Year
2002
Abstract
For the search and detection of concealed nuclear material we equipped a conventional car with a neutron measurement system. It consists of six neutron slab counters on each side and the appropriate electronics. The complete system can easily be fixed in most vehicles. The pulses of the six modules on each side are summed passively and each side is analyzed separately. The results are displayed on a handheld PC in the front of the car or, in case of a covered search, the data are stored in a non-volatile memory together with coordinate information from GPS. In order to verify the presence of fissionable material, we investigated the possibility to detect two or more neutrons in coincidence. In this way it is possible to distinguish industrial neutron sources like Am/Be(n) from Special Nuclear Material. First the material is localized by its total rate from the moving vehicle. Then the vehicle is stopped in the vicinity of the suspicious location and the coincidences are counted for a period of 10 to 1000 seconds depending on neutron intensity and distance. At a distance of 1 meter, a quantity of 3 g of weapon-grade plutonium can be detected in 1000 seconds, corresponding to 0.5 g reactor plutonium, in 10 seconds the detection limit is 200 g. In case of a concrete shielding of 10 cm thickness the detection limit is increased to 6.3 g in 1000 seconds and 0.47 kg for an interval of 10 seconds. With 20 cm concrete shielding these values are approximately doubled. Larger quantities of weapon-grade or reactor plutonium (kilograms) can also be identified out of the moving vehicle. This system may be used to discover illicit trafficking of nuclear material and thus prohibit nuclear proliferation.