The Neutron Bloodhound Project: Autonomous Mobile Directionally Sensitive Neutron Sensors for Managed Access Inspections

Abstract

Recent arms control and non-proliferation treaties call for the inspection of secure storage facilities and uranium enrichment plants[1, 2]. This study explores the ap- plications of a directionally sensitive neutron detector robotic system (a “neutron bloodhound” or “inspector bot”) to these inspection activities. We use extensive MCNP6 simulations to demonstrate that a 1’ diameter 2’ long cylinder of polyethylene moderator housing three 1” diameter by 1’ 8” long Boron Coated Straw proportional counters in a triangular formation offers highly directionally sensitive neutron flux information. According to our simulations, a detector of these specifications is expected to produce total count rates of 0.3685 ± 0.0025 nps per ng Cf-252 at 2 m regardless of the angle. When the detector is pointed at a source so that there is one detector closest and the other two are equidistant, we found a count rate ratio between the closer and the further two detectors to be 5.89 ± 0.12, reducing to 1 as the detector turns 60◦ relative to the source. This ratio changed dramatically with distance to and height above the source, but maintained directional sensitivity throughout all locations. We confirm that if front to back ratio is used in conjunction with rate, the bot will have enough information to guide itself to a source as long as the count rates from the item in question are above the neutron background. Additional tests show that environmental factors such as large steel neutron-reflecting surfaces have the potential to confuse the detector’s estimated source location up to 10◦. The detector’s use as a stationary monitor for gas centrifuge feed and withdrawal stations is also confirmed via simulation. These results are a comprehensive founda- tion on which to build more realistic simulations and compare future experimental tests, working towards eventual implementation of these bots in the field, a valuable addition to our national safeguards.