FUV quantum efficiency degradation of cesium iodide photocathodes caused by exposure to thermal atomic oxygen
2011
The color dependence of the measured decline of the on-orbit sensitivity of the FUV channel of the HST Cosmic Origins
Spectrograph (HST-COS) indicated the principal loss mechanism to be degradation of the cesium iodide (CsI)
photocathode of the open-faced FUV detector. A possible cause of this degradation is contamination by atomic oxygen
(AO), prompting an investigation of the interaction of AO with CsI. To address this question, opaque CsI photocathodes
were deposited on stainless steel substrates employing the same deposition techniques and parameters used for the
photocathodes of the HST-COS FUV detector. The as-deposited FUV quantum efficiency of these photocathodes was
measured in the 117-174 nm range. Several of the photocathodes were exposed to varying levels of thermalized, atomic
oxygen (AO) fluence (produced via an RF plasma). The post AO exposure QE's were measured and the degradation of
sensitivity versus wavelength and AO fluence are presented.
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