Searching radioactive material with hand-held gamma detectors

Reliable measurement techniques are decisive for early detection of illicit use of nuclear and radioactive material. In the context of the global threat of nuclear terror, detection of illicit nuclear and radioactive material is of high importance in order to prevent possible terrorist acts with dirty bombs or improvised nuclear devices (IND). Acquisition of nuclear or radioactive material is generally considered to be the major obstacle for terrorists which is most difficult to overcome on their way to assemble and ultimately use a dirty bomb or an IND. Therefore, sophisticated detection methods need to be employed to localize and identify illicit radioactive and nuclear material. As far as border crossings are concerned, portal monitors with large-volume detectors are best suitable for these purposes. But detection of such covert radioactive or nuclear material in places where the material is temporarily stored prior to the composition of the device or detection of thecomposed device requires a different type of detection equipment. New electrically cooled hand-held systems featuring high resolution gamma spectroscopy with a reasonably low weight to accomplish thorough surveys of suspicious locations became available recently. Germanium detectors with liquid nitrogen cooling were commonly used in that respect before. In case liquid nitrogen is not available, electrically cooled Germanium detectors are the only type of detector to fulfill the requirement of high energy resolution whereas light-weight detectors for on-site surveys are usually equipped with low-resolution crystals. We have performed measurements to investigate the characteristics of such a detector as well as measurements examining the quality of identification by automatic routines of the material in question. The results of identification and search tests will be shown. A comparison with other detectors for mobile search will be presented including an assessment of theirapplicability for realistic search tasks. New easy to use high-resolution search detectors will be a valuable completion to the widely used low resolution search devices and may thus help to prevent nuclear terrorism at an early stage.