Currently at the Idaho National Engineering and Environmental Laboratory, a handheld device is being developed to measure fast neutrons and gamma rays using a single detector. The handheld detection system presented here uses a single 12.7 mm (diameter) by 12.7 mm (length) liquid scintillator detector (BC501). The detection system can be made small and light. A small and light device can be used in several applications such as customs inspection, border security, and environmental radiation monitoring. The use of only one detector requires that the neutrons and gamma rays be distinguished by the shape of their pulses in the detector. Two methods of pulse shape discrimination (PSD) are presented here: charge integration and zero crossing. Figures of merit were calculated for both methods for a threshold energy range of 50-600 keVee. Results show that the zero crossing method gives much better PSD for 100 keVee and lower, whereas the charge integration method leads to better separation above 100 keVee. However, the neutrons and gamma rays are totally separated for energies of 100 keVee and above in both techniques. We are currently designing a miniaturized electronic system to be incorporated into the handheld device.Index Terms-Pulse shape discrimination (PSD).
Abstract--Recent advancements in digital signal processing (DSP) using fast processors and a computer allows one to envision using it in pulse shape discrimination. In this study, we have investigated the feasibility of using a DSP to distinguish between neutrons and gamma rays by the shape of their pulses in a liquid scintillator detector (BC501). For neutron/gamma discrimination, the advantage of using a DSP over the analog method is that in an analog system, two separate charge-sensitive ADCs are required. One ADC is used to integrate the beginning of the pulse rise time while the second ADC is for integrating the tail part. In DSP techniques the incoming pulses coming directly from the detector are immediately digitized and can be decomposed into individual pulses waveforms. This eliminates the need for separate ADCs as one can easily get the integration of two parts of the pulse from the digital waveforms. This work describes the performance of these DSP techniques and compares the results with the analog method.Index terms-Digital signal processing, gamma ray/neutron pulse shape discrimination.
Abstract--A variation of the basic Cockroft-Walton (C-W)Voltage Multiplier circuit design may be used to generate multiple voltages at sufficient currents to drive the dynodes of a photomultiplier tube. In a battery-operated handheld device, the current draw on the batteries must be kept to a minimum. Several other parameters must be considered carefully during the design as well. Components must be chosen based on size restrictions, expected load current, expected output voltage range, and the maximum allowable ripple in the output voltage. A prototype surface mount C-W board was designed and tested to power two photomultipliers. The whole system, including the detectors, draws less than 15mA of supply current with the outputs at 1000VDC.
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