Instrumentation Division Seminar

We present the first solid state avalanche amorphous selenium (a-Se) radiation detector optimized for medical imaging applications. Low dose medical x-ray imaging is currently limited by the electronic noise and the limited size of detectors. Our design utilizes a layer of avalanche amorphous selenium called High Gain Avalanche Rushing Photoconductor (HARP) to amplify photogenerated charges prior to electronic readout. A-Se allows for large area, uniform deposition compared to crystalline photoconductors. Additionally, it is the only amorphous material capable of avalanche multiplication (at electric fields > 70 V/um). We have previously developed the first solid state HARP multi layer structure. Our current detector is an indirect sensor referred to as Scintillating HARP Active Matrix Flat Panel Imager (SHARP-AMFPI), which multiplies photogenerated holes produced by the optical photons from a high-resolution x-ray scintillator. A structured 150 um CsI:Ti scintillator coupled to a fiber optic face plate was used to convert x-rays to optical photons. The optical photons are absorbed into the 15 um HARP layer capable of producing reliable and uniform avalanche gain of 75 over the 24 cm x 30 cm active area. The AMFPI consists of a commercial TFT array with an 85 um pixel pitch. For this work we used a 30 kVp Mo/Mo spectrum which is similar to that used for mammography. Images at low exposure levels without avalanche gain are dominated by the electronic noise of the TFT array. Quantum noise limited images are obtained as the electric field is increased above the avalanche threshold.