New developments in full-field radiography detectors: Direct conversion selenium detector with avalanche gain layer

摘要

Amorphous selenium direct-conversion x-ray detectors have been used successfully for full field digital mammography (FFDM) and digital radiography (DR.). Such detectors characteristically exhibit high spatial resolution and conversion efficiency that is a function of the applied electric field. About 50 electron volts of photon energy are required to generate each electron-hole pair in a typical amorphous selenium x-ray conversion layer biased at 10 volts per micron. At FFDM and DR imaging x-ray energies each absorbed photon can generate only about 250 to 1000 electron-hole pairs. Medical imaging applications must therefore employ low noise thin film transistor (TFT) arrays and charge integration amplifiers to achieve high signal-to-noise ratio (SNR) and detective quantum efficiency (DQE). To assure quantum-noise limited imaging results with the lowest practical x-ray exposure dose, it is desirable to include an additional low-noise gain stage in the x-ray conversion layer. We have proposed and studied a new structure for an amorphous selenium detector that employs an internal biased gain grid to cause avalanche-gain within the x-ray conversion layer. An amplification of at least 10X can be achieved without introducing excessive noise. Quantum-limited image detection should then be attainable for even very low exposures.