The other end of the scale: Coded apertures in the near field for highresolution 3D gamma event localization in bulk scintillators

Coded apertures were originally developed by the high-energy astrophysical community for use in imaging high-energy photons (x- and γ-rays) for which focusing optics are ineffective. We are now taking what was developed as a tool for use in the extreme far field at high energies to encode spatial information at optical wavelengths in the extreme near field to enhance the performance of position-sensitive x- and gamma-ray scintillator detectors. Spatial resolution for events within bulk scintillators is limited by the size of the light "spot" available at the sides of the scintillator, where phototransducers convert the light to an electrical signal. The ability to localize an event is determined by how well one can determine the centroid and the size of the spot. Generally, performance is limited to many millimeters in all three spatial dimensions, and one cannot resolve simultaneous events that are closer together than the width of the light spot (frequently of order 10 mm). For this reason, many applications requiring the finest spatial resolution subdivide the scintillator into tiny elements and use a digital approach to determine event location. However, that technique significantly complicates the overall instrument and sacrifices energy resolution because the light collection efficiency varies with event location within the subdivided scintillator. We are building a device that overcomes these shortcomings by using an optical coded-aperture shadow mask between a bulk crystal and a position-sensitive phototransducer. Simulations indicate that we can achieve millimeter-scale localization in all three spatial dimensions while resolving simultaneous energy depositions. The technique and progress toward its realization will be presented.