Kaleidoscopic Scintillation Event Imaging

TL;DR

This work tackles the challenge of imaging individual scintillation events with cameras under photon-starved conditions. It introduces a kaleidoscopic scintillator to boost light collection while preserving spatial information and formulates a Gaussian mixture model whose components encode the event and mirror reflections, solved with EM to recover the 3D target location. Experimental validation with a SPAD camera and gamma source demonstrates sub-millimeter localization accuracy across brightness levels, and simulations show substantial improvements over non-kaleidoscopic methods. The approach enables high-resolution, camera-based radiation imaging and paves the way for advanced detectors like Compton or neutron cameras that rely on imaging individual events.

Abstract

Scintillators are transparent materials that interact with high-energy particles and emit visible light as a result. They are used in state of the art methods of measuring high-energy particles and radiation sources. Most existing methods use fast single-pixel detectors to detect and time scintillation events. Cameras provide spatial resolution but can only capture an average over many events, making it difficult to image the events associated with an individual particle. Emerging single-photon avalanche diode cameras combine speed and spatial resolution to enable capturing images of individual events. This allows us to use machine vision techniques to analyze events, enabling new types of detectors. The main challenge is the very low brightness of the events. Techniques have to work with a very limited number of photons. We propose a kaleidoscopic scintillator to increase light collection in a single-photon camera while preserving the event's spatial information. The kaleidoscopic geometry creates mirror reflections of the event in known locations for a given event location that are captured by the camera. We introduce theory for imaging an event in a kaleidoscopic scintillator and an algorithm to estimate the event's 3D position. We find that the kaleidoscopic scintillator design provides sufficient light collection to perform high-resolution event measurements for advanced radiation imaging techniques using a commercial CMOS single-photon camera. Code and data are available at https://github.com/bocchs/kaleidoscopic_scintillator.

Figures (18)

• Figure 1: Method overview. An image is composed of light that is emitted from the scintillation event and reaches the camera either directly or after reflecting off mirrors of a kaleidoscopic scintillator. The spatial relationship between the event and mirror reflections is embedded in a Gaussian mixture model whose likelihood is maximized to estimate the event's location using the EM algorithm.
• Figure 2: Imaging parameters and coordinate systems. The figure only shows light emitted directly from the event to the camera.
• Figure 3: Image truncation example in 3D. The image of an event and one mirror reflection is shown. Light corresponding to the mirror reflection is truncated at the mirror's edge, resulting in a truncation line in the image. The truncation line only applies to that mirror reflection.
• Figure 4: Simulated kaleidoscopic image with theoretical acceptance zones. Acceptance zones are derived for each mirror reflection and overlaid in gray on the image for the (a) +$x$, (b) +$y$, (c) -$x$, and (d) -$y$ mirror reflections.
• Figure 5: Selected experimental images. Experimental images overlaid with the algorithm's estimated Gaussian components. Each dashed red circle is centered on the Gaussian component's mean. The inner and outer circles are one and two standard deviations in radius, respectively. Pixels with a photon are enlarged with a $3 \times 3$ filter for visualization purposes. The number of counts in the image and the algorithm's estimated event location are shown in each image.