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 LaBr₃: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.