Hard X-ray spectroscopy using a small-format TlBr array

Abstract

We report X-ray measurements on a prototype 3 � 3 TlBr pixel array, produced to assess the technological feasibility of making a Fano limited imager, which can operate near room temperature. The device was fabricated on monocrystalline material of size 2.7 � 2.7 � 1.0 mm 3 . It has a pixel size of 350 � 350mm 2 and pitch of 450mm. Measurements were carried out on all pixels over the energy range 5.9–662 keV using radioactive sources. The leakage currents were found to be low enough to allow room-temperature operation, with typical energy resolutions of B20 keV FWHM at 59.95 keV under full-area illumination. At a reduced detector temperature of –301C, these fell to B4 keV FWHM. Although the spectral performance of the present array is currently impaired by material limitations, its spectral acuity was found to be greatly enhanced by the small pixel effect. Additional photon metrology was carried out at the Hamburger Synchrotron-strahlungslabor radiation facility. Under monochromatic pencil beam illumination, the measured energy resolutions at 20 keV were B3 keV FWHM at � 301C. The spatial uniformity of the array was measured using a 50 � 50mm 2 , 20 keV monoenergetic X-ray beam, raster scanned over the entire active area. The response, in terms of count rate, gain and energy resolution was found to be uniform at the few percent level, consistent with statistics. It was observed during these measurements, that the X-ray response of the pixels was unstable, showing time-dependent gain shifts indicative of polarization effects. The magnitude of the effect was proportional to the total energy deposition per unit time. Lastly, the use of TlBr arrays in nuclear medicine applications is discussed with particular emphasis on radio-guided surgical probes. Recommendations for an optimized design are given. r 2002 Elsevier Science B.V. All rights reserved.