Dark matter detector radioimpurities ¹²⁹I and ²¹⁰Pb measured with accelerator mass spectrometry

Sodium iodide crystals doped with thallium NaI(Tl) can be used as detector material for direct dark matter detection by taking advantage of their particle detection properties of scintillation. In order to achieve this, it is crucial that these crystals are of ultra-high purity. Radioimpurities within the crystals may potentially mimic dark matter signals and thus must be quantified, minimised where possible and distinguished from real events. Abundances of radionuclides 129I and 210Pb, which are dominant sources of radioimpurities in NaI(Tl) crystals, were measured using accelerator mass spectrometry at the Australian National University (ANU) and the Australian Nuclear Science and Technology Organisation (ANSTO). NaI powder chemically processed to AgI, and, for the first time, unprocessed NaI powder, were shown to be suitable as AMS targets. A consistent 129I/127I ratio of (2.0 ± 0.3) × 10−13 was measured in three different commercially available NaI powders, one order of magnitude higher than the measured blank with (1.4 ± 0.3) × 10−14. Therefore, it was concluded that the choice of NaI powder has a negligible influence on the 129I contribution to low-background dark matter experiments. For 210Pb, different Pb molecular ion species were assessed with PbO2− being the preferred species and applied to investigate different lead oxide compounds for their suitability as Pb carriers. A 210Pb/Pb isotopic ratio of (3.6 ± 1.41.7) × 10−15 was measured in Pb3O4 powder. This met the required lower activity limit of 210Pb when adding 1 mg of stable lead into 1 kg of NaI(Tl) powder with a desired maximum 210Pb/Pb isotopic ratio of 1 × 10−14. These results indicate the suitability of the investigated Pb3O4 as a potential carrier for incorporation with Pb extracted from NaI used for dark matter experiments.