Cerenkov Photon Detection

Interaction Codes
Code	Description
400000	Photon lost (left tracking region)
401000	Photon discarded (CERFAC cut)
402000	Photon reached and fired PMT
404000	Photon absorbed (any media)
405000	Photon dropped (geometry traching error)

Whenever a Cerenkov photon reaches the PMT boundary the vertex processor VXPMT is called to determine if a PMT hit has occurred. How the photon is handled depends on the PMT model:-

Black Disk

gives 100% probability of generating a hit.

Grey Disk

uses information from the titles bank PMT that contains the angular response of a PMT/Concentrator vs. the angle of incidence and wavelength of the photon and is used to decide if an electronic PMT hit was generated. However the probabilities for reflection and for scoring a hit are renormalised to incorporate CERFAC and the Cerenkov photon emission probability reduced to save computation time, see discussion of CERFAC in 13.4.5). For photons that do not generate a hit, the possibility of a reflection is considered (currently rather crudely) and VXPMT_BOUNCE called if they do.

Transparent Disk

passes the photon to the PMT simulation code to track within the concentrator and PMT. This is discussed further in 12.7.3

If a hit was generated the time at which the photon hit the photocathode is determined. In order to properly generate the proper PMT charge and time it is necessary to take into account multiple hits on a single tube, so their generation is delayed until the Monte Carlo data acquisition routines.