Simple, Authenticable Measurement Of Fissile Materials

Future nuclear weapons treaties may limit the number of warheads a party possesses, which will require verification measurements of those warheads. To allow such measurements, innovative methods are needed that balance confidence in a measurement with safety and information protection. To this end, we are developing methods for measuring fissile materials using a moderated neutron source and neutron bubble detectors. The moderated neutron source provides neutrons of energy below the fission cross-section of fissionable materials, such as 238U and 240Pu, so that the resulting fission neutrons are due to interactions with the fissile material in the interrogated item, such as 235U and 241Pu. Bubble detectors are well-suited to this measurement since they are gamma blind and can be designed to have an energy threshold to minimize the detection of the moderated neutron source. Furthermore, measurement of the bubble detector response to the neutron flux can span from qualitative, using human senses to see or hear the bubbles being generated, to quantitative, counting bubbles with image processing and even spectral measurements with multiple neutron thresholds. Neutron-sensitive sheets paired with photo-sensitive film are also being considered for neutron detection. We are exploring conducting both attribute measurement for presence of fissile material and template measurements following the Zero-Knowledge Protocol. We present simulation studies to determine the feasibility of the fissile material measurements, including the design of the moderated neutron source to achieve an acceptable neutron spectrum and flux, optimization of the bubble detector thresholds, and consideration of authentication of the measurements. This work is supported by the US Dept. of Energy, National Nuclear Security Administration, Office of Defense Nuclear Nonproliferation Research and Development.