Ultra-low activity material screening with in-house ICP-MS

令人难以置信的是，宇宙质量的85%是“暗物质”--一种神秘的物质，它将星系聚集在一起，防止它们飞离。它被认为是由弱相互作用的大质量粒子(WIMP)组成--但这些粒子还没有被实验检测到。这是因为，尽管它们被预测会与正常物质相互作用，从原子上反弹，但它们这样做的次数很少，而且强度很弱，因此它们的存在极难被检测到。尽管如此，科学家们一直致力于构建能够记录WIMP相互作用的灵敏探测器。问题是，在这些探测器中还有许多其他粒子相互作用，这可能会掩盖少数微弱的WIMP。由于WIMP相互作用如此罕见，因此实验必须屏蔽从太空轰炸地球的所有宇宙射线。这迫使他们深入地下，在矿山或山下。然而，这仍然不足以提供所需的安静环境。接下来，这些实验将不受地下岩石中发现的天然微量放射性的影响--对我们无害，但对敏感设备来说是灾难性的。然而，仍然有一个最终的背景，来自制造探测器的材料。微量的铀和钍通过放射性衰变产生信号，这些信号通常与WIMP的预期信号难以区分。这意味着必须进行广泛的材料筛选活动，以选择最纯净的材料来建造探测器。为了寻找WIMP，开发更灵敏的探测器要求越来越清洁的建筑材料。然而，我们现在已经达到了技术成熟，我们的下一个探测器可以具有理论上预测的最终探测暗物质的灵敏度。问题是，用来建造它们的材料的筛选能力没有跟上步伐。我们传统上依靠“高纯锗”探测器(HPGe)来测量材料，然后再用它们来建立实验。然而，HPGe需要数周的时间来筛选单个样本--在未来几年我们需要筛选数百种材料时，这是不可接受的。此外，HPGe实际上不能直接测量U和Th。相反，它测量U和Th衰变成的元素。电感耦合等离子体质谱(ICPMS)能够直接测量U和Th。此外，每个样本都可以在几天内进行筛选，并达到比HPGe好得多的水平。ICPMS不能像HPGe那样告诉我们U和Th的衰变产物，但它们一起提供了我们需要的完整图像。然而，首先筛选材料所需的时间正是建立下一代暗物质实验所需要的。这项提议是利用ICPMS发展英国的低本底材料筛选能力，以支持英国的暗物质研发计划。这样的新能力将为U和Th筛查提供所需的前所未有的灵敏度，并具有数天的周转时间。这将加强英国的研发计划，使我们拥有世界级的能力，而此时国际上的HPGe和ICPMS设施正努力满足需求，不具备我们所需的灵敏度。我们将开发的ICPMS将有助于其他需要超低活动水平的罕见事件搜索实验，例如那些寻求观察无中微子双β衰变的实验。这些实验可以告诉我们中微子的基本性质，并以此解释大爆炸后不久粒子和反粒子之间的微小不平衡，这是今天任何物质存在所必需的。ICPMS还在食品安全、制药、环境、法医和临床研究中有重要应用，在这些研究中，低水平污染物的元素分析是一个具有重大社会和经济影响潜力的丰富研究领域。

An incredible 85% of the mass of the Universe is 'Dark Matter' -- a mysterious substance that holds the galaxies together, preventing them from flying apart. It is believed to be made up of Weakly Interacting Massive Particles (WIMPs) - but these particles have not yet been detected experimentally. This is because, although they are predicted to interact with normal matter, bouncing off atoms, they do so only very rarely and weakly such that their presence is extremely difficult to detect. Despite this, scientists have been working on constructing sensitive detectors capable of registering WIMP interactions should they occur. The trouble is that there are many other particles interacting in these detectors that might mask the few and faint WIMPs.Since WIMPs will interact so rarely the experiments must be shielded from all the cosmic-rays bombarding Earth from space. This forces them deep underground, in mines or under mountains. However, this is still not enough to provide the quiet environment required. The experiments are next shielded from natural trace radioactivity found in underground rock - harmless to us but catastrophic to the sensitive devices. Yet still a final background remains, coming from the very materials the detectors are made from. Tiny amounts of uranium and thorium produce signals by radioactive decay that are often indistinguishable from those expected from WIMPs. This means that extensive material screening campaigns must be conducted to select only the purest materials in constructing the detectors.Developing ever more sensitive detectors in the hunt for WIMPs has demanded ever cleaner construction materials. However, we have now reached a technological maturity such that our next detectors could have the sensitivity theoretically predicted to finally detect Dark Matter. The trouble is that capability to screen materials from which to construct them has not kept pace. We have traditionally relied on 'High Purity Germanium' detectors (HPGe) to measure materials before using them to build experiments. However, HPGe requires many weeks to screen a single sample - unacceptable when we need to screen several hundreds of materials in the coming years. Furthermore, HPGe cannot actually measure U and Th directly. Instead it measures elements that U and Th decay into.Inductively-Coupled Plasma Mass Spectrometry (ICPMS) is capable of measuring U and Th directly. Additionally, each sample can be screened in a matter of days, and to levels much better than HPGe. ICPMS cannot tell us about the decay products from U and Th as HPGe can, but together they produce the complete picture we need. The time taken to screen materials in the first place, however, is just what is called for in the building the next generation of Dark Matter experiment.This proposal is to develop the UK's low-background material screening capability with ICPMS to support the UK's Dark Matter R&D programme. Such new capability would provide the required and unprecedented sensitivity to U and Th screening, and with turnaround times of days. This would enhance the UK's R&D programme to the point that we would have world-class capability, at a time when internationally HPGe and ICPMS facilities are struggling to cope with demand and do not possess the sensitivity we need.The ICPMS that we will develop will aid other rare event search experiments that require ultra-low levels of activity, such as those seeking to observe neutrino-less double beta decay. These experiments could tell us about the fundamental properties of neutrinos, and in doing so explain the tiny imbalance between particles and anti-particles shortly after the Big Bang needed for any matter to exist today. The ICPMS also has significant application in food safety, pharmaceutical, environmental, forensic and clinical studies, where elemental analysis of low levels of contaminants is a rich area of research with significant societal and economic impact potential.