Synchrotron studies of X-ray detectors.
thesisposted on 2015-11-19, 09:17 authored by Anthony Peter. Nichols
This thesis is an account of research into various aspects of X-ray detection, with a common thread being the use of synchrotron radiation. An investigation into the so called "X-ray vectorial effect" is described. The aim being to design a detector able to determine the degree of polarisation in astronomical X-ray sources and the angle of the polarisation vector. Two experiments are reported, the first showing marked polarisation sensitivity, but not totally answering doubts. The second experiment demonstrated that the X-ray vectorial effect does not exist and with it the critical importance of accurate calibration of angles of incidence. Monte Carlo modelling of electron transport in photocathodes resulting from X-ray absorption is presented alongside compatible experimental results. An alternative polarimetry technique, also based on utilising the known polarisation sensitivity of photoelectron creation, is then derived and predicted to be competitive with the best current polarimeters and offering greater scope for improvement. A novel efficiency calibration technique for bare Microchannel plates (MCPs) using the Daresbury synchrotron with very low beam current is examined allowing us to conduct photon counting measurements, while utilising the energy tunability of the synchrotron. In quantum efficiency measurements this technique brought out absorption edge fine structure and EXAFS from the constituents of MCP lead glass with the energy resolution required in calibration the MCP spectrometer readout on the AXAF satellite. Complimentary measurements of the quantum efficiency of alkali halide (Csl and KBr) photocathodes are presented over the continuous energy range between 2 and 8 keV and at a range of incidence angles. These alkali halides are generally coated onto the MCP channels to enhance the detector efficiency. Synchrotron radiation was also utilised to examine the radiation damage characteristics of alkali halide photocathodes. Both Csl and KBr are shown to suffer significant degradation of photoelectric efficiency after minutes of irradiation with of order 1010 photons s-1. The way X-ray flux, angle of incidence and energy affect the degree of degradation is investigated. A solid state model of lattice defect production, diffusion and trapping incorporating surface effects and photoelectron emission is described. This first attempt at modelling the physical basis of photoyield degradation is shown to accurately recreate much of the observed behaviour including the shapes of the signal decay curves and recovery out of the synchrotron beam yielding a clear insight into the degradation process and suggesting ways its effects can be reduced.
Date of award1996-01-01
Author affiliationPhysics and Astronomy
Awarding institutionUniversity of Leicester