Spatial Localization of Radioactive Sources for Homeland Security

Abstract

In general localization of radioactive sources or hotspots is a major issue for radiological safety of operators in nuclear facilities and in National Security. For this purpose, portable Gamma-Ray Imaging (GRI) systems allow remote localization of radioactive sources from greater distances than conventional rate meters, leading to significant reductions of the dose received by operators.

The difficulty with GRI is the same difficulty with gamma ray detection. Gamma rays are quite high in energy and do not interact with matter with high efficiency. As a result large detectors are needed which are made of expensive heavy elements In addition pixelated array detectors with a lot of pixels are very expensive to be made, in contrast with the normal optical wavelength light cameras. Furthermore, gamma rays do not interact with lenses and focusing equipment, which results in a limited field of view of the gamma ray detectors.

A way to go around this is to have Mechanical Collimation. In this methodology a raster scanning is used with a single detector of one pixel that is moving around. This is relatively inexpensive but the whole process is rather slow (Hughes and Lightfoot, RadScan 600 - a portable instrument for the remote imaging of gamma contamination: Its design and use in aiding decommissioning strategy. 1996 IEEE nuclear science symposium - conference record (Vol. 1, 2 and 3). BNFL Instruments Ltd; RadScan800, https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.561.3193&rep=rep1&type=pdf). They have been used extensively in the past but nowadays are becoming obsolete.

A more advanced technique is to have pixelated arrays like code aperture devices. Nowadays, portable coded aperture gamma cameras for industrial applications are undergoing impressive developments and improvements in terms of lightness, usability, response sensitivity, angular resolution, and spectrometric capabilities (Amgarou et al JINST. 11: P08012, 2016; Dubos et al Nucl Instrum Methods Phys Res A, 787:302, 2015; Jeong et al Rev Sci Instrum 89:033106, 2018). These devices are quite fast but often expensive due to the pixilated array and limited by the correct choice of the MURA type mask to detect close or large distance sources.

Another methodology that is commonly used is the Compton Cameras (CC). Compton imaging is a visualization technique that uses the kinematics of Compton scattering for the reconstruction of a gamma radiation source image (Llopart et al Nucl Instrum Methods Phys Res A 581(1–2):485–494, 2007). They have been successfully used for detecting illicit nuclear materials, recovering orphan radioactive sources, and delimiting suspicious radiological contaminated areas.

In this chapter the majority of radioactive localization methods will be covered together with state of the art devices and future prospects in this field.