High-Rate Neutron Detector for Nuclear Material Accountancy and Control (NMAC)

Safeguards applications for nuclear fuel management and control impose demanding detector requirements beyond high detection efficiency. These include very high neutron counting rates, effective discrimination against high gamma fluxes, very high stability, long life, and robust operation in the field which includes operating in high temperature fume hoods filled with Argon gas. The ability to count both neutrons and gamma rays is of interest as well. Neutron coincidence and multiplicity counters have been traditionally serviced by detectors based on 3He gas. However, 3He detectors have a limited count rate capability, due to their high gas pressure, and large tube diameter, resulting in slow signals, on the order of microseconds. High pressure operation requires operating at very high voltages making it impossible to operate in the fume hoods where multiplicity counters are must useful.Unlike 3He tubes, boron-coated straw detectors operate with proportional gas below 1 atm, and due to their small diameter, of 4-5 mm, they can generate very fast signals. Moreover, the smaller diameter tubes can be dispersed more uniformly inside the moderator, resulting in very short neutron die-away times.A new generation of neutron multiplicity counters, based on boron-coated straw detectors, is proposed, capable of high count rates, and very high gamma rejection. At the same time, a front-end amplifier is proposed, that can produce fast signals with minimal dead time, and can read both neutron and gamma signals generated in straw detectors. We also propose use of a powerful commercial CAEN 8 channel digitizer DT5730S that receives the amplified signals from the 6 sector amplifiers and digitizes them at 500 MHz. The unit also performs real time pulse shape discrimination, rejecting gamma ray events as well as EMI pulses. Thanks to the very high speed of the new amplifier subtle differences in the fall time of gamma events versus neutrons can be detected.The Phase I projected investigated the technical feasibility of the proposed counter and electronics, and in particular, their ability to effectively discriminate gamma rays in very intense fields. The Phase II project will deliver a fully operational neutron multiplicity counter that satisfies the requirements outlined above. The counter will be comprehensively tested with actual spent nuclear fuel.If the proposed objectives are met, a new generation of multiplicity counters, based on boron-coated straws, will address the DOE challenge to develop accountability instruments and techniques that advance the ability to inventory fissile materials in domestic fuel cycle systems, to detect diversion and prevent misuse. The public will benefit from nuclear power as a resource capable of making major contributions in meeting the nation’s energy supply, environmental, and energy security needs.