Engineering of a Blocking Layer Structure for Low-Lag Operation of the a-PbO-Based X-Ray Detector
Oleksandr Grynko, Tristen Thibault, Emma Pineau, Alla Reznik
Abstract
Direct conversion flat panel detectors are of great significance to the field of medical X-ray imaging since they offer imaging performance and diagnostic capabilities not achievable with other methods. Currently, mammographic direct conversion detectors employ a layer of amorphous selenium (a-Se) photoconductor. Although its properties ideally fit the requirements of mammography, where “soft” X-rays are used, a-Se cannot be used in high-energy X-ray procedures. To extend the diagnostic capabilities of the direct conversion detectors, amorphous lead oxide (a-PbO) is proposed as an alternative photoconductor. It is a high effective atomic number material and thus has a higher X-ray stopping power over the wide X-ray energy range. a-PbO is, therefore, a suitable candidate for applications in radiography, fluoroscopy, and digital tomosynthesis. Here, we report on the development of a blocking structure with a polyimide (PI) layer needed to maintain low dark current at high electric fields. We demonstrate that a 1- μm-thick PI blocking layer allows the operation of the detector at strong electric fields (≥10 V/ μm) while suppressing the dark current to an innocuous level (<; 1 pA/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). It also improves temporal performance by reducing signal lag. No ghosting effect was observed at exposure rates up to 1 R/s; however, at high radiation levels, the detector's sensitivity degraded. This degradation is not permanent as the detector restores its original sensitivity after several hours of rest in the dark without bias applied.