Time-coded apertures for imaging in high-background environments

There is an increasing need for systems capable of imaging gamma-ray sources at energies up to 1 MeV and beyond. Conventional collimation becomes ineffective at these energies because a significant fraction of the higher energy radiation penetrates the shielding resulting in a high level of background on the detector. To reduce effects of background, the authors examine the use of a time-coded pinhole aperture array. At each step of the temporal code, the source is viewed by a number of pinholes, multiplexing the images onto the detector. Since several holes are open simultaneously, the effective signal-to-background ratio is improved, and furthermore, the array of smaller pinholes preserves spatial resolution over a single, larger pinhole. Using simulations, the authors have compared the performance of a temporal code having 27% mean transmission with both a single pinhole system and a scanning pinhole code (one hole open per time step and a background measurement with all holes closed). Variance in source estimates demonstrates a significant improvement for the 27%, transmission code over the scanning and single pinholes at all background levels