Simulation and optimization of a novel multilayer lead micro wires converter of a gas detector for mammography applications

Abstract

The position sensitive gas detectors for hard X-ray imaging usually consist of a photon to electron bulk converter and gas electron multiplier. But the bulk converter has its own limitation. The Anodizing Aluminum Oxide (AAOs) membrane with higher surface to volume ratio in comparison to bulk ones is a good candidate to fabricate arrays of Micro Wires (MWs) as porous converter. In order to use this MW converter for mamography applications, first of all for the X-ray energies in the range of 15 to 35 keV, the optimum thickness of the bulk Lead converter for having the maximum quantum efficiency (QE) was estimated. Then for the optimum thickness (1 μm), the diameter of Pb wires of MW converter was optimized to achieve the maximum QE. The optimum diameter of MW converter at this energy range is 360 nm. In second step, the QE of MW converter with optimum thickness and diameter which are 2 μm and 300 nm, respectively, are compared with the 2 μm thickness of bulk ones. For one layer of bulk converter with the optimum thickness of 1 μm, at 25 keV the maximum QE is around 0.85%. For 2 μm thickness, the maximum QE of MW converter occurs at 25 keV (1.05%) and for the bulk ones at 30 keV (0.8%). Although, the QE of MW converter is greater than the bulk ones, but for medical imaging applications this QE is not sufficient. Therefore, the idea of multilayers of Pb MW is proposed. The simulation results for the multi-layers of 2 μm Pb MW converters, show that in the energy range of 15 to 35 keV, the QE is maximum. So, at this energy range, the MW converter with 2 μm thickness and 300 nm pore diameter is considered as the optimum dimensions of converter. The simulation results show that for 40 layers of MW converter, the maximum QE is around 4.5 times greater than bulk ones. For 15, 18, 20, 25, 30 and 35 keV X-ray energies, the QEs of 40 layers of MW converter are around 9.6%, 12.5% 18.8%, 20.3%, 25% and 20%, respectively. These results clearly show that the proposed multilayer MW has reasonable QE for mammography applications.