Pixelated Cadmium Zinc Telluride (CZT) detectors provide the opportunity to perform spectroscopic imaging for discriminating one radioactive material from another. Although Compton interactions provide a means for imaging high energy gamma sources, identification of materials emitting lower energy signatures are better suited to collimator imaging techniques. This paper specifically considers a multiple pinhole method for its simplicity of pinhole focusing combined with straightforward processing methods for incorporating multiple apertures to reduce photon collection time while retaining image resolution. Multiple pinhole image detections are combined using an iterative Maximum- Likelihood Expectation-Maximization (MLEM) synthetic imaging algorithm. To enable subsequent field operations, the imaging system matrix is computed using an imaging model with adjustable parameters rather than one experimentally acquired from point sources. The system model includes an object space, a multiple pinhole collimator plane, and a pixelated detection plane. The modeled object space is implemented in two dimensions to reduce image reconstruction burden since 3D imaging is not practical for single view stand-off imaging. Focusing is modeled by a function computing photon trajectory and passage through the pinhole patterned barrier plane. Results show that a MLEM processed image will achieve resolution approaching that of a single pinhole imaged onto the full detector. The multiple pinhole advantages of simple implementation with shorter focal lengths combined with the availability of portable CZT detectors would be useful in short stand-off applications. Work is currently in progress to experimentally quantify spatial resolution and imaging timelines using an eV Products D-Matrix 4x4 array of pixelated CZT modules.

Date Published: 30 September 2016
PDF: 14 pages
Proc. SPIE 9968, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVIII, 996813 (30 September 2016); doi: 10.1117/12.2239948

Published in SPIE Proceedings Vol. 9968:
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVIII
Ralph B. James; Michael Fiederle; Arnold Burger; Larry Franks, Editor(s)