Air Light-Guide for CASTOR EM-H Calorimeter

The results obtained on the basis of Monte-Carlo simulation of an Air Light-Guide for the CASTOR EM-H fibre calorimeter are presented. Calculations have been performed in the framework of geometrical optics using the Frenel formulae for the reflection coefficients from all glass surfaces and the value 0.8 for the reflection coefficient of mirror parts. Absorption of Cherenkov light in the internal fibre volume has also been taken into account together with the possibility of its reflection from the input quartz photocathode window. It was found that the angular distribution of light emission from the read-out ends of the fibres is enhanced at very forward angles with respect to the longitudinal fibre axis. This property was employed in the design of the optimal light gathering. The efficiency of light collection on the photomultiplier photocathode was found to depend on the parameters of the light-guide. The partial derivative equations used to determine the 3-dimensional surface which would best focus the light rays from every fibre onto the photocathode are also presented, along with the numerically calculated shape of this surface (in section).