an ellipsoid. But the effect also appears if just an ellipsoid used.


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