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. This paper describes some of those requirements and reports the results of more general interest achieved during 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; its chemical composition and a method to measure the curing time in realistic conditions are reported.