Imaging performance of Lanthanum Bromide scintillators with wavelength shifting fiber readout

Astrophysical X-ray/gamma-ray telescopes and standoff detectors for national security applications may require detector areas on the order of a meter squared or more. A fine grained meter squared scale scintillator with mm-scale resolution may require ~10⁶ pixels and electronics channels. In many applications, this can be prohibitive. Since LaBr₃:Ce produces significantly more scintillation light (63 photons/keV) than other materials, it offers the possibility of a crossed optical fiber readout approach needing only a few thousand channels. A layer of 2 mm square, double clad waveshifting fibers can be laid in the x-direction across the top of a LaBr₃ detection plane (separated from the LaBr₃ by a thin glass seal that provides a moisture barrier), with a second layer of fibers in the y-direction laid across the bottom. With an absorption peak matched to the emission of LaBr₃, the fibers will absorb the scintillation light and re-emit it isotropically, a portion of which will be trapped in the fiber and propagated down the fiber axis, where the fiber ends are viewed by 64-channel MAPMTs. The crossed fiber layers are intended to measure x- and y-position only. Since only a small fraction of the light is trapped in the fibers, the energy is measured in nine large "energy measuring" PMTs viewing the scintillator through the bottom fiber layer. A lab-scale crossed-fiber LaBr₃:Ce imager has been constructed as a protype for this concept. Results of measurements of energy, position, and angular resolution are given. The use of pre-processed Detector Pixel Source Image (DPSI) files, which provide a significant speedup in the computationally intensive reconstruction process, is explained in detail.