Absorption and Reemission

Normally a medium can absorb or rayleigh scatter a photon. If a wavelength shifter is present, a photon could also be wavelength-shifted. For scintillators there is the additional possibility that a photon can be absorbed and reemitted (at a different wavelength and a later time). The fate of each photon is handled by PHINTL_PHOTON. In addition to calculating the probabilities for wavelength shifting, rayleigh scattering and absorption, for a scintillating medium PHINTL_PHOTON will also use the lookup tables generated by INPHI to determine the probability that absorption by any scintillator component occurs (using a combined attenuation length). It will then throw a random number to decide which process occurred for the photon. If scintillation is selected the interaction type is set to IDI_SCINT.

Note, the fact that absorption probabilities are calculated twice is a little confusing. For the bulk, absorption coefficients are specified for a given medium through the MEDA bank. However, these are effectively replaced by the absorption coefficients for each scintillator component so when implementing scintillation for a given medium, it is recommended to set these absorption lengths to be very long, such that this contribution is negligible.

When tracking photons, if VXINT finds a IDI_SCINT vertex, a call is made to VXSCINT, the routine which handles absorption and reemission by a scintillator. It takes an incoming vertex, decides which scintillator component did the absorbing, and then decides if the photon is reemitted. To do this it looks at how far up the emission spectrum the absorbed photon was, and then emits with probability proportional to the fraction of the emission spectrum remaining (The emission probability is also modified by the overall probability factor specified in SCCO). If reemission occurs, the time of emission is randomly selected from the reemission timing distribution.