Detection of ionising radiation using single photon avalanche diodes
Item statusRestricted Access
Embargo end date31/07/2022
Bulling, Anthony Frederick
Single photon avalanche diodes (SPADs) are highly sensitive solid-state photon detectors, and since their inception in 2003 into complementary metal oxide semiconductor (CMOS) technologies, have provided a platform of highly integrated and low-cost mass manufacture detectors, capable of rapid detection of single photons. The consequent commercial potential has led to the development and availability of a range of CMOS SPAD sensors and arrays. As a consequence of their mature development in advanced processes, detection sensitivity and timing capabilities, CMOS SPADs have found their way into multiple applications, most notably biomedical fluorescence lifetime imaging microscopy (FLIM), Light Detection and Ranging (LiDAR), Time-of-Flight (ToF) imaging, Single Photon Counting (SPC), high speed imaging, biological particle tracking and more recently visible light communications. Although, all current applications are constrained to the visible and near infrared spectrums. The advances and broad applications of CMOS SPADs has led to increased development towards further miniaturisation, performance improvements and developing highly integrated intelligent sensors. Therefore, the investigation of the application of SPADs in the detection of ionising radiation has significant commercial potential as an alternative technology to current detectors. The goal of this research is to explore the direct detection of ionising radiation with CMOS SPADs. The fundamental mechanism for detection is a depleted junction operated in Geiger mode, with an induced electric field that allows sensitive detection of generated electron-hole pairs as a result of incident radiation. Therefore, based on this principle it was hypothesised that SPADs can be applied beyond current photon specific detection applications into the detection of ionising radiation. Furthermore, high energy physics is transitioning towards CMOS processes with all the unprecedented advantages it provides. Therefore, it is believed that with SPAD advancement and maturity in CMOS technology, this parallel and continually developing technology may lend itself favourably towards progress and application in the detection of ionising radiation. This work reports on a 3D-stacked backside illuminated (BSI) CMOS SPAD image sensor for the detection of accelerated electrons, pions and X-rays, utilising a scanning electron microscope, synchrotron particle accelerators and X-ray tube sources respectively. For accelerated electron detection, electron energies from 5 to 30 keV were detected, and statistical significance was found that both SPAD excess bias voltage and/or incident accelerated electron energy result in a distinct output. Furthermore, the SPAD image sensor was able to achieve time-resolved imaging of the electron beam raster scan pattern. For X-ray detection, X-rays with peak photon energies from 30 to 160 keV were detectable using an X-ray tube, and it was found that an increase in either SPAD bias voltage, output beam voltage or beam intensity results in higher relative average counts per pixel, therefore demonstrating the potential application of SPAD image sensors in X-ray imaging. These results are the first demonstration and application of a CMOS SPAD in the detection of accelerated electrons and X-rays. Further investigation revealed that the attenuation of lower energy photons from an X-ray tube spectrum results in approximately 200 % increase in relative average counts. Furthermore, The BSI CMOS SPAD image sensor was irradiated with a high energy pion beam at 120 GeV, using the Super Proton Synchrotron (SPS) at CERN, and high energy electrons at 2.5 GeV, using an electron accelerator at ELSA. For the pion irradiation, pions were conclusively detected, and it was found that an increase in SPAD bias voltage results in higher relative average counts per pixel. No conclusive detection of higher energy electrons was observed as a result of low beam intensity. After the pion irradiation, radiation damage to the SPAD image sensor was observed. These results are the first demonstration and application of a CMOS SPAD in the detection of high energy charged particles.