Inorganic scintillator surface enhancements with 2-D photonic crystals to improve light collection

Abstract

Inorganic scintillators are widely used in various applications of gamma spectroscopy such as nuclear nonproliferation and safeguards, medical applications, space applications, and astronomy. This is due to good energy resolution, stable performance, somewhat low cost, and relatively high detection efficiency. However, many inorganic scintillators have high refractive indices and suffer significant light losses due to total internal reflection (TIR). This project proposes using optimized periodic nanostructures called photonic crystals to recover some of the light originally lost due to TIR. Photonic crystals provide an optical bridge (constructive interference) between the scintillator and the photosensor for the trapped light photons. Improving the light extraction can improve the energy and time resolutions of the scintillator, allowing for a wider range of research and industry applications. Photonic crystals can be optimized in terms of their dimensions, shapes, and materials to maximize the light extraction. Preliminary optimization tests were performed using a LYSO scintillator coupled with Si3N4 photonic crystals. First, a realistic light input source is obtained by simulating the scintillation process in Monte Carlo code Geant4. The simulated scintillation photons are collected at the LYSO-PMT boundary to obtain their energy and angular distributions. In the next step, a deterministic code OptiFDTD is used to simulate light interactions with different nanostructures. Currently, the simulations are limited to 2-D block nanostructures. The optimization tests vary the height, width, and spacing of the photonic crystals. Preliminary optimization tests show an improvement in the light transmission by more than 60%. The optimized geometry will be manufactured in the lab using various manufacturing techniques such as ion milling, electron beam lithography, or 3D printing. Various gamma sources will be used to experimentally characterize the LYSO scintillators with and without photonic crystals. These experiments will also be used to validate the simulations and demonstrate the effectiveness of the photonic crystals in improving the energy resolution. Once validated, the simulations will be used to determine optimized photonic crystals for other inorganic scintillators, such as bismuth germanate, sodium iodide, and lanthanum bromide.