Uniformity measurements across the area of the CMS ECAL avalanche photodiodes

The photodetectors which read out the scintillation light generated in the lead tungstate crystals in the barrel part of the Compact Muon Solenoid (CMS) electromagnetic calorimeter are avalanche photodiodes (APDs). Scanning the APD's active area with a collimated light spot, the spatial uniformity of their quantum efficiency and gain has been measured at four different operating gains. Details of the APD surface structure are examined by scanning with a fine light spot. These details help to explain the difference between the bias voltage for a given gain when the full APD area is illuminated and when only the central part is illuminated.