Improving the Confidence Associated with Passive Total Neutron Counting in the Nuclear Weapon Disarmament Verification Process

Abstract

Passive total neutron counting is an important tool in the nuclear weapon disarmament monitoring and verification process proposed by the International Partnership for Nuclear Disarmament Verification (IPNDV). In the process, neutron measurements of given treaty accountable items (TAIs) are repeated multiple times in different locations and environments, and the measured neutron count rates are expected to remain unchanged. However, neutrons are heavily scattered in the environment, and the change in location or geometry of the environment can produce varying results in neutron measurements which can deteriorate the confidence of passive total neutron counting. In this paper, we have studied different kinds of neutron detection instruments and methods in various environments to determine the effects of the environment on passive total neutron counting and to develop recommendations and procedures to minimize and take these environmental factors into account. As a result, the moderated 3He proportional counter was the most promising type of instrument in terms of how the change in an environment impacted the measured neutron count rates. However, even with the 3He counter, the environmental influence increased rapidly with the source to- detector distance (SDD). For example, with one-meter, two-meter, and three-meter SDDs, the maximum difference in count rates between outdoor and indoor measurements was 10.06(7) percent, 18.6(3) percent, and 28.1(5) percent, respectively. To mitigate the impact of the measurement environment, we propose to use a bare 252Cf reference source measured in the same geometries as the nuclear TAIs to estimate the influence of the environment on the measured neutron count rates. Using this technique in the same conditions as above produces indoor predictions that differ by 2.55(3) percent with a one-meter SDD, 0.334(5) percent with two-meter SDD and 1.93(4) percent with a three-meter SDD from the actual indoor measurements.