Sample preparation equipment for ultra low background screening with ICP-MS

'Dark Matter' - a mysterious substance that holds the galaxies together makes up an incredible 85% of the mass of the Universe. It is believed by many to be made up of Weakly Interacting Massive Particles (WIMPs) - but they have not yet been detected experimentally. This is because they bounce off atoms 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 may 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, forcing them deep underground. However, this is still not enough to provide the quiet environment required; they are therefore shielded from natural radioactivity found in underground rock - harmless to us but catastrophic to the sensitive devices. 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 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 check 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. It cannot tell us about the decay products from U and Th as HPGe can, but together they produce the complete picture we need. Moreover, the time taken to screen materials - about a day or less, is just what is called for in building the next generation of experiment. However, ICPMS requires all materials to be 'prepared' - dissolved in acid and introduced as a liquid. ICPMS will only achieve high sensitivity if the materials are digested, prepared and introduced cleanly. With traditional techniques, heating materials in open glassware exposed to the elements, sensitivity can suffer dramatically.

This proposal is to enhance the UK's existing ICPMS with closed microwave ashing and digestion ovens with ultra-pure, high concentration, aggressive acids, to support the UK's Dark Matter R&D programme. Such new capability would give incredible sensitivity, in only hours. This would elevate the UK's R&D programme to unique world-class status now and well into the future, at a time when internationally HPGe and ICPMS facilities are struggling to cope with demand and do not possess enough sensitivity.

It will also aid other rare event searches that require ultra-low 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. ICPMS also has powerful 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.

There now exists significant international demand for screening materials for trace levels of radioactive contaminants, especially U and Th, for next generation rare event particle physics experiments, particularly those seeking to detect Dark Matter or Neutrino-less Double Beta Decay. These experiments require construction from the purest materials, demanding radio-assaying at the ppt level, and with throughput sufficient to screen of order a hundred samples per year. We have demonstrated instrument sensitivity at ppt-levels as part of our Dark Matter R&D programme using an Agilent 7900 ICPMS. Achieving such sensitivity for material assays, however, depends critically on sample preparation techniques and cleanliness, since solids must be digested completely in acid before assay, a process that traditionally exposes the sample to environmental contamination. With closed microwave technology and ultra pure acid systems proposed here, the potential impact of the ICPMS system is extended powerfully. There exists no system in the UK dedicated to low-background screening with microwave sample preparation technology. To our knowledge, this is true internationally.

ICPMS is a technology that is already used extensively outside of particle physics in diverse fields such as food safety, pharmaceuticals, environmental sciences, forensics and clinical research. Most commercial and research based ICPMS systems are used extensively with high activity materials that cause background interferences and limit the sensitivity of the instruments to low levels of particular elements. The in-house Agilent 7900 we are using for our R&D programme delivers unique ultra-low background capability that is of significance to these fields in trace elemental or contamination identification and quantification. To allow exploration and initiate programs to realise impact in such fields, we have recently been awarded funds (£5,000) through the STFC Impact Acceleration scheme, administered through UCL Business (UCLB), the technology transfer arm of UCL. Development of sample preparation equipment, proposed here, that accepts virtually all materials, delivers ppt-level sensitivity reliably and reproducibly, retains volatile gases, and enables high concentration HF use, all for a system reserved for ultra-low background screening, considerably enhances the already significant potential for economical and societal impact within these areas.

Food safety is an immediate area of focus for impact. Analyses for food safety now require ever increasing sensitivity to titanium dioxide and zinc oxide concentrations, as well as any potential particles of O(100 nm) diameter or less. This must be achieved measuring absolute concentrations, which is difficult given high background devices. However, titanium sample preparation for ppt-level sensitivity is extremely difficult unless microwave digestion and HF acid is used, as we are demonstrating as part of our R&D. We are working with Analytix/Milestone, the UK's leading manufacturer and supplier of microwave technology for sample digestions, to develop methods and techniques for titanium preparation, as well as plastics and quartz materials that are also not readily digested without microwaves or ashing. Even where some of these materials may be digested eventually, prolonged open systems result in loss of analytes and influx of contaminants, increasing systematic uncertainty and lowering sensitivity, respectively.

Similarly, geochemistry and environmental radioactivity measurements are pushing on sensitivity limits due to the nature of the samples regularly screened and their inherent contaminations. In the UK, ICPMS user's meetings, most prominently hosted by Agilent Technologies, highlight the potential for rich commercial and industrial collaboration in these areas using a dedicated low-activity system with capability to rapidly sample any material reliably, with high precision, and to extremely low levels.