D.J. Bassford

A high resolution Small Field Of View (SFOV) gamma camera: a columnar scintillator coated CCD imager for medical applications

We describe a high resolution, small field of view (SFOV), Charge Coupled Device (CCD) based camera for imaging small volumes of radionuclide uptake in tissues. The Mini Gamma Ray Camera (MGRC) is a collimated, scintillator-coated, low cost, high performance imager using low noise CCDs. The prototype MGRC has a 600 μm thick layer of columnar CsI(Tl) and operates in photon counting mode using a thermoelectric cooler to achieve an operating temperature of - 10°C. Collimation was performed using a pin hole collimator. We have measured the spatial resolution, energy resolution and efficiency using a number of radioisotope sources including 140 keV gamma-rays from 99mTc in a specially designed phantom. We also describe our first imaging of a volunteer patient.

SiC X-ray detectors for harsh environments

We have characterised a number of SiC Schottky Diodes as soft X-ray photon counting detectors over the temperature range -30°C to +80°C. We present the spectroscopic performance, as measured over the energy range ~ 6 keV–30 keV and correlate the data with measurements of the temperature dependence of the device leakage current. The results show that these detectors can be used for X-ray photon counting spectroscopy over a wide temperature range. Measurement of the radiation tolerance of Semi Transparent SiC Schottky Diodes (STSSD) has shown that these devices can still operate as photon counting X-ray spectrometers after proton irradiation (total dose of 1013 cm−2, 50 MeV). We present measurements on proton irradiated STSSDs that indicate that radiation induced traps, located in the upper half of the bandgap, have reduced the mobility and concentration of charge carriers. X-ray spectra predicted using a Monte Carlo model for SiC diodes are compared with measured spectra.

Avalanche Gain and Energy Resolution of Semiconductor X-ray Detectors

Realistic Monte Carlo simulations for the avalanche gain of absorbed X-ray photons were carried out in a study of the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes (APDs). The work explored how the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and, hence, the photon energy), the relationship between ionization coefficients, and the mean gain itself. We showed that the conventional notion of APD gains significantly degrading energy resolution is incomplete. If the X-ray photons are absorbed outside the avalanche region, then high avalanche gains with little energy resolution penalty can be achieved using dissimilar ionization coefficients. However, absorption of X-ray photons within the avalanche region will always result in broad gain distribution (degrading energy resolution), unless electrons and holes have similar ionization coefficients.

Design and use of mini-phantoms for high resolution planar gamma cameras.

There is now wide spread interest in the application of small field of view gamma cameras for clinical use in operating theatres, intensive care units and bedside imaging. The development of these imaging systems some of which have improved spatial resolution has necessitated a reappraisal of the suitability of conventional phantoms intended for use with gamma cameras. The testing of imagers having increased system resolution requires phantoms and test objects of smaller dimensions. We have investigated the use of high resolution mini-phantoms for the evaluation of planar imaging devices with spatial resolution of the order of 1 mm. We present images of a number of phantoms that show their suitability for evaluating high resolution planar gamma cameras. It is also apparent that there are a number of practical difficulties that have not been previously reported when using liquid filled phantoms with hole sizes between 1 and 4 mm. In particular problems of filling small diameter holes and liquid surface tension effects can limit the utility of these phantoms. Initial observations when imaging mini-phantoms with a high resolution CCD based gamma camera are presented.

A Hybrid Camera for simultaneous imaging of gamma and optical photons

We present a new concept for a medical imaging system, the Hybrid Mini Gamma Camera (HMGC). This combines an optical and a gamma-ray camera in a co-aligned configuration that offers high spatial resolution multi-modality imaging for superimposition of a scintigraphic image on an optical image. This configuration provides visual identification of the sites of localisation of radioactivity that would be especially suited to medical imaging. An extension of the new concept using two hybrid cameras (The StereoScope) offers the potential for stereoscopic imaging with depth estimation for a gamma emitting source.

Temperature dependence of the average electron-hole pair creation energy in Al0.8Ga0.2As

The temperature dependence of the average energy consumed in the creation of an electron-hole pair in the wide bandgap compound semiconductor Al 0.8Ga0.2As is reported following X-ray measurements made using an Al0.8Ga0.2As photodiode diode coupled to a low-noise charge-sensitive preamplifier operating in spectroscopic photon counting mode. The temperature dependence is reported over the range of 261 K-342 K and is found to be best represented by the equation e AlGaAs 7.327-0.0077 T, where eAlGaAs is the average electron-hole pair creation energy in eV and T is the temperature in K.

A hybrid camera for locating sources of gamma radiation in the environment

We present a new concept for a portable environmental imaging system, the Compact Hybrid Gamma Camera (CHGC). This combines an optical and a gamma-photon camera in a co-aligned configuration that offers high spatial resolution multi-modality imaging for superposition of gamma ray and optical images. We report on its potential use for surveying, monitoring and clean-up of radioactive sources in the environment.

Determination of the electron-hole pair creation energy in Al0:8Ga0:2As

The average energy consumed in the creation of an electron-hole pair (commonly called the electron-hole pair creation energy) in the compound semiconductor Al0.8Ga0.2As has been experimentally measured for the first time at X-ray energies using 55Fe and 109Cd radioisotope sources and a GaAs X-ray photodiode as a reference detector. The value measured (5.1 eV ± 0.08 eV at 294 K) is compared to values previously estimated in the literature.

First spectroscopic X-ray and beta results from a 400 μm diameter Al0.8Ga0.2As photodiode

Results characterising the performance of thin (1 μm active layer) 400 μm diameter Al0.8Ga0.2As mesa photodiodes operated at room temperature are presented showing the spectral response under illumination with moderate energy X-rays (5.9 keV and 22.16 keV) from radioisotope X-ray sources (55Fe and 109Cd) and beta particles from a 14C source (endpoint energy = 156.48 keV). The X-ray FWHM is established as 1.95 keV at 5.9 keV and 22.16 keV, primarily limited by electronics noise from the preamplifier. The diodes show a spectral response to 14C beta particles, but the endpoint energy is not reached due to the detectors thinness. The X-ray results are compared with 200 μm diameter Al0.8Ga0.2As photodiodes previously characterised using X-ray radioisotope sources at our laboratory.

Direct detection of Tritium and Carbon-14 beta particles with GaAs photodiodes

New measurements are reported which show beta particles emitted from 3H and 14C sources being directly detected with GaAs mesa photodiodes without use of scintillators. Spectra accumulated with GaAs diodes which were originally developed for X-ray spectroscopy are presented which show that individual beta particles from these sources can be counted and the energy they deposit in the detector measured. Potential longer term applications of this technology as it is developed further and improved include space missions, autoradiography, monitoring tritium produced by fusion reactors and nuclear decommissioning.