Light collection uniformity of lead tungstate crystals for the CMS electromagnetic calorimeter

A technique has been developed to measure the light collection uniformity of lead tungstate crystals for the CMS electromagnetic calorimeter. The use of a hybrid photomultiplier tube allows a precision measurement despite the intrinsically low light yield. It is found that shading the chamfered edges of the crystals with a pencil can achieve a reduction in the front-non-uniformity of approximately 0.1%/X0. Similar effects are observed by de-polishing the chamfers.