Study of some optical glues for the Compact Muon Solenoid at the Large Hadron Collider of CERN

Two Avalanche Photodiodes will measure the light produced in each of the 61,200 PbWO4 crystals composing the barrel part of the electromagnetic calorimeter of the Compact Muon Solenoid (CMS) at the Large Hadron Collider of CERN. To improve the collection of the photons, these detectors will be glued to the crystal. To be used in CMS, the optical glue must fulfil several requirements. The paper describes those requirements and reports the results of the investigation of several commercial optical glues. In particular, refractive index, absorption length, radiation hardness and forecast ageing after 15 years are reported. The most promising glue for CMS was more deeply investigated, in particular its chemical composition, chemical compatibility with the other parts of the calorimeter and curing time in realistic conditions.