Lead oxide (PbO) for direct conversion detectors
Doctor of Philosophy
DisciplineChemistry and Materials Science
SubjectDirect conversion flat panel X-ray detectors
Lead oxide for direct conversion detectors
Medical imaging detectors
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There is great interest in the utilization of non-crystalline photoconductors for direct conversion medical X-ray imaging detectors. Lead Oxide (PbO) is one of the most promising candidates for application in general radiography and fluoroscopy since it possesses high theoretical X-ray-to-charge conversion gain and high X-ray stopping power due the high Z of Pb (Z = 82). A further advantage of PbO compared with other photoconductors (like polycrystalline layers of HgI2, PbI2, CZT) is the absence of heavy absorption edges up to 88 keV, which inherently offers higher spatial resolution. Therefore, PbO exhibits many of the requirements for an effective detector material. However, although very promising, poly-PbO layers are known to exhibit a relatively high dark current, an incomplete charge collection and a residual signal after the end of X-ray exposure, called signal lag. The reported lag was the main obstacle of this poly-PbO-based detector prototype, since this restricts applications to static imaging only and obscures the full potential of PbO in medical imaging. Another disadvantage is the high porosity of poly-PbO and its structural instability in air, which makes this material challenging for practical utilization in X-ray detectors. To combat the above problems (i.e. lag, porosity and structural degradation in air) we have advanced PbO deposition technology and developed a novel type of amorphous lead oxide (a-PbO). The obtained a-PbO layers have near single-crystal bulk densities and are stable in air. In addition, the layers are capable of withstanding higher electric fields, while at the same time iii exhibiting lower dark currents than previously achieved. Also, the temporal response and charge yield were significantly improved and compare favorably with published results on conventional films of polycrystalline PbO and even a-Se – the only photoconductor currently utilized in direct conversion detectors. Our measured X-ray response show almost complete elimination of signal lag to a level sufficient for the high-speed operations. Our advances in PbO technology allow us to utilize the full potential of PbO for medical X-ray imaging applications.