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

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.

Internationally the ability to screen materials for trace levels of radioactive contaminants, especially U and Th, is a high priority for the next generation of rare event particle physics experiments, particularly those seeking to detect Dark Matter or Neutrino-less Double Beta Decay. They demand construction from the purest materials, requiring radio-assaying at the parts-per-trillion (ppt) level, and with throughput sufficient to screen of order a hundred samples per year. Development of an Inductively-Coupled Plasma Mass Spectrometry (ICPMS) system that meets these requirements would mark a step change in the UK's screening capability, coupling mid- and late-chain U and Th measurements from underground HPGe detectors to progenitor U and Th direct assays from ICPMS to deliver unprecedented sensitivity, as well as meeting sample throughput demands.

However, ICPMS is a technology that is already used extensively outside of particle physics in diverse fields such as food safety, pharmaceutical, environmental, forensic and clinical research. Development of equipment dedicated to low-activity measurements at the ppt level has genuine potential for economical and societal impact within these areas. 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 relatively high concentrations of particular elements. An in-house ICPMS, with controlled low background environment for sample preparations and measurements would fill the growing need for more sensitive elemental screening. 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. 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. Collaboration would also benefit research institutions such as, for example, the Department of Chemistry at UCL and the London Geochronology Centre where similar measurements are being performed but where again the ultimate sensitivity is considerably lower than would be available through this proposal.

Commercial companies developing low activity products represent particularly favourable avenues for realising impact. Potential beneficiaries would be ETEL as well as Hamamatsu Photonics. Building on previous work with each of them in screening materials for their development of photosensors (including the R11410 model to be used in LZ, QUPIDs, and ZEPLIN-III phototubes), we would be able to rapidly screen ultra-low background components from their R&D programme within the photosensor divisions. Similarly, with the University of Edinburgh and the Scottish Universities Environmental Research Centre (SUERC) we would enlarge activity with underground Ge detector screening to incorporate ICPMS for unprecedented sensitivity to U/Th in silicon electronics. Alpha particles from U/Th chains can cause 'Single Event Effects' and faults in integrated circuits, such that for critical systems involving many such integrated circuits, extremely low levels of U and Th are becoming a necessity. However, the ability to screen components for such activity across the full decay chains does not exist in the UK at present. ICPMS with ppt level sensitivity in U/Th opens up this avenue of research given the throughput and turnaround times ICPMS would deliver for progenitor U and Th isotopes (days rather than weeks required for Ge screening of U/Th daughters).