Nuclear Instruments and Methods in Physics Research A 447 (2000) 614 } 618 Structural transformation of CsI thin "lm photocathodes under exposure to air and UV irradiation A.S. Tremsin*, S. Ruvimov, O.H.W. Siegmund Experimental Astrophysics Group, Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA Lawrence Berkeley National Lab, Berkeley, CA 94720, USA Received 16 November 1999; accepted 27 November 1999 Abstract Transmission electron microscopy has been employed to study the structure of polycrystalline CsI thin "lms and its transformation under exposure to humid air and UV irradiation. The catastrophic degradation of CsI thin "lm photocathode performance is shown to be associated with the "lm dissolving followed by its re-crystallization. This results in the formation of large lumps of CsI crystal on the substrate surface, so that the "lm becomes discontinuous and its performance as a photocathode is permanently degraded. No change in the surface morphology and the "lm crystalline structure was observed after the samples were UV irradiated.  2000 Elsevier Science B.V. All rights reserved. Keywords: CsI photocathodes; Quantum e$ciency; Crystalline structure 1. Introduction CsI thin "lms have become widely used as UV and soft X-ray photocathodes [1,2] for their high e$ciency. They also appear to be relatively stable under short exposure to ambient air. However, due to the hygroscopic properties of the CsI material, photocathode e$ciency degrades very quickly after exposure to humid air, and this phenomenon is more pronounced at longer wavelengths [1}3]. It was shown that the quantum e$ciency (QE) of an air-exposed photocathode degrades substantially after about 1 h exposure to air of 50% humidity [1,4]. The mechanism of photocathode degrada- * Corresponding author. E-mail address: ast@ssl.berkeley.edu (A.S. Tremsin).  Present address: Mitsubishi Silicon America, Salem, OR 97303, USA. tion is likely to be associated with CsI hydrolysis, since its oxidation is inhibited due to the large bandgap of CsI (about 6 eV). Once exposed to moderately humid air, a degraded CsI photo- cathode can still be recovered by heating [4] (soft degradation), however its damage would be per- manent after exposure to extremely humid air (catastrophic degradation). In the latter case the photocathode would usually turn `milkya and be- come transparent to UV light [4]. The humid air exposure results in the irreversible structural trans- formation of a CsI "lm, which then conglomerates into lumps and becomes discontinuous [5,6]. However, so far this structural transformation has not been studied in detail. On the other hand, the stability of photocathode performance under UV irradiation [7}10] is another concern for some detector applications with high radiation dozes. 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 3 0 5 - 4