First results of trace-uranium detection using newly developed γ-γ coincidence neutron activation analysis

Neutron activation analysis (NAA) is a powerful trace-element analysis technique able to detect long-lived radioactive and stable isotopes at sub-ppt concentrations. NAA is widely used in rare event searches, such as neutrinoless double-beta decay, where identification of radiopure materials is required. Some materials, such as sapphire (Al₂O₃), are particularly difficult to radioassay due to side activities produced by the matrix and its impurities after neutron activation. Moreover, sapphire is insoluble in most mineral acids, making inductively coupled plasma mass spectrometry (ICP-MS) problematic. However, radioactive screening of sapphire is necessary in preparation of the nEXO double-beta decay experiment, where sapphire is considered as a structural component for the field cage.

To meet this challenge, we developed a new counting scheme for NAA which makes use of gamma cascades emitted in ²³⁹Np decay, the neutron activation product of ²³⁸U. The new counting scheme is expected to improve the detection limit of ²³⁸U in sapphire by a factor of 8, compared to single-γ counting achieving about 10 ppt sensitivity. We re-activated a previously studied sapphire sample at the MIT research reactor. The measurements were performed using two HPGe detectors at the Triangle Universities Nuclear Laboratory (TUNL). In this talk, I will present first results of γ-γ coincidence NAA for sapphire and compare them to previously obtained single-γ NAA results.