Enhanced sensitivity to trace 238U impurity of sapphire via coincidence neutron activation analysis

Sapphire has mechanical and electrical properties that are advantageous for the construction of internal components of radiation detectors such as time projection chambers and bolometers. However, it has proved difficult to assess its 232Th and 238U content down to the picogram per gram level. This work reports an experimental verification of a computational study that demonstrates γγ coincidence counting, coupled with neutron activation analysis (NAA), can reach ppt sensitivities. Combining results from γγ coincidence counting with those of earlier single-γ counting based NAA shows that a sample of Saint Gobain sapphire has 232Th and 238U concentrations of < 0.26ppt and <2.3 ppt, respectively; the best constraints on the radiopurity of sapphire.