Finding and identifying radioactive material by carborne search for OSI deployment: Poster at Comprehensive Nuclear-Test-Ban Treaty: Science and Technology, S&T2011, Vienna, Austria

The CTBTO verification system comprises an On-Site Inspection (OSI) to verify the suspicion of a banned nuclear test. An OSI comprises different methods of verification, one of them being the radiological survey of the inspection area. The radiological survey is done by airborne survey, carborne survey and environmental sampling, thereby narrowing the inspected area with each step. Our institute operates a measurement car with highly sensitive neutron and gamma detection systems. We investigated search strategies with the gamma detectors of this system. Of course large area surveys should be done by gross gamma counting. One problem with gamma survey are variations in background which may lead to incorrect identification of relevant spots or, even worse, to a miss of relevant spots due to high background. Therefore, our gamma detectors are equipped with special software, called NBR (natural background reduction). This is a proprietary technique which allows discriminating artificial from natural gamma radiation in a simple way without qualified spectral information and is therefore suitable, e. g., for plastic scintillators. This technique has proved to be valuable for the detection of artificial radiation for example at border crossing stations. This procedure was investigated for OSI operations where different nuclides are involved (see e.g. the list of relevant nuclides in the On Site Inspection Test Manual). After locating a refined search with hand-held devices can only be done with blinded instruments according to the treaty