Moisture proof columnar cesium iodide (CsI) layers for gas avalanche microdetectors

Cesium iodide columnar layers having a diameter of /spl ap/3 /spl mu/m, and wall spacing of /spl ap/1 /spl mu/m act as secondary electron emitters and can be used for detection of radiation: charged particles, X-rays and gamma rays. With a large enough electric field across the columnar layers, /spl ap/400 /spl mu/m in thickness, gas avalanche gain is evident when placed in a suitable gas, such as P10 or argon-ethane mixtures. The cesium iodide columns are damaged by ambient moisture. This damage can be prevented by evaporating protective layers of insoluble, low boiling point inorganic materials, such as mercuric iodide. Columnar layers with 20 nm coatings of mercuric iodide yield more than 30000 electrons on average when traversed by electrons from a /sup 90/Sr beta source.