Candidate: Sahar Adnani
Date: October 3, 2023
Time: 10:00 AM
Location: Remote Attendance
Supervisor(s): Karim, Karim Sallaudin
Abstract:
This work is an investigation of amorphous Selenium (a-Se) based semiconductor detectors for high-energy radiation imaging applications. The work focuses on enhancing the detection efficiency of a-Se while maintaining spatial resolution and linearity. Additionally, various detector configurations are explored along with materials for reduced dark current and the implementation of novel readout methods.
Simulation results demonstrate that stacking a-Se based detectors with a pixel size of 1 mm can significantly enhance detection efficiency without substantial degradation of spatial resolution for detection of single photon emission computing tomography (SPECT) gamma radiation. A careful balance between energy thresholds and the number of layers reveals that 52% detection efficiency is achievable for a 110 keV energy threshold using 10 detector layers. This approach effectively reduces noise counts arising from interlayer scattering. In addition, a 50 µm pixel pitch detector exhibited lower detection efficiency for high energy thresholds, hence requiring more detector layers to be comparable to the 1 mm pixel detector. The findings indicate that proper design of a-Se stacked detectors can offer a cost-effective solution for large-area radiation imaging applications with the potential to achieve sub-millimeter resolution.
A resistive microstrip detector design coupled with charge division readout is introduced to overcome the complexity of stacking multiple a-Se detector layers in practice. The experimental results verified 1 mm spatial resolution with high linearity over the microstrip length. The proposed bottom microstrip design, along with demonstration of this design on a flexible substrate not only simplifies readout circuit complexity but also facilitates the realization of curved multi-layer detectors for high-energy radiation imaging applications.
To address dark current issues associated with the novel detector design, this work explores the utilization of low-temperature SU-8 as a hole-blocking layer for a-Se detectors. Results showcase the viability of achieving dark currents below 1 pA/mm2 even at increased electric fields of 40 V/µm. The adoption of the bilayer configuration, incorporating both SU-8 and Cs-doped a-Se, further enhances sensitivity and minimizes lag. This advancement holds promise for the development of high-performance selenium radiation detectors suitable for dynamic medical imaging.
The new detector design with resistive microstrips and the new hole-blocking bilayer was employed in a photon counting setup. Challenges related to noise are addressed through the incorporation of the commercially available CUBE charge preamplifier and the design of a custom PCB board. The successful capture of photons and the generation of Pulse Height Spectra (PHS) validate the effectiveness of the novel approach, opening the door for further work on charge division and multi-channel PHS results.
Overall, this work contributes to the utilization of a-Se semiconductor material for new radiation imaging applications and especially, for realizing high-energy radiation imaging applications that have not been traditionally associated with lower atomic number (Z=34) a-Se photoconductor. The proposed advancements include a comprehensive range of innovations, including higher detection efficiency, reduction in dark current at high voltages, noise mitigation, and a simplified large area image readout scheme that can expedite the commercial development of a-Se detectors for new higher radiation energy medical, industrial, and scientific imaging applications.