An investigation of soft X-ray imaging and polarimetry.

This thesis includes work on both microchannel plate X-ray detectors and X-ray polarimetry, which although essentially distinct, have a common link through X-ray photocathodes. The source(s) of the background noise count rate in microchannel plates are investigated. Various noise mechanisms assessed include outgassing, cosmic rays, field emission, internal radioactivity, ion feedback and thermal emission. Experimental measurements are compared with calculations from a Monte Carlo model based on the assumption that radioactive decay (by beta emission) of elements within the microchannel plate glass is the major source of dark noise. The performance of Caesium Bromide as an X-ray photocathode for enhancing the quantum detection efficiency of microchannel plates is reported and compared with that of Caesium Iodide. A new type of microchannel plate configuration, a Sandwich Plate, consisting of three standard microchannel plates bonded in permanent contact is examined for use as an X-ray photon counting detector. This investigation includes a study of the correlation between gain reduction with increased count rate and the size of the illuminated area. An evaluation is made of Galileo Long Life (L2) Microchannel plates operated in pulse counting mode with special emphasis on the stability of the gain as a function of abstracted charge. Further evidence for radioactivity as the major source of background noise is obtained; L2 plates contain different radioactive isotopes compared to the 'standard' (Mullard) microchannel plate glass. The design and performance of a new type of polarimeter, a Photoemission Polarimeter, for use in soft X-ray astronomy is presented. The polarimeter utilises the linear polarisation sensitivity in the photoemission from a CsI photocathode. Possible sources of instrumental modulation are evaluated by comparing experimental measurements with calculations from a Monte Carlo model. The sensitivity of the photoemission polarimeter is compared with X-ray polarimeters presently used in X-ray astronomy.