Investigation and optimisation of mobile NaI(Tl) and 3He-based neutron detectors for finding point sources

Neutron radiation produces high-energy gamma radiation through (n,γ) reactions in matter. This can be used to detect neutron sources indirectly using gamma spectrometers. The sensitivity of a gamma spectrometer to neutrons can be amplified by surrounding it with polyvinyl chloride (PVC). The hydrogen in the PVC acts as a moderator and the chlorine emits prompt gammas when a neutron is captured. A 4.7-l 3He-based mobile neutron detector was compared to a 4-l NaI(Tl)-detector covered with PVC using this principle. Methods were also developed to optimise the measurement parameters of the systems. The detector systems were compared with regard to their ability to find 241AmBe, 252Cf and 238Pu-13C neutron sources. Results from stationary measurements were used to calculate optimal integration times as well as minimum detectable neutron emission rates. It was found that the 3He-based detector was more sensitive to 252Cf sources whereas the NaI(Tl) detector was more sensitive to 241AmBe and 238Pu-13C sources. The results also indicated that the sensitivity of the detectors to sources at known distances could theoretically be improved by 60% by changing from fixed integration times to list mode in mobile surveys.