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

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Multiple astronomical observations have established that about 85% of the matter in the universe is not made of normal atoms, but must be otherwise undetected elementary "dark matter" particles that do not emit or absorb light. Deciphering the nature of this so-called Dark Matter is of fundamental importance to cosmology, astrophysics, and high-energy particle physics. A leading hypothesis is that it is comprised of Weakly Interacting Massive Particles, or WIMPs, that were produced moments after the Big Bang. If WIMPs are the dark matter, then their presence in our galaxy may be detectable via scattering from atomic nuclei in detectors located deep underground to help reject backgrounds due to cosmic rays. Detection of WIMP dark matter would solve a fundamental mystery in particle physics and cosmology, providing a unique window to learning about the primary matter constituent of the Universe and of physics beyond the Standard Model of particle physics. SABRE (Sodium-iodide with Active Background REjection) is an experiment that will search for dark matter signals using an array of Sodium Iodide (NaI) scintillating crystals. This award will provide funds to demonstrate the feasibility of fabricating ultra-low background NaI crystals, with lower levels of Potassium-40 and other radioactive impurities, by improving the methods to produce and purify NaI powder, and by developing methods to grow crystals with very low contamination of radioactive elements. The SABRE project provides ample opportunities for students and postdoctoral researchers to gain experience and training for careers in basic research. SABRE has promoted strong relationships with commercial companies and assisted in the development of special products available to all customers. These collaborations have brought improved high purity products and sensitive assay services to the market. The development of high purity NaI crystals is also making possible other new experiments with scintillating bolometers for dark matter research.The sensitivity of the search for rare dark matter events depends critically on the background in the detector, which in NaI(Tl) crystals is dominated by low levels of K-40 radioactivity in the crystal. This award will support on-going development of NaI(Tl) crystals with lower intrinsic radioactivity. In addition, by coupling efficient low background photomultipliers to the crystals and reducing dynode afterglow with lower PMT voltage, they will achieve a threshold energy lower than that of the DAMA experiment. Their goal is a flat background of ~ 0.10 cpd/kg/keV and a threshold energy less than 1 keVee. Measurements of K at the level of 10 ppb have been possible for some time; an improvement to 2 ppb is indicated in recent work. A preliminary measurement of U and Th at a level of ~­ 0.5 ppt is indicated in recent work. These measurements are much faster than growing crystal and gamma counting and are thus guiding research in a timely way toward production of crystals with low levels of these radioactive impurities.