Three-dimensional event localization in bulk scintillator crystals using optical coded apertures

Scintillator-based detectors are among the most commonly used methods for detecting ionizing radiation. Scintillators provide a reliable, cost-effective, and simple way to make large-volume detectors. Furthermore, localizing the position of the interactions in three dimensions within the crystals is useful to a wide array of fields. The most straightforward way of doing this is to pair the crystal with a position-sensitive phototransducer (PT). This allows for measurement of the shape of the light spot at the PT plane. Using this information, various methods exist to localize the gamma-ray interaction in the crystal; however, the position resolution worsens the farther the event occurs from the PT plane. To improve on the localization ability, this work uses an optical coded-aperture shadow mask between the crystal and the PT. The recorded detector response is used in reconstructing the event over the entire depth of the crystal, and the 'sharpest' reconstructed image gives an event's depth. The lateral position is given from the standard coded-aperture image reconstruction. Experimental results obtained by emulating a 26-mm-thick crystal using a thin 1-mm-thick NaI(Tl) crystal and different amounts of light pipe between the crystal and the PT plane achieved $\sim $1 to 2-mm resolution in all three dimensions throughout most of the 26-mm-thick crystal.