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

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

'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.

Planned Impact

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 throughUCL 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.

Publications

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Akerib D (2016) Tritium calibration of the LUX dark matter experiment in Physical Review D

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Akerib DS (2017) Results from a Search for Dark Matter in the Complete LUX Exposure. in Physical review letters

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Aprile E (2014) Observation and applications of single-electron charge signals in the XENON100 experiment in Journal of Physics G: Nuclear and Particle Physics

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Carmona-Benitez M (2016) First Results of the LUX Dark Matter Experiment in Nuclear and Particle Physics Proceedings

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Dobson J (2018) Ultra-low background mass spectrometry for rare-event searches in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

 
Title Perspectives On The Unknown 
Description Perspectives on the Unknown is a short documentary film that investigates a scientific discovery known as dark matter and dark energy, which fills up over 95% of the Universe and yet remains as the unknown. The film tries to present different perspectives on this invisible matter by questioning how scientists deal with this mysterious matter, and how artists interpret this unknown. The film features two artists, Alison Gill and Julie Mecoli, with a physics academic, Dr. Chamkaur Ghag from department Physics and Astronomy at the University College London. This project is a collaborative project with Roger Stabbins and Thomas Deacon. 
Type Of Art Film/Video/Animation 
Year Produced 2015 
Impact This work has led to further enquiries from students in the arts, and organisation of a visit tot the Boulby Underground Laboratory in 2016 where several national artists will group and develop proposals for further collaborative work. The intention is that this would result in a travelling exhibit. 
URL https://vimeo.com/123613759
 
Description Trace contaminant measurements have been performed.
Exploitation Route performing trace radioactivity measurements
Sectors Other

 
Description Yes - to perform low activity measurements for commercial companies
First Year Of Impact 2014
Sector Other
Impact Types Economic

 
Description Chair of STFC's Particle Astrophysics Advisory Panel
Geographic Reach National 
Policy Influence Type Participation in a advisory committee
 
Description Impact Acceleration Award
Amount £5,000 (GBP)
Funding ID ST/M003981/1 
Organisation Science and Technologies Facilities Council (STFC) 
Sector Academic/University
Country United Kingdom
Start 10/2015 
End 12/2015
 
Description International Exchanges Award
Amount £11,029 (GBP)
Funding ID IE141517 
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 03/2015 
End 02/2017
 
Title Development of lead shields 
Description In partnership with UK industry (Lead Shield Engineering) we have developed new designs for low background radiation shields for use with gamma spectroscopy detectors. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact These designs have been realised with lead and copper shields manufactured by Lead Shield Engineering UK for the Boulby Underground Laboratory, the AWE, and others. 
URL http://www.lead-shield.co.uk/
 
Title Trace radioactivity analyses with ICP-MS 
Description Development of methods for trace uranium and thorium radioactivity analyses with ICP-MS using microwave digestion 
Type Of Material Technology assay or reagent 
Year Produced 2016 
Provided To Others? Yes  
Impact We have been able to assay materials to levels of less than 10 ppt (g/g) for uranium-238 and thorium-232 
 
Title Trace radioactivity measurements 
Description Improvements to trace radioactivity measurements in material samples using underground gamma spectroscopy and surface mass spectrometry. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact We have provided low level radioactivity measurements for rare event search experiments with UK involvement but also to industry; in particular SSI, and Photek Ltd for the enhancement of their products supplied commercially to UK and international consumers. 
URL http://www.boulby.stfc.ac.uk/Boulby/Projects/39343.aspx
 
Description LUX-ZEPLIN 
Organisation Lyngby-Taarbæk Municipality
PI Contribution I am the co-lead of the LZ 'Background and Screening' Work Package (WBS1.10), one of 11 in the international project, and lead for the equivalent Work Package (WP4) in the UK project. These contain tasks related to the radio-assay of all materials for construction of the instrument to meet low-background requirements unprecedented in the field to-date, and characterization of the experiments radiation field from materials and the environment to provide accurate input for the background model. These are crucial tasks necessary for LZ to achieve its science reach in terms of Dark Matter sensitivity, and to assess any potential signal in case of discovery. In addition to overall leadership and management of these Work Packages, UCL is specifically responsible for delivery of approximately half of all project radio-assays together with radio-content characterization and modeling. This is performed with state-of-the-art technologies and infrastructure I have been developing in the UK since 2012.
Collaborator Contribution All other activities in this international collaboration, from design through to construction.
Impact Construction of the LZ dark matter experiment
Start Year 2012
 
Description Event Horizon Cafe Scientifique 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact 30 people attended a Cafe Scientifique event organised by UCL undergraduate students where I talked about the search for Dark Matter in our Universe. Impact that has arisen since includes requests for more information on how to conduct further studies or apply for PhD and postgraduate positions in the field of Dark Matter and particle physics in general.
Year(s) Of Engagement Activity 2015
URL https://www.facebook.com/events/197495787252785/
 
Description Article in Cosmos magazine 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Featured in a Cosmos Magazine article about the search for Dark Matter.
Year(s) Of Engagement Activity 2015
URL https://cosmosmagazine.com/physical-sciences/ghost-traps-hunt-dark-matter
 
Description BBC Today programme feature 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Radio feature discussing UK particle physics and involvement in 'Beyond Standard Model' experiments including Dark Matter searches.
Year(s) Of Engagement Activity 2013
URL http://www.bbc.co.uk/programmes/b03kpnjl
 
Description Forbes magazine article 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact 5000 views of this article discussing Dark Matter experiments with UK involvement, where I discuss the implications of new results. This article led to changes in opinion of the readers as documented by the comments on the website, and further enquiries.
Year(s) Of Engagement Activity 2013
URL http://www.forbes.com/sites/bridaineparnell/2013/11/07/why-the-lux-results-matter-to-dark-matter-and...
 
Description IOP Physics World article 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Article in Physics World magazine about underground science including Dark Matter searches. This article highlighted activities at the UK's Boulby Underground Laboratory.
Year(s) Of Engagement Activity 2015
URL http://physicsworld.com/cws/article/indepth/2015/may/07/subterranean-science
 
Description IOP Physics World article 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Article highlighting the selection of the next generation of experiments in the search for Dark Matter, including LZ.
Year(s) Of Engagement Activity 2014
URL http://physicsworld.com/cws/article/news/2014/jul/15/dark-matter-searches-get-us-government-approval
 
Description Light and Dark TV series 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact This was a major BBC production broadcast on TV in two-parts, hosted by Prof. Jim Al-Khalili. It featured a section on Dark Matter where I discussed underground experimental techniques.
Year(s) Of Engagement Activity 2013
URL http://www.bbc.co.uk/programmes/b03jrxhv
 
Description Naked Scientists Radio and Podcast feature 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Radio and Podcast feature by the Naked Scientists discussing Dark Matter searches. This feature led to several further media enquiries and questions from the general public.
Year(s) Of Engagement Activity 2015
URL http://www.thenakedscientists.com/HTML/podcasts/naked-scientists/show/20150602/
 
Description New Scientist Public Event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact 270 members of the general public attended a Particle Physics Experts Masterclass organised by New Scientist. I was one of the 5 experts, presenting Dark Matter including the LZ experiment, and I was a member of the panel for a Q&A session. There was significant interest during the ticketed event (£150 per person), and outcomes that have arisen since include contact from schools and students regarding Dark Matter for their school projects, and contact from the media.
Year(s) Of Engagement Activity 2016
URL http://www.eventbrite.co.uk/e/instant-expert-mysteries-of-particle-physics-tickets-19713709235?aff=n...
 
Description Public Lecture at Second Home 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Industry/Business
Results and Impact Approximately 70 people that work at or operate from Second Home, London, attended a talk about Dark Matter that focused on technologies. Second Home houses numerous technology firms and businesses and this talk led to several questions about potential socioeconomic impact of particle physics and Dark Matter experiments, and potential areas for collaborative research and spin-offs. It has also led to arrangement of a trip to Boulby Underground Laboratory in 2016, where I will host 10 artists from across the UK on an underground tour of the science facility.
Year(s) Of Engagement Activity 2015
URL https://www.facebook.com/events/1498288067129210/
 
Description Radio 5 Live feature 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Dark Matter feature on Radio 5 live, highlighting UK research. This feature led to enquiries from the general public and other media outlets.
Year(s) Of Engagement Activity 2015
URL http://www.bbc.co.uk/programmes/b043wvt9
 
Description Radio feature on Newsdrive 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Interview on BBC Newsdrive discussing UK involvement in international Dark Matter search experiments.
Year(s) Of Engagement Activity 2014
URL http://www.bbc.co.uk/programmes/b03zdylq
 
Description School Visit (Newham Collegiate College, London) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Particle physics seminar focussing on searched for beyond the standard model physics, particularly for dark matter signatures. Approximately 40 students attended, and there was considerable interest afterwards with questions, discussion, and follow up emails to me.
Year(s) Of Engagement Activity 2017