GEM: a new detector for scanned projection radiography

While digital radiography using energy-integration detectors is starting to replace screen/film radiography, digital radiography using photon-counting detectors will offer even more benefits. A prototype scanned projection radiography (SPR) system for Xe:CH₄ imaging has been constructed, which combines a Gas Electron Multiplier (GEM) and a Gas Microstrip Detector (GMD). The energy deposited by each incident photon can be measured when photon counting is done. Both the GEM and the GMD are novel gas-filled counting detectors developed for high-luminosity experiments in particle physics. The GMD is similar to the multiwire proportional chamber except that its anodes and cathodes are strips printed onto a substrate. A prototype photon-counting X-ray imaging system using a Xe:CH₄ GMD was previously built in our lab, and showed very promising spatial resolution and energy resolution. In order to improve the electrical stability of the GMD imaging system, the GEM is coupled to the GMD as a preamplification element. The GEM, a new concept for electron amplification in gas detectors, is basically a multilayer foil with many high precision holes. Electron amplification happens at the centres of these holes. With the gain contribution from the GEM, the GMD gain and operating voltage can be decreased, thereby improving its reliability. The physical characteristics of the GEM/GMD combination filled with 1 atm Ar:CH₄ (90:10, by partial pressure) are being tested