Spatially-Resolved X-Ray Spectroscopy of CsI Deposits

Using the X-ray spectromicroscopy technique, we studied the primary and secondary electron photoemission of cesium iodide (CsI) films deposited on different substrates, such as copper, aluminum, stainless-steel, and gold-coated quartz. We found an increase of more than a factor of 2 in the secondary emission after exposing, for a few seconds in ultra-high-vacuum (3 X 10(-10) mbar), the sample surface to an X-ray dose-rate of about 5 X 10(11) photons/s cm2 at the Al Kalpha photon energy (1486.7 eV). The activation effect, that lasts for about two days, is related to a change in the surface morphology: after the X-ray exposure the surface presents a single phase with respect to the presence of two phases in the as-deposited or aged samples (lower Quantum Efficiency (QE) condition). Furthermore, the 25 mum lateral resolution of our spectromicroscope allowed us to show that the secondary electron photoemission is strongly inhomogeneous on the plane of the surface, and that this inhomogeneity is more closely related to the spatial inhomogeneity of the Cs photoemission yield than to that of 1. Finally, we found a decomposition effect induced by the X-ray exposure. This photoetching is less effective for the CsI deposited on the copper substrate, probably due to the presence of Cu-I compounds at the interface with the substrate, which is revealed by X-ray diffraction measurements