29.06.2018 Amorphous lead oxide (a-PbO) for application in direct conversion X-ray detectors: use of polymorphism to suppress signal lag

Prof. Dr. Alla Reznik, Lakehead University and Thunder Bay Regional Health Research Institute, Canada

Date: Friday, 06.07.2018, 10:00 h - 12:00 h
Location: Department of Physics, Renthof 6, seminar room 00014

Abstract

Semiconductor-based direct-conversion X-ray detectors have been actively sought for a wide range of x-ray applications in the fields of domestic security and medical imaging. Lead Oxide (PbO) is one of the most promising high Z (atomic number) candidates for this purpose due to high X-ray stopping power, high theoretical X-ray-to-charge conversion gain and proven technological compatibility with a-Si electronics required for a large detector area coating. Polycrystalline modification of lead oxide (poly-PbO) has a long and successful history of applications in optoelectronics where it was employed as a phototarget in Plumbicons – the most extensively used pick-up tubes for broadcasting. The success of Plumbicons triggered great interest in utilizing PbO layers for X-ray medical imaging detectors. Unfortunately, poly-PbO photoconductive layers were reported to be challenging for applications in real-time X-ray medical imaging due to the presence of a residual signal after exposure termination, called signal lag. Signal lag leads to inadequate temporal characteristics and therefore it is a bottle neck of performance for many non-crystalline materials, considered for dynamic radiation sensing. Here we analyze causes of signal lag in poly-PbO layers and show that a technological step to replace nonhomogeneous disorder in polycrystalline PbO with homogeneous amorphous PbO structure suppresses signal lag and improves time response to X-ray irradiation. Our study suggests a way to make detector-grade PbO layers free of lag. The proposed advances of the deposition process are low cost, easy to implement and with certain customization might potentially be applied to other materials, thus paving the way to their wide-range commercial use.