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The Photoemission Polarimeter In Soft X-Ray Astronomy

Measurements in our laboratory have recently shown that the pulse quantum yield (counts/photon) of CsI X-ray photocathodes, measured near grazing incidence, depends on the linear polarisation state of the incident beam. These results have implications both for the fundamental understanding of the X-ray photoeffect and, since CsI is well-known as a cathode material of high (unpolarised) quantum efficiency in the 0.1-1.0 keV band, for the development of high throughput astronomical X-ray polarimetry. This paper, concentrating on the latter topic, describes the practical accomodation of a Photoemission Polarimeter at (or near) the focus of a satellite X-ray telescope. Problems of device format, instrumental polarisation and data compression are discussed with reference to the Composite X-ray Polarimeter recently proposed for the ESA XMM mission. Examples of the sensitivity of this instrument are given. In the second part of the paper, our most recent laboratory measurements are presented. These concern the variation in the number of X-ray photoelectrons with the polarimetric azimuth of a CsI cathode.