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A xenon ionization detector for scanned projection radiography: theoretical considerations.

Xenon ionization detectors have been used successfully in computed tomography (CT) scanners; however, the detector design used in CT scanners does not provide sufficient spatial resolution for scanned projection radiography. We have been investigating a new design of xenon detector with individual element widths of 0.5 mm. In this design, there are no metal septa separating individual elements. As a result, detection efficiency is better than the design with septa, and construction of an array with submillimeter element widths is simpler; however, crosstalk will now occur between elements. Theoretical calculations of efficiency and resolution for our septaless design of xenon detector are presented. Results of these calculations indicate that for a spectrum of 100 kVp, element dimensions of 0.5 mm X 0.5 mm X 10 cm, a front window of 0.5-mm aluminum, and a xenon pressure of 20 atm, quantum efficiency will be greater than 95%, detective quantum efficiency (DQE) will be approximately 75%, and both energy and conversion efficiencies will be limited to 50% by K-fluorescent escape. The calculations also predict that for the same design, the reduction in lesion contrast induced by crosstalk will be less than 10% for all typical spectra. These theoretical results have encouraged us to pursue the construction of a prototype septaless xenon detector for scanned projection radiography.