Neutron coincidence counters for disarmament verification: experiments and modeling

A measurement campaign in support of nuclear disarmament verification was carried out in 2019 on the premises of the Belgian Nuclear Research Centre, SCK CEN. Well-characterized MOX sources with different types of shielding materials were used during the campaign. The relative content of Pu was up to 14%wt and the isotopic abundance of 239Pu was up to 93%wt. One of the technologies that were deployed was neutron coincidence counting with 3He based slab counters.The obtained background and dead time corrected total and reals rates were used to determine the mass of the assayed sample by using the so-called point model equations. The obtained masses were systematically underestimated by 15 % in average. A calibration approach based on Monte Carlo simulations where the mass is estimated from the calculated number of reals per spontaneous fission event was also used. A Monte Carlo model of the measurements setup was therefore developed to account for the finite source size, multiplication, moderation and absorption in the detector and scattering in the environment, including the measurement room walls, floor and ceiling. By using a Monte Carlo based efficiency for the reals the masses were slightly overestimated by typically few percent. However, outliers were observed.In an effort to understand the obtained results, additional calculations were carried out to estimate the impact due to both the neutron emission by (α,n) reactions and spontaneous fission decay on the measured observables. The associated total and reals counts were obtained for various configurations and compared with the experimental data. The obtained results indicate the importance of (α,n) reactions when neutron multiplication is not negligible.