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12.1} Cherenkov transport and quantum efficiency


 In order to use quantum efficiency (using the QUEFFC routine)
 the user must input a sensitivity < -100 using the OPT-PROP  option with
= SENSITIV). That option sets the quantum efficiency as a function of photon energy OVERALL through the problem and it is not material/region dependent. The reason is that it is applied "a priori" at photon generation time (for obvious time saving reasons). Here below is an explanation taken directly from the code. * current particle is at a given position, in a given material * Cherenkov (or scintillation) photons are going to be produced * the photon generation probability is immediately reduced over the whole energy spectrum according to the maximum quantum efficiency of the problem. The latter, set as
in card OPT-PROD, IS MEANINGFUL EVEN IF ROUTINE QUEFFC IS USED, see below * the detailed efficiency, set again by the OPT-PROD card with
= SENSITIV or by routine QUEFFC, is used for a further reduction when the actual energy of each individual photon is selected (rejecting it against Q.E.(omega)/Q.E._max, where omega = photon angular frequency) Summarising, the yes/no detection check is done AT PRODUCTION and NOT AT DETECTION: this in order to substantially cut down CPU time. If one wants all photons to be produced the sensitivity must be set = 1. Then it is still possibile to apply a quantum efficiency curve AT DETECTION, by means of the user weighting routine FLUSCW (see 13}) or by a user-written off-line code. Since the quantum efficiency curve provided by OPT-PROD with
= SENSITIV is applied at production and not at detection, it is not known which material the photon will eventually end up in. Furthermore,
must be set anyway equal to the maximum quantum efficiency over the photon energy range under consideration. One cannot use the QUEFFC routine as a way to provide an initial screening on the produced photons, i.e. to use a "safe" initial guess for the quantum efficiency (say, for instance 20%) and then, at detection, to refine it through more sophisticated curves, i.e. rejecting against the actual quantum efficiency/0.2 (this again can be done in routine FLUSCW). This makes sense of course if the user has different quantum efficiency curves for different detectors (one should use in QUEFFC the curve that maximises all of them and then refine it by rejection case by case), or if the quantum efficiency is position/angle dependent upon arrival on the photomultiplier (again one should use inside QUEFFC the quantum efficiency for the most efficient position/angle and refine by rejection at detection time. Optical photons are absorbed in those material where the user selected properties dictate absorption, i.e. metals or materials with a non zero absorption cross section. These absorption events can be detected in different ways. For instance: * through energy deposition by particle -1 (optical photons have always id = -1), photons usually deposit all energy in one step (since only absorption and coherent scattering are implemented). So one can check for JTRACK = -1 and energy deposition (RULL) in a given region (e.g., the photocathode of the PMT). One can also apply an extra quantum efficiency selection, e.g. using the COMSCW user routine. * through boundary crossing of particles -1 into the given region, however this is correct if and only if absorption is set such that the photon will not survive crossing the region. Again further selections can be performed, e.g., using the FLUSCW user routine.

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