Development of High-Purity NaI(T1) Crystals for the SABRE Dark Matter Experiment

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Astrophysical observations and cosmological data give strong evidence that nearly a quarter of the Universe consists of dark matter. Weakly interacting massive particles (WIMPs) and axions have been proposed as theoretically favored candidates for dark matter. Despite the strong evidence for the existence of dark matter, neither WIMPs nor axions have been conclusively detected. The DAMA/LIBRA experiment claims for more than a decade to have detected dark matter with an array of low background NaI(Tl) crystal scintillators. However, other experiments with different detectors have not seen evidence for dark matter, even though they have very high sensitivity. Because the sensitivity to dark matter may depend on details of the detector material, a new search for evidence of dark matter with NaI(Tl) detectors with much lower background than DAMA/LIBRA achieved is needed. This award will fund the SABRE (Sodium-iodide with Active Background REjection) experiment which has been focused on the development of new crystals with lower background than DAMA/LIBRA. The development of high purity crystals can affect many fields of research, perhaps even semiconductors which presently have trace levels of radioactivity that affect their reliability. The effort also involves many fields and attracts students with a wide range of backgrounds. SABRE also promotes strong relationships with industrial partners, who then brought new products and services to all customers. Collaboration with RMD, Sigma Aldrich, Seastar Chemicals, and Sandfire has brought high purity products and sensitive assay services to the market. Collaboration with the Mellen Company has led to an invention that can be used to purify materials with higher efficiency.The research funded by this award will support the development and testing of a low-background NaI(Tl) crystal detector exploiting the new zone-refining technique developed by Princeton in collaboration with Mellen Company in order to improve the present record established with NaI-33; and the development of a crystal growth facility underground at the Canfranc Laboratory, Spain. SABRE will have two phases: phase 1 consists of one detector at the Gran Sasso Laboratory (LNGS), in Italy, which works as a proof-of-principle (PoP) for testing the radio-purity of crystals; in phase 2, twin detectors will be built in the northern and southern hemispheres. The SABRE-south is planned to be installed in the SUPL (Stawell Underground Physics Laboratory), a new underground laboratory in Australia under construction. The two detectors will enhance the potentiality to differentiate a dark matter signal from seasonal and local effects.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.