Sensitivity-optimized wide-field imaging with a CZT-based coded mask imager

Coded mask imaging can produce excellent images in the hard X-ray range from ∼10 to 200 keV, and up to several MeV in the gamma-ray range. Sub-degree resolution is obtained for hard X-rays and a few-degree resolution for gamma-rays. However, these capabilities are realized with large instruments, several meters in length, and limited fields of view, tens of degrees, which are not suitable for large-area surveillance to assess radiological threats. We have developed coded mask imaging techniques to address this deficiency. They operate into the MeV range and provide high sensitivity over a wide FOV, 140 degrees, with a single instrument. This is possible by designing for limited angular resolution, ∼20 degrees, in order to reduce the otherwise negative consequence of vignetting by the mask. With this resolution, few-degree localizations are obtained with detections of ∼10 sigma significance, which usually provides adequate positioning to associate a radiation source with a specific object for follow-up investigation. We report on the design, modeling, and tests of a hand-held CZT-based coded mask imager that uses these techniques to image a 140 degree FOV with a nearly constant resolution of 20 degrees. The 7.5 mm thick coded mask provides good modulation and, therefore, good sensitivity up to 1 MeV and beyond, and the sensitivity is nearly uniform across the FOV due to an optimized coded mask/detector array design. The imager employs CZT pixel detectors that provide ∼1% energy resolution at 662 keV