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. 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. These detectors are cylindrical tubes, typically 2.54 cm (1") in diameter, and 50 cm in length, containing up to 10 atm of 3He gas. They have enjoyed considerable success due to their stability, long life, ease of fabrication, low background rates, good gamma discrimination, and high detection efficiency. However, 3He detectors have a rather 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 additionally imposes safety issues and shipping concerns, and it is much more susceptible to loss of counting efficiency as a result of outgassing. Unlike 3He tubes, boron-coated straw (BCS) detectors operate with proportional gas below 1 atm, and due to their small diameter, on the order of 4 mm, they can generate very fast signals, on the order of tens of nanoseconds. Moreover, the smaller diameter tubes can be dispersed more uniformly inside the moderator, optimizing neutron absorption, resulting in very short neutron die- away times. The inherent capability of the BCS for fast signals must be supported by the front-end amplifier, in order to avoid deadtime limiting, and pileup of low-amplitude gamma signals. We propose the development of an amplifier that can read fast signals with minimal deadtime, and can read both neutron and gamma signals generated in BCS detectors. If the proposed objectives are met, a new generation of multiplicity counters, based on BCS detectors, will address the DOE challenge to develop accountability instruments and techniques that advance the ability to inventory fissile materials in domestic fuel cycle systems, in order 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.