Veiling-Glare Of A Linear Multichannel Si(Li) Detector

Abstract

Multichannel lithium-drifted silicon (Si(Li)) semiconductor detectors are well suited to line-scan medical imaging systems. They have excellent linearity, high efficiency, large dynamic range and good stability. A major limitation in many medical imaging systems using image intensifiers is the veiling-glare associated with the large differences in intensity between different parts of the image field. Images acquired with linear Si(Li) detectors should suffer less from veiling-glare, primarily because they are line-scan images, but also because the Si(Li) detector responds to X-rays directly rather than to the light produced in a scintillator. The effective veiling-glare has been measured in a linear, 64-channel Si(Li) detector of thickness 5 mm and center-to-center spacing between adjacent sensitive areas of 0.5 mm. Each elemental contact was 6 mm high and 0.4 mm wide. The measurement was made in two different ways, both employing a 33 keV beam of synchrotron X-rays at the Stanford Synchrotron Radiation Laboratory. In one method, a lead phantom in the shape of an isosceles triangle was illuminated across its width with a fan beam of 33 keV X-rays 0.5 mm high by 20 mm wide. The X-ray flux passing by the phantom was measured in each channel of the Si(Li) detector. In the second method, a highly collimated beam of 33 keV X-rays (0.5 mm high by 0.025 mm wide) was scanned across 15 channels of the detector in 0.01 mm steps. The measured intensity in each channel was recorded at each position of the beam and the photon flux fell to less than 0.1% at a distance of 3 mm from the edge of the beam. The results of both measurements were in mutual agreement. Both were compared also to calculated results based on the known total X-ray cross sections for Compton scattering and photoelectric absorption. To within the experimental error, the observed and calculated results were found to be generally in good agreement.