Low background screening facility at Boulby for rare event search experiments

世界各地的科学家正在接近物理学中两个最大的挑战：理解暗物质的本质和中微子的特性。暗物质占宇宙质量的85%，但我们不知道它是什么，因为它从未被观察到，尽管我们知道中微子有质量，但我们不知道确切的质量有多少，也不知道它们是否是自己的反粒子。如果是这样的话，就可以解释宇宙大爆炸后不久物质和反物质之间的微小不平衡。这些问题对我们对宇宙及其演化的理解有着深远的影响——回答这些问题是科学中最优先考虑的问题之一。希望观察暗物质粒子相互作用或中微子双衰变事件的实验，都有一个共同的要求，这些实验将告诉我们中微子的存在。由于这两种过程都极为罕见，因此探测器需要屏蔽可能掩盖信号的所有背景辐射源。第一道防线是将探测器放置在矿井或山底深处，这样可以减少来自太空的宇宙射线轰击地球表面的速度。接下来，探测器被铜、铅、塑料和水包围，以阻挡地下岩石发出的辐射。虽然这是自然的低水平辐射，不会对人类造成伤害，但对于我们极其敏感的罕见事件搜索来说，这是灾难性的！最后也是最困难的一步是只用最纯净的材料来建造探测器本身，这些材料在可能释放辐射的微量污染物方面特别“干净”。几十年来，英国在罕见事件地下物理方面有着非常强大的记录和国际地位，我们继续在最先进的实验中发挥主导作用。我们现在站在发现下一代暗物质和中微子实验的边缘。不幸的是，在英国，我们在用材料制造探测器之前筛选材料以检查其放射性纯度的能力已经不够了。随着探测器变得越来越灵敏，对材料的要求也越来越高，要求材料更加清洁，没有哪怕是最小的辐射。以前我们可以依靠位于博尔比地下实验室的锗探测器来进行筛选，但它已经无法满足暗物质和无中微子双β衰变实验的灵敏度要求。这是一个双重问题，因为除了在探测器结构中接受材料之前进行审查外，我们还必须准确地了解每个组件的预期辐射量。只有当我们知道我们对所有这些材料的期望时，我们才有希望观察到一个过剩的信号，并声称发现了一个特殊的信号。世界上很少有设施拥有足够灵敏度的锗探测器来满足这些需求。我们将在博尔比安装一个新设备，配备最先进的锗探测器，英国15个研究所将使用该探测器进行低本底筛选活动，以进行领先的暗物质和无中微子双β衰变实验。有了这种提供世界上最敏感的材料筛选测试的迫切需要的能力，博尔比设施将重新成为该领域的领导者，我们将能够构建下一代实验，这将有助于解开宇宙的奥秘。这样的设施将对粒子物理学和宇宙学以外的各种应用非常有用。它将大大改善博尔比正在进行的环境放射性研究，并将提供研究气溶胶生长和云形成的气候的能力，以及下一代电子技术。在国际上，对这种具有我们将提供的灵敏度的仪器有工业和商业需求。

Scientists around the world are closing in on two of the biggest challenges in physics: understanding the nature of dark matter and the properties of the neutrino. Dark matter accounts for 85% of the mass of the Universe yet we do not know what it is since it has never been observed, and although we do know neutrinos have mass we do not know precisely how much nor if they are their own anti-particles. If they are it could explain the tiny imbalance between matter and antimatter shortly after the Big Bang. These questions have profound impact on our understanding of the Universe and its evolution - answering them is amongst the highest priorities in science.Experiments that hope to observe dark matter particle interactions or neutrinoless double beta decay events that will tell us about the neutrino share a common requirement. Since both of these processes are extremely rare the detectors need to be shielded from all sources of background radiation that might mask the signal. The first line of defence is to place the detectors deep underground in mines or under mountains - this reduces the rate of cosmic rays from space that bombard the surface of the Earth. Next the detectors are surrounded with copper, lead, plastics and water to block the radiation emitted by the underground rock. Although this is natural low-level radiation that causes no harm to humans, it is catastrophic for our extremely sensitive rare-event searches! The final and most difficult step is to construct the detectors themselves from only the purest materials that are exceptionally 'clean' in terms of trace contaminants that may emit radiation.The UK has a very strong track record and international standing in rare-event underground physics, going back several decades, and we continue to take leading roles in the most advanced experiments. We now stand on the edge of discovery with the next generation of dark matter and neutrino experiments. Unfortunately, here in the UK, our capability to screen materials to check their radio-purity before using them to build detectors is no longer sufficient. As detectors have become ever more sensitive, so has the requirement for the materials to be ever cleaner and free from even the smallest amount of radiation. Whereas previously we could rely on a germanium detector located at the Boulby Underground Laboratory to perform our screenings, it can no longer keep up with the sensitivity requirements of the dark matter and neutrino-less double beta decay experiments. This is doubly problematic because in addition to vetting material before accepting it in detector construction, we must also understand precisely the amount of radiation expected from each and every component. Only if we know what we expect from all of these materials can we hope to observe an excess and claim discovery of an exceptional signal. Very few facilities exist worldwide with germanium detectors with sufficient sensitivity to satisfy these needs.We will install a new facility at Boulby with a state-of-the art germanium detector that will be used by 15 institutes across the UK in their low background screening campaigns for the leading dark matter and neutrino-less double beta decay experiments. With this much needed capability to deliver amongst the world's most sensitive material screening tests, the Boulby facility will be reinstated as a leader in the field, and we will be able to construct the next generation of experiments that will help unravel the mysteries of the Universe.Such a facility would be very useful to a wide variety of applications well beyond particle physics and cosmology. It would significantly improve environmental radioactivity studies already in progress at Boulby, and would provide capability for studying the climate with aerosol growth and cloud formation, and next generation electronics. Internationally there is an industrial and commercial demand for such instruments with the sensitivity we would provide.