SECONDARY-ELECTRON EMISSION FROM THIN CSI FILMS INDUCED BY RELATIVISTIC ELECTRONS

The sensitivity to minimum ionizing particles, of CsI-based gaseous de tectors currently developed for X-ray and UV-photon imaging in future colliders, was studied in detail. The detection efficiency of minimum ionizing electrons was measured with a thin CsI film coupled to a low- pressure gaseous electron multiplier. The contributions from secondary electrons emitted from the solid and from ionization electrons in the gas, were studied as function of the gas pressure, the CsI thickness and the electron incidence angle. Monte Carlo simulations of the solid and gas contributions are in good agreement with the experimental res ults. It was found that the secondary emission from the CsI film contr ibutes less than 5% to the detection efficiency of minimum ionizing el ectrons, over a thickness range of 200 to 2000 nm. We demonstrated tha t the direct ionization of the gas is dominant, even in He-rich mixtur es, unless very low gas pressures are used.