The mean number of generated photo-electrons depends on the index of refraction of the radiator gas, its transmissivity , especially at short wavelengths, quantum efficiency of PMT, and mirror reflectivity. The intensity and spectrum of photons is given by the Frank-Tamm relation :
where Z is the particle charge, 
is the index of refraction,
L is particle trajectory length and
. The expected
number of obtained photoelectrons is then
where 
is the detection efficiency
of the photons
with energy E.
depends on
Cerenkov gas transparency,
reflection losses, quantum
sensitivity of PMT and probability to miss this PMT because of
geometry factors.