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Developments in GaAs pixel detectors for X-ray imaging

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13 Author(s)
Manolopoulos, S. ; Dept. of Phys. & Astron., Glasgow Univ. ; Bates, R. ; Campbell, M. ; Da Via, C.
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Position sensitive hybrid pixel-detectors have been fabricated by bump bonding silicon or bulk grown semi-insulating gallium arsenide pixel detectors to CMOS read-out chips. Their performance as X-ray imaging sensors, in the energy range of 10-70 keV, was evaluated in terms of spatial resolution. For the GaAs device a fit was made to the line spread function (LSF) obtained from the image of a narrow slit and the corresponding modulation transfer function (MTF) and noise equivalent passband (Ne) evaluated. A value of 5.7 line pairs per mm (1p/mm) was found for the latter, with a modulation of 10% at the Nyquist frequency (Ny). A comparison is also given of the performance of these devices with state-of-the-art scintillator on silicon CCD dental X-ray sensors. In a bid to improve detector performance, thick layers of high quality GaAs have recently been grown by low pressure vapour phase epitaxy (LP-VPE). Hall measurements on initial samples gave free carrier concentration of 1-8×1011 cm-3 From the C-V dependence of a reverse-biased Schottky diode this material, however, a space charge density of 2×1013 cm-3 was measured. The observed temperature and frequency dependency of the capacitance is characteristic of the presence of deep levels and so the material is believed to have a small degree of compensation due to these levels. The measured charge collection efficiency determined (c.c.e.) for 60 keV gamma rays showed an increase with reverse bias, reaching a plateau value of 93% for 100 V. The limitations of present detectors are discussed and possible future developments indicated

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Nuclear Science, IEEE Transactions on  (Volume:45 ,  Issue: 3 )