David H. Lumb

Proton Damage Effects in EEV Charge Coupled Devices

An examination is conducted of the effects of low-energy protons on CCD performance to evaluate the potential effectiveness of space-borne observational instruments. Degradation is described as a function of incremental dose, irradiation temperature, or proton energy for several device architectures, some of which incorporate design features to minimize signal-charge/trapping-site interaction. Degradation of the charge transfer is studied for very low proton doses, and dark current is found to vary directly with proton dose. Displacement damage in the signal-transfer channels generates charge-trapping sites that have a negative effect on EEV CCD performance. Degradation of charge-transfer performance is shown to be the most significant hindrance to effective CCD operations for X-ray spectroscopic applications.

Applications of charge coupled devices to X-ray astrophysics missions

Abstract The utilisation of charge coupled devices (CCDs) for astronomical observations is discussed, and the development of CCDs to meet the scientific requirements of future space-borne observatories are described. Some data are presented for the X-ray detection efficiency (up to 90%) and energy resolution (∼100 eV FWHM) obtained with latest EEV CCDs developed for enhanced X-ray response. Some designs of CCD focal planes for future X-ray spectroscopy missions are illustrated.

Calibration and X-ray spectroscopy with silicon CCDs

Abstract It is shown that the novel mode of operation of charge coupled devices (CCDs) allows a new method of measuring the X-ray energy-to-charge conversion factor, without requiring externally calibrated circuits etc., thereby reducing possible contributions to systematic uncertainties in this measurement. In addition, the low noise operation of CCDs is shown to lead to possibilities for measuring the Fano factor in silicon with improved precision. Advances in CCD X-ray detection performance are described, including energy resolutions of 80 eV FWHM at a temperature of 180 K, and detection efficiencies of greater than 90%. Such improvements are shown to have potential benefit for various X-ray spectroscopic applications.

X-Ray Imaging Spectroscopy With EEV CCDS

The requirements for CCDs which may be used in future X-ray astronomical missions are discussed. Results from a programme designed to enhance commercially available CCDs for X-ray spectroscopic applications are presented. The use of deep depletion technology allows a combination of good X-ray detection efficiency (> 50% at 6 keV) and X-ray energy resolution (e, 100eV FWHM). The effects on the scientific performance of a radiation dose of as little as 14 krads are shown to be potentially serious. Further developments in the topics of back illumination and large area arrays are described.

Event recognition techniques in CCD X-ray detectors for astronomy

Abstract Charge coupled device (CCD) image sensors are rapidly gaining in importance as candidate focal plane detectors for X-ray astronomy. Methods of event selection and recognition are described, which allow excellent X-ray energy resolution (e.g. 133 eV FWHM at 4.95 keV) to be attained. The recognition and discrimination of charged particle background events is also shown to be efficient (up to 99.9%). Changes in CCD design to allow optimisation for X-ray imaging spectroscopy are discussed, and it is demonstrated that CCDs may offer, simultaneously, high spatial resolution, good energy resolution, high detection efficiency and good background rejection.

X-Ray Measurements Of Charge Diffusion Effects In EEV Ltd. Charge-Coupled Devices

Most scientific imaging applications of CCDs require low noise and high charge transfer efficiency. These attributes have been exhibited in devices produced by a number of manufacturers, but not all perform well as x-ray imaging spectrometers. This application requires that all charge liberated by an x-ray photon be completely collected within the original pixel. Processes of charge diffusion may prevent this from occurring and degrade the obtainable energy resolution. Measurements of charge diffusion and its effect on x-ray performance are presented for three types of English Electric Valve Co. (EEV) CCDs. These are fabricated on bulk and epitaxial wafers with resistivities of 1000 and 1011 cm. A range of different pixel sizes is employed. Measurements of detection efficiency are compared with predictions of a theoretical charge-diffusion model. Implications for optimizing the CCD structure for use in an x-ray astronomical spectroscopy application are noted.

European Photon Imaging Camera for x-ray astronomy

The capabilities of the European Photon Imaging Camera (EPIC), the main instrument of ESAs Cornerstone mission in X-ray astronomy with multiple mirrors (XMM), are discussed. The CCD characteristics, spatial resolution, energy bandpass and faint source sensitivity, spectral resolution and sensitivity, and timing capability are addressed, and the scientific rationale of the EPIC is summarized. The EPIC instrument system concept is briefly described.

Performance of CCDs for X-ray imaging and spectroscopy

CCD image sensors offer the potential for X-ray imaging with high spatial resolution, simultaneously with good energy resolution and quantum efficiency. Recent work at Leicester has concentrated on investigating the properties of commercially available TV sensors provided by GEC. It has been shown that single photon counting with good noise performance is achievable. Noise levels ∼10–15 electrons rms should be possible corresponding to energy resolutions ∼100 eV fwhm. However, it may be shown that with conventional CCDs the resolution is degraded by incomplete collection of the X-ray generated charge arising from the undepleted substrate. Methods for minimising this effect and for increasing the quantum efficiency at energies > 2 keV are described. Results are presented for the efficiencies of a range of GEC devices measured at energies of 2–22 keV, and for the degradation of energy resolution resulting from charge diffusion.

Further Development Of CCD X-Ray Detectors For Astronomy

A number of future space-borne X-ray astronomy missions have now been proposed, which will utilise CCD imaging devices in their focal planes. The CCDs will offer simultaneously high spatial resolution, excellent energy resolution and detection sensitivity. However, a number of technological developments are still to be demonstrated before the X-ray performance of CCDs is optimised. The development of CCDs fabricated on high resistivity silicon wafers, combined with back thinning and annealing processes, allows the production of fully depleted detectors. These devices are capable of detecting single X-ray photons , with high efficiency throughout the 0.1-15keV energy band. Efficient coverage of focal planes is facilitated with the introduction of new large area CCDs. The EEV P88000 series CCDs have a range of formats up to 1242 x 1152 pixels (6.5 sq cm), and have included process changes to optimise noise (5.5 electrons rms) and dark current. Results of a programme to investigate the tolerance of EEV CCDs to ionising radiation are also presented.

Charge Coupled Devices (CCDS) For X-Ray Spectroscopy Applications

The X-ray performance of two new types of GEC P8600 CCDs have been measured. One, a conventional thin depletion layer device, possesses very low noise characteristics enabling good X-ray energy resolution to be achieved, but with only modest quantum efficiency (10% at 5.9 keV). The other, a deep depletion device fabricated on high resistivity silicon also possesses the desired low noise performance (8 electrons rms), combined with much improved quantum efficiencies (60% at 5.9 keV). Degradation of energy resolution due to charge spreading effects has also been overcome through use of the deep depletion layer and the new device has a capability for rejection of background charged particles of around 96%. The development is for spectroscopic instrumentation for X-ray astronomy.