Neutron detection by measuring capture gammas in a calorimetric approach

Radiation detector systems for homeland security applications have been usually equipped with 3 He tubes to detect the distinguished neutron signature of Special Nuclear Materials (SNMs). The serious shortage of 3 He gas, however, recently initiated substantial efforts to develop alternative neutron detectors, particularly for large-area Radiation Portal Monitors (RPMs). Most activities are currently directed to detectors comprising 6 Li or 10 B — beyond doubt with remarkable success. Nevertheless, their broad deployment poses an economic challenge. Our contribution presents a different technique — the detection of neutron capture gammas. In contrast to other attempts we do not focus on characteristic gammas or conversion electrons in the low-energy range, or on the detection of single high-energy capture gammas. Rather we propose to measure the sum energy of multiple gammas released after neutron capture reactions in a semi-calorimetric approach. This method allows simultaneous measurements of neutron and gamma radiation with a single detector (even including spectroscopic information for nuclide identification). A first prototype of such a Neutron Capture Detector (NCD) was developed based on proven standard detector materials and technologies. It consists of thin Cadmium sheets surrounded by four BGO scintillation crystals. The detector response was studied in measurements with cold neutrons extracted at the BER II reactor, and with fission neutrons from 252 Cf. The NCD performance is discussed in comparison with those of a 3 He tube. Simulation calculations have been performed to estimate the detection efficiency as a function of the detector size. A complete database to model the multiple-gamma emission from excited 114 Cd nuclei was composed by a semi-empirical approach which combines the gamma energies and yields of well resolved transitions with information from integral measurements. The simulation results are validated experimentally and allow optimizing more complex NCD systems for RPM applications.