Secondary electron emission from thin CsI films induced by relativistic electrons

The sensitivity to minimum ionizing particles, of CsI-based detectors 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 thin CsI film coupled to a low pressure gaseous electron multiplier. The contribution from secondary electrons emitted from the solid and from the ionization electrons in the gas, were studied as a function of the gas pressure, the CsI thickness and electron incidence angle. Monte Carlo simulations of the solid and the gas contributions are in good agreement with the experimental results. It was found that the secondary emission from the CsI film contributes less than 5% to the detection efficiency of minimum, ionizing electrons, over a thickness range of 200 to 2000 nm. We demonstrated that the direct ionization of the gas is dominant, even in the HE-rich mixtures, unless very low gas pressures are used. (authors) 25 refs, 9 figs, 2 tabs