Radionuclide Identification Devices (RIDs) or Backpack Radiation Detection Systems (BRDs) are often equipped with NaI(Tl) detectors. We demonstrate that such instruments could be provided with reasonable thermal-and fast-neutron sensitivity by means of an improved and sophisticated processing of the digitized detector signals: Fast neutrons produce nuclear recoils in the scintillation crystal. Corresponding signals are detectible and can be distinguished from that of electronic interactions by pulse-shape discrimination (PSD) techniques as used in experiments searching for weakly interacting massive particles (WIMPs). Thermal neutrons are often captured in iodine nuclei of the scintillator. The gammaray cascades following such captures comprise a sum energy of almost 7 MeV, and some of them involve isomeric states leading to delayed gamma emissions. Both features can be used to distinguish corresponding detector signals from responses to ambient gamma radiation. The experimental proof was adduced by offline analyses of pulse records taken with a commercial RID. An implementation of such techniques in commercial RIDs is feasible.

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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.

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<p>Searching digitized detector signals for piled-up delayed components with distinct energy and delay time signatures is a smart method to provide common NaI(Tl) detectors with additional neutron detection capabilities at no extra cost. This technique nicely complements the idea of neutron detection by analyzing events with high energy depositions above the range of common gamma-ray energies. In combination, both approaches can provide half of the neutron sensitivity offered by a commercial ⁶Li co-doped NaI(Tl) (NaIL™) scintillator of the same size, at the price of higher and load-dependent background contributions. Delayed-coincidence techniques are most suitable for neutron monitoring or long-term measurements, where the statistics of the acquired delay-time distributions allows separate fitting of the effect and background contributions. In this case, the thermal neutron flux can be quantified in parallel to gamma-ray spectroscopy at overall detector loads exceeding 10 kcps.</p>

<p>Searching digitized detector signals for piled-up delayed components with distinct energy and delay time signatures is a smart method to provide common NaI(Tl) detectors with additional neutron detection capabilities at no extra cost. This technique nicely complements the idea of neutron detection by analyzing events with high energy depositions above the range of common gamma-ray energies. In combination, both approaches can provide half of the neutron sensitivity offered by a commercial ⁶Li co-doped NaI(Tl) (NaIL™) scintillator of the same size, at the price of higher and load-dependent background contributions. Delayed-coincidence techniques are most suitable for neutron monitoring or long-term measurements, where the statistics of the acquired delay-time distributions allows separate fitting of the effect and background contributions. In this case, the thermal neutron flux can be quantified in parallel to gamma-ray spectroscopy at overall detector loads exceeding 10 kcps.</p>