Radiation-induced effects in glass windows for optical readout GEM-based detectors

In this paper we present irradiation measurements performed to select a transparent anode substrate that best meets the requirements of an optical readout for a novel detector, the LaGEMPix. The modification of the optical properties of the material due to proton irradiation were studied in soda-lime, fused quartz and fused silica glasses coated with an Indium Tin Oxide layer. The irradiations were performed using the research Beam Transfer Line (BTL) of the IBA Cyclone 18 MeV cyclotron of the Bern University Hospital (Inselspital). We recorded visible scintillation light generated by proton irradiation in the soda-lime and fused quartz samples. We also investigated the darkening of these three glasses and observed radiation-induced colour centres in the soda-lime glass sample. The optical transmission spectra of the samples were measured before and after irradiation. Reductions of 45%, 1% and 0.4% were observed for soda-lime glass, fused quartz and fused silica, respectively (with an associated error of 0.25%). We conclude that the best option for our specific application is the fused silica substrate, which will be the transparent anode for the next generation of the LaGEMPix detector.