12.1} Cherenkov transport and quantum efficiency

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 In order to use quantum efficiency (using the QUEFFC routine)
 the user must input a sensitivity < -100 using the OPT-PROP  option with
 
SDUM
 = 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 
WHAT(5)
 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
    
SDUM
 = 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
 
SDUM
 = SENSITIV is applied at production and not at detection, it is not known
 which material the photon will eventually end up in.

 Furthermore, 
WHAT(5)
 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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