Collaborative Research: DarkSide-20k

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。直接探测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。该奖项是为了支持nsf资助的DarkSide （DS）小组建造DarkSide-20k (DS-20k)，这是一个使用有效（基准）质量为23吨（20吨）的液态氩时间投影室（LAr TPC）直接搜索WIMP的项目。DarkSide的合作者提出了一种在大型氦-4生产工厂分离氦-3的技术，可以缓解未来稀有氦同位素的短缺。“铀”项目将建立一个能够生产数百吨低放射性地下氩（UAr）的长期设施，并将为DarkSide-20k以外的几个项目提供UAr，如辐射测量、地下水Ar-39年龄测定和国家安全条约核查。DarkSide项目将推进用于禁止核试验核查的Ar-37和用于放射性测年的Ar-39探测技术。Aria项目将通过同位素分离进一步降低UAr中的Ar-39，从而产生分离重要稳定同位素（如O-18、N-15和C-13）的能力。这些被用来生产用于医学正电子发射断层扫描（PET）的放射性核素。先进气体净化、精密清洁、超纯钛生产、硅芯片放射性降低等技术也将得到发展。以液态氩为目标的WIMP反冲探测，特别是在DarkSide-10和DarkSide-50项目中，包括使用大气氩（AAr）和地下氩（UAr）填充的DarkSide-50 LAr TPC的操作，使灵敏度的显着提高成为可能。液态氩能有效地将核反冲产生的能量转化为电离和闪烁，是一种极好的WIMP探测介质。氩闪烁时间分布（“脉冲形状”）取决于电离粒子的性质，提供强大的粒子识别来抑制自然放射性的背景。利用DarkSide-50中的AAr，证明了脉冲形状分析抑制β / γ背景的因子超过1.5 x 10^7，这是所有暗物质探测器中最强大的背景抑制因子之一。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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. Direct 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. This award is to support the NSF-funded DarkSide (DS) groups for the construction of DarkSide-20k (DS-20k), a direct WIMP search using a Liquid Argon Time Projection Chamber (LAr TPC) with an active (fiducial) mass of 23 tons (20 tons).DarkSide collaborators have proposed a technology for separation of He-3 at large He-4 production plants that could alleviate future shortages of the rare helium isotope. The Urania project will establish a long-term facility that will be able to produce hundreds of tons of low-radioactivity underground argon (UAr), and will supply UAr for several programs beyond DarkSide-20k, such as for radiometry, Ar-39 age dating of ground water, and National Security treaty verification. The DarkSide program will advance technologies for detection of Ar-37, used for nuclear test ban verification, and Ar-39, used for radiometric dating. The Aria project will further reduce the Ar-39 in UAr by isotope separation, yielding as by-product the ability to separate important stable isotopes such as O-18, N-15, and C-13. These are used to produce radionuclides for medical positron emission tomography (PET). Technologies for advanced gas purification, precision cleaning, production of ultra-pure titanium, and reduction of radioactivity in silicon chips will also be developed.The significant improvement in sensitivity is made possible by technology developed within several world-wide programs toward WIMP recoil detection with liquid argon as the target, and in particular within the DarkSide-10 and DarkSide-50 programs, including operation of the DarkSide-50 LAr TPC with both atmospheric argon (AAr) and underground argon (UAr) fills. Liquid argon is an excellent medium for WIMP detection due to its efficient conversion of energy from nuclear recoils into both ionization and scintillation. The argon scintillation time profile ('pulse shape') depends on the nature of the ionizing particle, providing powerful particle discrimination to suppress background from natural radioactivity. Using AAr in DarkSide-50, it was demonstrated that pulse shape analysis suppresses beta/gamma backgrounds by a factor in excess of 1.5 x 10^7, one of the most powerful background rejection factors among all dark matter detectors.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.