Development of a scintillation-based CMOS Quantitative Autoradiography imager for Safeguard Applications
Ardelia Michelle Clarke
Dissertation under the direction of Professor Todd Peterson
This dissertation involves an interdisciplinary approach involving physics, materials science, and imaging science focused on the development of a large-area ionizing-radiation Quantum Imaging Detector (LAiQID) as a real-time, portable, event-counting, large-area, high spatial resolution and sensitivity, and good energy response, digital autoradiography imager for safeguards and nuclear nonproliferation applications with particle and energy discrimination capabilities. The LAiQID consists mostly of commercial off-the-shelf (COTS) components, with a scintillator or phosphor screen in direct contact with the large end of a fiber-optic taper, which is optically coupled to a microchannel plate (MCP) image intensifier, the output of which is lens-coupled to a complementary metal-oxide-semiconductor (CMOS) camera with the intensified scintillation events displayed. Through a series of characterization measurements, I showed several beneficial performance characteristics of the LAiQID for safeguards and other imaging applications.
First, the LAiQID offers high intrinsic spatial resolution (100-200 um) for alpha and beta particles. This beneficial finding is based on the phosphor screen chosen, particle type, and large area due to the fiber-optic taper. The taper was found to contain non-uniformity as the CMOS light collection efficiency drops by a factor of two from the center to the edge of the phosphor’s screen. Despite such nonuniformity, the detector’s collection efficiency was not impacted because detected events were higher than the low detection threshold of 60 pixels. The intrinsic efficiency of the LAiQID for alphas and high-energy betas was measured at nearly 100%. Secondly, the LAiQID was found to have a sensitivity to a range of particles and energies from >50keV electrons and gammas to 6 MeV alpha particles. Minimum detectable activity (MDA) ranges for the LAiQID were characterized between 0.01- 0.1 Bq/cm2 for beta-emitters and 4 mBq/cm2 for beta-emitters. The count-rate limits for the large-area iQID were explored and compared to a previously-smaller area iQID system. As a result, the large-area iQID has a greater count-rate capability (lower theoretical and experimental spatial pileup) than the previous iQID. Both smaller and large-area iQID systems have different imaging area of 40 cm^2 and 100
2cm^2 , respectively.
Next, I developed a spatial pileup Monte-Carlo (MC) algorithm to simulate whether the LAiQID can handle count-rates (107 Bq) related to the high priority samples found in safeguards. This was validated experimentally using a gated image intensifier (provides the ability to control how much light is detected on CMOS camera) and 506 uC 241Am to induce spatial pileup. Quantitative imaging with LAiQID occurred at 1% duty cycle (percentage of time the CMOS camera can detect radiation). Performance tradeoffs were observed between CMOS sensor ability to detect events and post-processing algorithms throwing out oversaturated counts or missing counts when duty cycle was either higher (5%) or lower (0.1%) than 1%.
Future work should investigate discrimination capabilities between gamma and beta events due to its usefulness in safeguard applications to detect trace amounts of nuclear material and prevent nuclear material proliferation. It would also be worth investigating whether the optimal duty cycle for quantitative imaging changes based on its radiation interaction (alpha, beta, photon).||