Sealed GEM photomultiplier with a CsI photocathode: ion feedback and ageing

We present the performance of a sealed gaseous photomultiplier consisting of a cascade of 3 or 4 Gas Electron Multiplier (GEM) elements coupled to a semitransparent CsI photocathode, in Ar/CH4 (95/5). A few-month stability study of the photocathode in a sealed mode is presented. Increasing the number of GEMs in cascade substantially reduces the ageing of the detector under strong irradiation. The ion feedback to the photocathode has probably a minor effect on the ageing rate.