Imaging results with a LaBr3-based rotational modulator

A gamma-ray imager based on rotational modulation is capable of obtaining significantly better angular resolution than the fundamental geometric resolution defined by the ratio of detector diameter to mask-detector separation. Compared to a standard coded aperture or Compton telescope, a Rotational Modulator (RM) requires a detection plane with only modest spatial resolution, and is consequently less complex. A prototype of this concept developed at LSU features high sensitivity and energy resolution, and a simple readout system. The instrument consists of a single grid of 1.5" wide lead slats separated by 1.5" and spaced 1.2 m in front of an array of 19 1.5" × 1" thick LaBr
₃
:Ce scintillators in a concentric circular layout. 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. A novel reconstruction technique deconvolves this image with the point-spread function to reduce the effects of noise and resolve sources to within 20', or about 6x better than the geometric resolution of the instrument. We describe this reconstruction technique, and present imaging and spectral results for the RM prototype.