Microscopic Characterisation of Photo Detectors from CMS Hadron Calorimeter

The CMS hadron Calorimeter is made of alternating layers of scintillating tiles and metals, such as brass or iron. The original photo detectors were hybrid units with a single accelerating gap called Hybrid Photo Diodes (HPD). Scintillating light was transmitted to the HPDs by means of optical fibers. During data taking at the Large Hadron Collider (LHC), the signal strength of scintillator tiles of detector units in the forward region degraded significantly due to the damage related to the amount of radiation to which the scintillator was exposed to. Scintillators suffer damage when exposed to radiation, however, the amount of damage observed was more than originally estimated. Several HPDs were removed during a detector shut down period. Microscopic scans of relative quantum efficiencies for few of these HPDs were made. The damage of the photocathode was determined to vary with the amount of optical signal transmitted by optical fibers to the HPD. Imprints of each fiber (1 mm) on the photocathode with varying damage within the same pixel were observed. Most of the observed reduction of the calorimeter signal can be attributed to localised damage of the photocathode.