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

多次天文观测已经确定，宇宙中大约85%的物质不是由正常的原子组成的，而是由不发射或吸收光的未被探测到的基本“暗物质”粒子组成的。破译这种所谓的暗物质的本质对宇宙学、天体物理学和高能粒子物理学至关重要。一个主要的假设是，它是由大爆炸后瞬间产生的弱相互作用大质量粒子（wimp）组成的。如果wimp是暗物质，那么它们在我们星系中的存在可以通过位于地下深处的探测器的原子核散射来检测，以帮助排除宇宙射线造成的背景。探测到WIMP暗物质将解决粒子物理学和宇宙学中的一个基本谜团，为了解宇宙的主要物质组成和粒子物理学标准模型之外的物理学提供一个独特的窗口。SABRE（主动背景抑制碘化钠）是一个使用碘化钠（NaI）闪烁晶体阵列搜索暗物质信号的实验。该奖项将提供资金，通过改进生产和纯化NaI粉末的方法，以及开发具有极低放射性元素污染的晶体的方法，来证明制造具有较低钾-40和其他放射性杂质水平的超低本底NaI晶体的可行性。SABRE项目为学生和博士后研究人员提供了充足的机会，以获得基础研究方面的经验和培训。SABRE促进了与商业公司的牢固关系，并协助开发适用于所有客户的特殊产品。这些合作为市场带来了改进的高纯度产品和敏感的分析服务。高纯度NaI晶体的发展也为暗物质研究的闪烁测热仪的其他新实验提供了可能。寻找罕见暗物质事件的灵敏度在很大程度上取决于探测器的背景，在NaI（Tl）晶体中，背景主要是由晶体中低水平的K-40放射性决定的。该奖项将支持具有较低本征放射性的NaI（Tl）晶体的持续发展。此外，通过将高效的低背景光电倍增管耦合到晶体上，并以较低的PMT电压减少dynode余辉，可以实现比DAMA实验更低的阈值能量。他们的目标是实现~ 0.10 cpd/kg/keV的平坦背景和小于1 keee的阈值能量。在10 ppb水平上测量K已经有一段时间了；在最近的工作中，表明将改进到2 PPB。在最近的工作中，初步测量了~ - 0.5 ppt水平的U和Th。这些测量比晶体生长和伽马计数要快得多，因此可以及时指导研究生产低水平放射性杂质的晶体。

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.