Physics aspects of the ray-tracing procedure

The Cerenkov detector response is calculated by a special ray-tracing procedure ( CCVPHO).

The main features of this procedure are:

• For every step the effective number of photons is calculated. This number is proportional to the step length and depends on the Cerenkov gas refraction index , its transparency as a function of wavelength, the average quantum efficiency ( ) of the PhotoMultiplier Tubes (PMT) and the particle velocity. This is discussed in sections 3.1 and 6.0 in more detail.
• Ray-tracing is performed for each effective photon. In case the photon reaches the PMT, it generates ONE additional photo-electron for this PMT. Possible losses due to less then perfect reflection off mirrors and Winston cones, absorption on the surface of magnetic shields or support frames, are taken into account.
• The Ray-tracing subroutine uses realistic shapes and positions of all mirrors, Winston cones, magnetic shields and support frames. Particle trajectories can begin at any point inside the Cerenkov detector - this is important in case of secondary particle production.
• The Cerenkov gas, elliptical and hyperbolic mirrors, magnetic shields, Winston cones are described in GEANT also as different materials. This is important for an accurate estimation of the detection efficiency of slow particles.