Monitoring LSO/LYSO Crystal Calorimeters

LSO/LYSO crystals were chosen by the SuperB, Mu2e and COMET experiments to construct total absorption electromagnetic calorimeters. They were also proposed as the active medium for a Shashlik calorimeter which was one of the two options proposed for the CMS forward calorimeter upgrade at the proposed HL-LHC. Because of the severe radiation environment expected at the HL-LHC, precision light monitoring is crucial for keeping excellent energy resolution promised by LYSO crystals. This paper discusses the wavelength choice for monitoring LYSO based crystal calorimeters. $\gamma $ -ray induced absorption and light output loss were measured for 20 cm long crystals from five different vendors. Monitoring sensitivity and divergence are analyzed. Two approaches using excitation and emission light and their practical implementations in a beam test at Fermilab are discussed. The response of an LYSO/W Shashlik calorimeter cell after a $\gamma $ -ray irradiation of 90 Mrad is also presented.