The Small Angle Calorimeter
View of the Small Angle Calorimeter before its installation.
The Small Angle Calorimeter (SAC) consists of a 5×5 matrix of PbF2 Cherenkov crystals, each with a front face of 3×3 cm2 and a length of 14 cm.
It covers the Ecal inner hole and has the role to detect the very forward photons emitted following the positrons interaction in the diamond target.
The rate of photons from beam Bremsstrahlung in this region is up to O(40) particles per bunch and then a fast detector is necessary to veto them. This motivated the choice of a Cherenkov detector combined with fast photomultipliers, Hamamatsu R13478UV with an ultraviolet transparent window.
Tipical signal from the SAC registered with a FADC sampling at 2.5 GS/s (1024 samples). The different peaks represent different photons detected in a 250 ns time window.