Photo-current scanner system for in situ quality assessment of large area CsI photocathodes

The facility for the production of the large area (64×42cm2) CsI photocathodes (PCs) for the ALICE/HMPID detector has been equipped with a photo-current scanner system for the in situ measurement of the CsI response over the full photo-sensitive area before transfer to the detector. This paper provides a detailed description of the experimental setup. Measurements of variations in the photo-current due to the pad structure of the PCs as well as due to heat treatment and ageing effects of the CsI PCs demonstrate the sensitivity of the instrument to changes in quantum efficiency (QE). The usability of the scanner to perform quality monitoring during the series production of PCs for the detector is shown in a comparison of scanner measurements and beam test results for 17 PCs.