Speaker
Description
Inorganic scintillators are commonly used in various gamma spectroscopy applications due to their excellent energy resolution, reliable performance, relatively low cost, and high detection efficiency. Nevertheless, many inorganic scintillators have high refractive indices and experience significant light losses at the collection surface caused by total internal reflection (TIR). This project uses optimized periodic nanostructures, also known as photonic crystals (PHCs), to recover some of the light initially lost due to TIR. The PHC layer creates an optical pathway through constructive interference between the scintillator and the photosensor for the originally trapped light photons. Enhancing the light extraction from an inorganic scintillator also improves its energy resolution, time resolution, and overall detection efficiency. This work has many applications, particularly in nuclear security and nonproliferation, where enhanced energy resolution is crucial. It also holds significant potential for medical applications requiring an improved time resolution. During this project, we first simulated an optimized PHC geometry to maximize the light output using codes Geant4 and OptiFDTD. The optimization simulations for a 10 x 10 x 3 $mm^3$ LYSO scintillator coupled with 2D block structure $Si_3N_4$ PHC show an improvement in light output of more than 60% for a single light pass. Following the simulations, manufacturing efforts were made to reproduce the proposed PHC geometry and then characterize the scintillators with and without the PHC deposition. The manufacturing process for the block structure PHC geometry involves electron beam lithography followed by reactive ion etching. The required electron beam lithography resolution depends on the spacing between the PHC. For the initial tests, an unoptimized PHC geometry was selected to troubleshoot and optimize the manufacturing process for our scintillator substrate (LYSO). The LYSO samples deposited with PHC were then characterized through radiation measurements with a Cs-137 source. The radiation measurements showed an improvement in pulse size and total counts, which suggests an improvement in the scintillator's light output. The results also showed an improvement in the energy resolution of the scintillator, which for a specific LYSO sample improves from 12.5% for a bare sample to 11.9% for a PHC deposited sample. Air coupling between LYSO and a PMT was used for simplicity. The work so far demonstrates the potential of PHC structures for improving the overall light output and energy resolution of a LYSO scintillator. The reported results are for a LYSO sample with approximately 80% PHC coverage and unoptimized PHC geometry. Future work will include further improving the PHC manufacturing quality and manufacturing fully optimized nanostructures with greater surface coverage. Future work will also include simulations for lanthanum bromide and sodium iodide scintillators; due to their high refractive indices their light output and energy resolution should be improved more substantially than for LYSO. Finally, other PHC manufacturing methods will be explored, such as spin coating to produce PHC nanospheres.
