Lanthanum bromide-based rotational modulation gamma ray imager

A time modulation imaging device uses a periodic structure which translates or rotates above one or few position-insensitive detectors. One common design, a Rotational Modulation Collimator (RMC) uses a bi-grid collimator which rotates above a single detector, and is able to attain very good angular resolution. The two grids cost sensitivity and weight, however, making the RMC unattractive for certain applications. A Rotational Modulator (RM) consists of a single grid of transparent and opaque slats of width a, above an array of several detectors with diameter d, subject to the constraint a = d. The sensitivity, weight, and angular resolution can be comparable to that of a coded aperture device. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. This count profile is cross-correlated with pre-calculated modulation profiles to produce an initial source image. Further processing of the image with a “cleaning” technique that incorporates information from the point-spread function can accurately resolve point sources. In an RMC imager recently constructed at LSU, LaBr3:Ce detectors are used, which produce significantly more light than other common scintillators, offering ≪ 3% FWHM energy resolution at 662 keV. The instrument features high sensitivity and energy resolution, angular resolution of 0.8° (1σ), and a simple readout system. The detector array consists of 19 1:5″×1″ thick LaBr3:Ce detectors in a concentric circular layout. A grid spaced ∼1.2 m from the detection plane with slat width 1:5″ offers a field of view radius of 6:9°. We present our reconstruction technique, cleaning algorithms, and imaging results for the RM prototype.