New results of X-ray polarisation detection efficiencies are presented for two small pixel devices. EEV Ltd. have designed the first CCD X-ray polarimeter with an improved deep depletion layer of 80 urn, to maximise the device quantum efficiency at energies above 1 keV. A novel design concept has been utilised to minimise the pixel dimension whilst maximising production, the CCD has 4x9 um2 pixels. This device was tested and has been shown to be greater than a factor of two more efficient for polarisation measurements above 10 keV, than previously tested CCDs. The first successful measurements below 10 keV show a 5 - 10 % polarisation detection efficiency (modulation factor, M(E)), a significant result, due to X-ray optics having greater effective area below 10 keV. The smallest pixel CCD to date, with 2.4 um2 pixels, designed for optical purposes by Philips Ltd., is shown to have 26.2 % modulation at 10 keV. From the analysis of the data from these devices it has been recognised that charge diffusion, and thus the depletion depth, has a significant effect on the polarisation measurements. A model has been written to simulate polarised X-ray interactions in CCDs, and is shown to successfully model thinly depleted devices (Kodak KAF1400), in terms of polarisation detection efficiency. Further improvements have been identified to provide an accurate model for deeply depleted devices, in order to create a tool for optimising future CCD polarimeters. The modulation factors, M(E), for a 0.5 um2 pixel CCD have been estimated using the model, and it has been shown that a device of this type would provide a further factor of 2 improvement in M(E), with a significant measurement as low as 5 keV. The feasibility of a future mission including a CCD X-ray polarimeter has been investigated in detail, in terms of current and future technology. The XMM and the XEUS optics are considered in conjunction with both the EEV polarimeter and the modelled 0.5 urn2 pixel CCD. Two types of Astronomical sources are taken into account over a 5 - 20 keV energy range: bright galactic objects, e.g. The Crab Nebula faint extragalactic objects e.g. Active Galactic Nuclei (AGN). Using the XEUS optics, a 25 m focal length and an effective area maximised to 1 m2 at 1 keV yields observation times of less than 105 sees for a Crab measurement (polarisation 10 - 20 %, below 12.5 keV) and of the order 104 sees or less using a 0.5 urn2 pixel CCD. For measurements of the more faint AGN (polarisation 10 - 20 %) observation times are of the order 106 sees or below for the EEV CCD (7.5 - 10 keV) and the 0.5 um2 device (5 - 12.5 keV). Employing the XEUS optics for the measurement of the polarisation of AGN requires no further technological advancement if the EEV polarimeter is utilised, and the production of a 0.5 um2 pixel CCD is said to be within easy reach according to manufacturers.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:696658 |
| Date | January 1999 |
| Creators | Hill, Joanne E. |
| Publisher | University of Leicester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2381/30620 |
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