Abstract
The photoconductor layer is an important component of direct conversion flat panel X-ray imagers (FPXI); thus, it should be carefully selected to meet the requirements for the X-ray imaging detector, and its properties should be clearly understood to develop the most optimal detector design. Currently, amorphous selenium (a-Se) is the only photoconductor utilized in commercial direct conversion FPXIs for low-energy mammographic imaging, but it is not practically feasible for higher-energy diagnostic imaging. Amorphous lead oxide (a-PbO) photoconductor is considered as a replacement to a-Se in radiography, fluoroscopy, and tomosynthesis applications. In this work, we investigated the X-ray sensitivity of a-PbO, one of the most important parameters for X-ray photoconductors, and examined the underlying mechanisms responsible for charge generation and recombination. The X-ray sensitivity in terms of electron–hole pair creation energy, W±, was measured in a range of electric fields, X-ray energies, and exposure levels. W± decreases with the electric field and X-ray energy, saturating at 18–31 eV/ehp, depending on the energy of X-rays, but increases with the exposure rate. The peculiar dependencies of W± on these parameters lead to a conclusion that, at electric fields relevant to detector operation (~10 V/μm), the columnar recombination and the bulk recombination mechanisms interplay in the a-PbO photoconductor.
Highlights
The ever-growing demand for advanced radiation medical imaging techniques sustains continued research interest in novel materials and technologies for imaging detectors, based on the direct conversion of diagnostic X-rays
In direct conversion flat panel Xray imagers (FPXIs), a uniform layer of the photoconductor is deposited over large area readout electronics based on either thin-film transistor (TFT) arrays or complementary metal-oxide-semiconductor (CMOS) active-matrix arrays
Stabilized amorphous selenium (a-Se) is the most successful, commercially viable, large-area-compatible X-ray photoconductor used in direct conversion FPXIs for medical imaging due to its several distinct advantages over other potentially competing photoconductors [1,2]
Summary
The ever-growing demand for advanced radiation medical imaging techniques sustains continued research interest in novel materials and technologies for imaging detectors, based on the direct conversion of diagnostic X-rays. Stabilized amorphous selenium (a-Se) is the most successful, commercially viable, large-area-compatible X-ray photoconductor used in direct conversion FPXIs for medical imaging due to its several distinct advantages over other potentially competing photoconductors [1,2] Both X-ray-generated electrons and holes can drift in a-Se under appropriate conditions [3,4]. For higher energy applications such as radiography, fluoroscopy, and tomosynthesis, a-Se does not have sufficient X-ray stopping power, and alternative materials are needed to expand the success of the direct conversion concept over the diagnostic energy range In this regard, one should focus on the development of high-Z (atomic number), widebandgap photoconductors that can efficiently attenuate high-energy X-ray photons and are compatible with the FPXI technology. A trapping mechanism can be excluded from the reasons for the carrier loss in a-PbO and will not be discussed further
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