Sensitivity analysis towards trace-uranium detection with γ-γ coincidence NAA

Neutron activation analysis (NAA) is a powerful trace-element analysis technique widely used in radioassay for rare event searches, such as neutrinoless double beta decays, where extremely radiopure materials are often required. Thus, the ability to detect radioactive isotopes at sub-ppt concentrations is crucial for material screening. However, some materials are particularly hard to radioassay, such as sapphire (Al₂O₃) which is being considered as a structural component for the field cage in the nEXO detector. Sapphire is resilient to most acids making inductively coupled plasma mass spectrometry (ICP-MS) difficult. NAA of sapphire is also challenging due to the side activities produced by the matrix and its impurities. We have developed a new counting scheme for NAA which makes use of gamma cascades emitted by ²³⁹Np, the neutron activation product of ²³⁸U. We found that the new counting scheme would improve the detection limit of ²³⁸U in sapphire by about a factor of 8 compared to the usual single-γ counting scheme. In this talk, I will discuss the new counting scheme and the statistical analysis that determined its detection limit.