A spot-focusing Cerenkov detector

Tests on a new type of Cerenkov counter have been carried out at CERN in a 50 GeV c/sup -1/ positive beam. This detector can be seen as a ring-focusing counter in which a subtraction of a fixed angle theta /sub 0/ has been performed optically on the Cerenkov angle theta . For a particular value gamma /sub 0/ of the relativistic factor of the particle detected, the Cerenkov angle theta is equal to theta /sub 0/, and all the Cerenkov light is focused on a spot, which gives its name to the detector. For all other values of gamma the spot degenerates into a small ring, much smaller than it would have been in a conventional focusing counter. The radius of the ring provides a determination of the velocity of the particle, and the position of the centre is related to its direction. This counter can be used for the detection of multi-particle events. (6 refs).