Study of photocathode protection with thin dielectric films

Long-term stability of air-sensitive solid photocathodes operating in gas could be attained by coating their surface with a thin solid dielectric film. The protective film should have good electron transmission properties. We present data on LiF, NaF, CsF, MgF2, BaF2, SiO2 and Al2O3 films. The electron escape length and the filmʹs protection properties were studied on relatively stable photocathodes, namely CsI, CuI and Al. We observed a reduction of the electron affinity due to adsorption of polarized water molecules on alkali fluorides. We report on the stability of CsI photocathodes coated with thin films, under exposure to air and under gas multiplication conditions. The possible protection of visible photocathodes is discussed.