An investigation of fine structure effects in CCDs developed for JET-X
thesisposted on 15.12.2014, 10:41 by Adam. Keay
Charge Coupled Devices (CCDs) are low power, robust detectors promising medium spectral resolution, high spatial resolution and a high signal to noise ratio. It is for this reason that CCDs have become the preferred focal plane detector in all x-ray astronomy missions due for launch in the 1990s and early in the next millennium. The absorption of x-rays by a CCD is modulated by the transmission of the complex electrode structures and passivation layers on its front surface. Since the path length of soft x-rays (0.l1 - 2 keV) is of the same order as the thickness of these layers, fine structure in the quantum efficiency (Q(E)) around absorption edges can have a profound influence on the response of the CCD. This thesis presents the work done in investigating fine structure effects in the Q(E) of x-ray CCDs and the impact that these effects will have on astronomy missions. The state of x-ray astronomy as we move into the CCD-mission era is reviewed in Chapter 2. the operation of a CCD as an x-ray detector is then described.;The extensive set of experiments carried out at the Daresbury Synchrontron Radiation Source (SRS) to map the Q(E) of JET-X CCDs are detailed, including a description of the first use of the new, low beam current mode of SRS operation. Chapter 5 describes the origins and expression of X-ray Absorption Fine Structure (XAFS) in the CCD response. The instruments that make up the JET-X Focal Plane Cameras are then reviewed. In Chapter 7 the importance of calibrating JET-X accurately is shown by analysing the impact of XAFS in the detector sub-systems. It is shown that XAFS threaten the scientific return from current and future x-ray astronomy missions. In Chapter 8 the information contained by the fine structure in the CCD Q(E) is extracted and analysed to obtain the thickness and structure of the inert layers on the surface of the CCD, and also to improve models of the CCD response. Finally the work is summarised and additional further work is suggested.