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Circular mesa GaAs p+–i–n+ diodes for photon counting soft X-ray spectroscopy have been fabricated and characterised over a temperature range of +80 to –30 °C. The spectroscopic performance of the diodes, as measured by the FWHM of the Mn Kα X-ray line from an 55Fe radioisotope, is reported. In addition, we compare the GaAs diodes with previously fabricated and characterised Al0.8Ga0.2As p+–i–n+ diodes of similar geometry.

A Monte Carlo model has been developed to predict the X-ray spectroscopic performance of silicon carbide detectors over the energy range 1–60 keV. Consideration of the charge creation and diffusion in the different active layers along with provision for fluorescence and Compton scattering produce spectra that are in good agreement with measurements. Direct comparison of modelled spectra with X-ray data from a novel detector, the Semi-Transparent SiC Schottky Diode (STSSD) and comparison of SiC data from the literature also shows good agreement between the reported spectra and the model. Predicted quantum efficiencies are also presented.

Al0.8Ga0.2As p+–p−–n+ diodes with low reverse leakage currents ( 20 V), we observed avalanche multiplication which, while improving the signal-to-noise ratio, degraded the spectroscopic performance.

We describe a novel SiC Schottky diode architecture. The semi-transparent SiC Schottky diode has an “ultra-thin” (18 nm Ni/Ti) Schottky contact, a gold annular overlayer and a gold corner-contact pad. We show that the new architecture exhibits the same essential characteristics as a more conventional ‘thick-contact’ Schottky diode (≥ 100nm). Such diodes will have a higher efficiency for low energy (< 5 keV) X-rays than that of conventional structures combined with minimal self-fluorescence from the electrode materials. We present X-ray spectra from 55 Fe, 109 Cd and 241 Am radioactive sources that show these diodes can be used for spectroscopy with promising energy resolution (1.47 keV FWHM at 22 keV) at room temperature (23oC). The reduction in contact thickness, however, does reduce the barrier height of the new diodes in comparison to those fabricated using the conventional process, and requires a trade-off between the low energy detection threshold and the noise in the detector.

The high resolution gamma imager (HRGI): a CCD based camera for medical imaging. We describe the High Resolution Gamma Imager (HRGI): a Charge Coupled Device (CCD) based camera for imaging small volumes of radionuclide uptake in tissues. The HRGI is a collimated, scintillator-coated, low cost, high performance imager using low noise CCDs that will complement whole-body imaging Gamma Cameras in nuclear medicine. Using 59.5 keV radiation from a Am-241 source we have measured the spatial resolution and relative efficiency of test CCDs from E2V Technologies (formerly EEV Ltd.) coated with Gadox (Gd2O2S(Tb)) layers of varying thicknesses. The spatial resolution degrades from 0.44 to 0.6mm and the detection efficiency increases ( x 3) as the scintillator thickness increases from 100 to 500 mum. We also describe our first image using the clinically important isotope Tc-99m. The final HRGI will have intrinsic sub-mm spatial resolution (similar to 0.7 mm) and good energy resolution over the energy range 30-160 keV. (C) 2003 Elsevier B.V. All rights reserved.

We have fabricated and characterised unpassivated 200 μm diameter circular mesa Al0.8Ga0.2As p+–i–n+ diodes as soft X-ray photon counting detectors. The spectroscopic performance, as measured by the FWHM of the 5.9 keV X-ray line from an 55Fe radioisotope, shows that these detectors can be used for X-ray photon counting spectroscopy with promising energy resolution (0.9–2.5 keV) over a −30 to +90 °C temperature range.