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 ${}^{232}\mathrm{Th}$ and ${}^{238}\mathrm{U}$ content down to the picogram per gram level. This work reports an experimental verification of a computational study that demonstrates $\gamma \gamma \text{coincidence}\text{counting}$, coupled with neutron activation analysis (NAA), can reach ppt sensitivities. Combining results from $\gamma \gamma \text{coincidence}\text{counting}$ with those of earlier single-$\gamma$ counting based NAA shows that a sample of Saint Gobain sapphire has ${}^{232}\mathrm{Th}$ and ${}^{238}\mathrm{U}$ concentrations of $<0.26\text{ppt}$ and $<2.3$ ppt, respectively; the best constraints on the radiopurity of sapphire.

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• Dark matter
• Neutrinoless double beta decay
• Rare decays
• A ≥ 220
• Gamma ray spectroscopy
• Spectrometers & spectroscopic techniques