Charge recombination and electron-hole pair creation energy in amorphous lead oxide X-ray photoconductor

The X-ray sensitivity is one of the most important parameters for X-ray photoconductors; it is usually described in terms of the electron-hole pair creation energy W-+/- - the average energy required to generate a single detectable pair of electron and hole. This parameter accounts only for the detected carriers, reflecting that only a fraction of the X-ray generated charges are collected, while the rest are lost through the recombination processes. The charge recombination results in an effective W-+/- that is not a material parameter but rather a characteristic of the detection system, which remains larger than its theoretical value W-+/-(0) and depends on the applied electric field F, X-ray energy E and exposure X. In this work, we characterize the X-ray sensitivity of amorphous lead oxide (a-PbO) photoconductor, which is considered as one of the most promising replacements for amorphous selenium (a-Se) for radiographic, fluoroscopic, and tomosynthesis applications, and examine the underlying mechanisms responsible for charge recombination. W-+/- was measured in a wide range of electric fields (1-20 V/mu m), X-ray exposures (0.02-2 R) and energies (29-51 keV). W-+/- decreases with both the electric field and X-ray energy, saturating at 18-32 eV/ehp depending on the energy. W-+/- increases with radiation exposure, especially at lower fields and higher energies. The particular dependencies of W-+/- on these parameters, especially X-ray energy-dependent W-+/-, conclude that the columnar recombination mechanism dominates in the a-PbO photoconductor with a secondary contribution of the bulk recombination.