Study of the quantum efficiency of CsI photo-cathodes exposed to oxygen and water vapour

The operation of CsI photocathodes in gaseous detectors requires special attention to the purity of the applied gas mixtures. We have studied the influence of oxygen and water vapour contaminations on the performance of CsI photocathodes for the ALICE HMPID RICH prototype. Measurements were done through comparison of Cherenkov rings obtained from beam tests. Increased levels of oxygen and water vapour did not show any effect on the performance. The results of this study found a direct application in the way of storing CsI photocathodes over long periods and in particular in the shipment of the HMPID prototype from CERN to the STAR experiment at BNL.