{"id":7629,"date":"2017-10-16T14:04:54","date_gmt":"2017-10-16T18:04:54","guid":{"rendered":"https:\/\/newsroom.carleton.ca\/?post_type=cu_story&p=7629"},"modified":"2025-10-17T10:57:54","modified_gmt":"2025-10-17T14:57:54","slug":"neutrino-hunter","status":"publish","type":"cu_story","link":"https:\/\/carleton.ca\/news\/story\/neutrino-hunter\/","title":{"rendered":"Neutrino Hunter"},"content":{"rendered":"\n
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\n Neutrino Hunter\n <\/h1>\n \n <\/header>\n <\/div>\n\n \n <\/path>\n <\/path>\n <\/svg>\n <\/div>\n\n \n\n <\/div>\n<\/section>\n\n

David Sinclair stands at the bottom of a cylindrical cavern two kilometres beneath the rolling scrubland of Sudbury, Ont. The walls of the Cryopit at SNOLAB<\/a> \u2014 a world-class science laboratory accessed via a spur tunnel from an active nickel mine \u2014 are shiny white trowel-smooth shotcrete. The concrete floor is covered with an epoxy finish to seal in surface dust and create a washable surface. But other than some blue ventilation tubes hooked up to the facility\u2019s air purification system, a few bundles of cable and a couple pallets of shrink-wrapped electronics equipment being stored here temporarily, the enormous space \u2014 20 metres high, with a diameter of 15 metres \u2014 is empty for now.<\/p>\n\n\n\n

That doesn\u2019t deter Sinclair<\/a>, an internationally renowned particle physicist and a distinguished research professor at Carleton University. He\u2019s focused on what the Cryopit could soon contain: one of the proposed uses for the space is the cutting-edge nEXO experiment<\/a>, a detector that will use five tonnes of enriched xenon in an attempt to measure the exact mass of neutrinos and determine whether the elusive elementary particles are their own antiparticles. Sinclair is also thinking about the past.<\/p>\n\n\n\n

\"\"<\/a>
Sinclair greets Arthur B. McDonald, co-winner of the 2015 Nobel Prize in Physics.<\/figcaption><\/figure>\n\n\n\n

He\u2019s reminiscing about the mid-1980s, when he was a young faculty member at the University of Oxford \u2014 before the Sudbury Neutrino Observatory<\/a> (SNO) experiment was built and helped a Canadian win the Nobel Prize in Physics, before the federal government decided to double down on its initial investment and fund the construction of SNOLAB, before his Carleton colleague Mark Boulay began the DEAP-3600 project<\/a> in the adjacent cavern to search for an unobserved form of matter that comprises most of the mass in the universe \u2014 and all of this was solid rock.<\/p>\n\n\n\n

Although he is always careful to share credit and stresses that it takes a lot of people to make a project of this scale feasible, Sinclair made a crucial decision while at Oxford that ultimately begat the SNO experiment and the thriving research legacy that followed. There had been talk in Canada\u2019s particle physics community about the possibility of an underground lab in the Sudbury area, with hundreds of metres of norite shielding sensitive detection systems from the cosmic radiation that bombards the planet. Sinclair was excited about the idea \u2014 and the opportunity to probe fundamental questions about the nature of the universe on home turf \u2014 but knew it would never proceed if people only discussed it over coffee and chipped away at the concept while concentrating on their day jobs. He had a sabbatical leave coming up in 1984-\u201885 and planned to spend it working in Australia, an appealing adventure for an ex-pat Canadian and his family.<\/p>\n\n\n\n

\u201cI was free to go anywhere in the world and do anything that I wanted,\u201d recalls Sinclair, looking up at the domed ceiling of the Cryopit. \u201cBut what the SNO project needed was somebody with a year of free time to take on the feasibility study. So I called up my wife and said: \u2018We\u2019re not going to Australia after all \u2014 we\u2019re going back to Canada.\u2019 And I\u2019ve been working on this ever since.\u201d<\/p>\n\n\n

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The Hunt for the Neutrino Begins<\/h2>\n\n\n\n

Funded by a collection of agencies, including Canada\u2019s Natural Sciences and Engineering Research Council (NSERC) and National Research Council, Industry Canada, the Northern Ontario Heritage Fund Corporation, the U.S. Department of Energy and the U.K. Particle Physics and Astronomy Research Council, the SNO Institute was formed in 1990. About 100 scientists from Canada, the U.S. and the U.K. comprised the team; its director was Queen\u2019s physicist Art McDonald \u2014 who would go on to win the 2015 Nobel Prize<\/a> \u2014 and Sinclair served as one of his deputies. Excavation began in March 1990 and was completed just over three years later.<\/p>\n\n\n\n

Sinclair \u2014 who was born in Montreal and grew up in Ottawa before doing a BSc and PhD in physics at Queen\u2019s and then a postdoc at the Niels Bohr Institute in Copenhagen \u2014 lived in Sudbury with his family during construction and commissioning of the SNO experiment. An avid fisherman who frequently goes casting in Quebec and northern Canada, he brought a rod and reel and occasionally dropped a line into local lakes and nearby Georgian Bay. But the real prize he was seeking was not so simple.<\/p>\n\n\n\n

The neutrino was first postulated by theoretical physicist Wolfgang Pauli in 1930 to explain why energy did not appear to be conserved in certain types of radioactive decay, which is defined as \u201cthe spontaneous transformation of an unstable atomic nucleus into a lighter one.\u201d The missing energy, Pauli suggested, was carried off by a tiny subatomic particle with zero electrical charge (neutrino is Italian for \u201clittle neutral one\u201d). Thought to be among the most abundant particles in the universe, they were exceedingly difficult to detect. \u201cAbout 100 billion neutrinos from the sun pass through your thumbnail every second,\u201d according to the Nobel Prize website, \u201cbut you do not feel them because they interact so rarely and so weakly with matter.\u201d<\/p>\n\n\n\n

The Sudbury Neutrino Observatory consisted of 1,000 tonnes of heavy water<\/a> (on loan from Atomic Energy of Canada Ltd.) inside a spherical acrylic vessel with a 12-metre diameter. This vessel was immersed in ultra pure water in a barrel-shaped cavern 2,070 metres below the Earth\u2019s surface; 30 metres tall, with a diameter of 22 metres, it\u2019s the largest cavern at this depth in the world. SNO was operated as a cleanroom lab: scientists, technicians and visitors had to shower and put on clean coveralls, shoes and hairnets before entering the facility, eliminating the trace levels of radiation that are emitted from the everyday dirt on our clothing, skin and hair.<\/p>\n\n\n\n

\"\"<\/a>
SNO+ detector at SNOLAB. Photo courtesy of the SNO+ Collaboration.<\/figcaption><\/figure>\n\n\n\n

The 10 or so neutrinos that interacted with the detector every day would be stopped or scattered by the heavy water, Sinclair and his colleagues hypothesized, producing flashes of light called Cherenkov radiation. This light would be detected by an array of 9,600 photomultiplier tubes (PMTs) mounted on a geodesic support structure<\/a> surrounding the heavy water vessel. The flashes would be recorded and analyzed, allowing scientists to extract information about the neutrinos that caused them. The lab included electronics and computer facilities, a control room, and water purification systems for both heavy and regular water. A trailer near the mine shaft on the surface would function as its office.<\/p>\n\n\n\n

As often occurs with experiments of this scale and complexity, there was a hiccup when the SNO detector was turned on for the first time in 1999. PMTs are a widely used technology in particle physics experiments but are not normally submerged in water. The PMTs for SNO were water tested at Carleton and seemed to work fine, but in the detector, running at about 2,000 volts, they started to spark. \u201cIt was a dark day for the SNO experiment,\u201d recalls Sinclair, who was approached by McDonald and asked to find a solution. Sinclair had designed SNO\u2019s innovative water purification system \u2014 the experiment required water with about a million times less radioactivity than drinking water \u2014 and had the technical experience (and engineering and chemistry knowledge) to address the problem. Eventually, he realized that because the PMTs were immersed in water from which all the standard dissolved gasses had been extracted, the gas-permeable seal in the PMTs allowed gas in the tubes to diffuse into the water. This was creating a vacuum at the base of the PMTs, and as the pressure dropped, the voltage capability also dropped. Sinclair came up with a way to introduce very clean nitrogen into the water, and the sparking stopped.<\/p>\n\n\n

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Solving the Neutrino Mystery<\/strong><\/h2>\n\n\n\n

With the detector functioning properly, data collection and analysis could begin. In 2001, the SNO team released its first results \u2014 and, right out of the gate, solved a 30-year-old mystery.<\/p>\n\n\n\n

Since the early 1970s, scientists have known that electron-neutrinos \u2014 one of three types of the particle, along with the muon-neutrino and the tau-neutrino \u2014 are emitted in vast numbers by the nuclear reactions that fuel the sun. But experiments that detected neutrinos reaching Earth \u201cfound only a fraction of the number expected from detailed theories of energy production in the sun. This meant there was something wrong with either the theories of the sun, or the understanding of neutrinos.\u201d SNO researchers discovered that some of the electron-neutrinos change into other types of neutrinos as they travel to the Earth.<\/p>\n\n\n\n

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<\/p>\n\n\n\n

\u201cThe determination that the electron neutrinos from the sun transform into neutrinos of another type is very important for a full understanding of the universe at the most microscopic level,\u201d SNO scientists said in a news release. \u201cThis transformation of neutrino types is not allowed in the Standard Model of elementary particles. Theoreticians will be seeking the best way to incorporate this new information about neutrinos into more comprehensive theories. The direct evidence for solar neutrino transformation also indicates that neutrinos have mass.\u201d<\/p>\n\n\n\n

That finding meshed with the results from the Super-Kamiokande detector in Japan, earning McDonald and Takaaki Kajita the Nobel Prize. \u201cThe discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe,\u201d the Royal Swedish Academy of Sciences said in a release. \u201cNow the experiments continue and intense activity is underway worldwide in order to capture neutrinos and examine their properties. New discoveries about their deepest secrets are expected to change our current understanding of the history, structure and future fate of the universe.\u201d<\/p>\n\n\n

\"The<\/figure>\n\n\n

Descending into the Western Hemisphere’s Deepest Mine<\/h2>\n\n\n\n

To visit SNOLAB, a half-hour drive west of downtown Sudbury, you must arrive early in the morning at the facility\u2019s surface building, a 3,000-square-metre, three-storey complex completed in 2005 to replace the original office trailer. In the gear room, you\u2019re given heavy-duty neon yellow coveralls, boots, a hardhat, headlamp and safety glasses and, after changing, you walk a couple hundred metres to the No. 9 shaft \u2014 the main access portal to Vale\u2019s Creighton Mine and, at 2,175 metres, the deepest continuous mine shaft in the Western Hemisphere.<\/p>\n\n\n\n

Scientists and guests squeeze into a double-decker elevator (known as a cage) with a few dozen miners beginning their shift, the gate is closed, and you descend at up to 50 kilometres an hour to 2,070 metres below ground. You step out of the cage into a dark tunnel (or drift) carved out of the rock, with walls covered in metal screening, and walk about a kilometre and a half, stepping aside to let mine vehicles pass, until reaching the entrance to the lab. Then you shower, put on clean clothing and enter a bustling 5,000-square-metre warren of scientists at work.<\/p>\n\n\n\n

SNOLAB has space for about 120 people per shift. The cavern that contained SNO \u2014 which stopped taking data in 2006 \u2014 is now home to the SNO+ experiment, which repurposed the original detector (replacing the heavy water with liquid scintillator, an organic liquid similar to mineral oil that gives off light when charged particles pass through it) for a study of low-energy solar neutrinos and other physics phenomena. There\u2019s also a new Cube Hall cavern that\u2019s ground zero for Carleton researcher Mark Boulay\u2019s DEAP experiment. The Cryopit is SNOLAB\u2019s other large space, but the cleanroom facility also includes smaller \u201cladder labs\u201d in the drifts that connect the caverns, a lunchroom and meeting room.<\/p>\n\n\n\n

\"\"<\/a>
The DEAP-3600 experiment. Photo courtesy of DEAP.<\/figcaption><\/figure>\n\n\n\n

There was no guarantee that any of this would be built. SNO was intended to be a single experiment, not a facility, says Sinclair, because the latter implied a long-term commitment that funders were not prepared to make. \u201cBut SNO was such a success that we changed that idea,\u201d he says. \u201cMaking a facility for a series of experiments was seen as the right thing to do. Doing a single experiment has two problems. First of all, these experiments take a lot of planning and design, so there\u2019s a long period where you\u2019re testing ideas. For SNO, that was over a decade, and it took about a decade to build the experiment, and a couple more years before we had the answers we were looking for. But if you want a facility that attracts students and young scientists, having a sequence of experiments so there\u2019s always something that\u2019s coming up with exciting results \u2014 and something that\u2019s big enough so there are always different groups coming together \u2014 that\u2019s a wonderful fertile environment for students and young scientists.\u201d<\/p>\n\n\n\n

It cost $70 million to build SNOLAB, with excavation taking place in 2007 and 2008, and cleanroom status achieved in 2010. The Cryopit \u2014 the third-largest space after the SNO cavern and Cube Hall \u2014 was designed specifically for cryogenic detectors; it\u2019s isolated within the facility in case the detector warms and water needs to be vaporized and expelled.<\/p>\n\n\n\n

Darryl Boyce, Carleton\u2019s assistant vice-president of Facilities Management and Planning, played a critical role in the construction of SNOLAB. \u201cHe provided outstanding management of the project,\u201d says Sinclair, \u201cjust as if it were being carried out on the Carleton campus.\u201d<\/p>\n\n\n\n

More than half the total funding was provided by the Canada Foundation for Innovation (CFI). The remainder came from the Ontario Innovation Trust, the Northern Ontario Heritage Fund and FedNor, with the CFI, NSERC and member institutions supplying operating funding, and the City of Greater Sudbury supporting public education. In January 2017, the CFI provided $28.6 million for three years of operations, and six months later the Ontario government added a $28.8- million five-year boost.<\/p>\n\n\n

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Focusing on Neutrino Mass with the nEXO Experiment<\/strong><\/h2>\n\n\n\n

Even though he is officially retired \u2014 a move he made three years ago to allow Carleton to hire new physics faculty \u2014 Sinclair is in line at a campus Tim Hortons at 8 a.m. after a late-night flight back to Ottawa from Sudbury. He\u2019s still working full time, going to his lab every day and supervising grad students, with the nEXO experiment occupying most of his research attention. Sinclair has been involved with the project since it started a dozen years ago, when EXO-200 \u2014 a prototype detector using 200 kilograms of liquid argon inside a vessel made from thin, ultra-pure copper \u2014 was built at Stanford. In 2007, the detector was moved to the Waste Isolation Pilot Plant, a geological repository for nuclear waste 650 metres below ground near Carlsbad, New Mexico.<\/p>\n\n\n\n

The detector is looking for evidence of a process known as neutrinoless double beta decay, a type of radioactive decay in which two protons<\/a> are simultaneously transformed into two neutrons, or vice versa, inside an atomic nucleus. Observing any signs of this process, says the Institute of Physics magazine Physics World<\/em>, would show that neutrinos are their own antiparticles (a particle with the same mass but opposite charge). \u201cThis would constitute discovering a new class of particles that lies beyond the Standard Model of particle physics,\u201d says Physics World<\/em>, \u201cand would be a major breakthrough in modern physics.\u201d Measurements of this decay process could also be used to determine the absolute mass of a neutrino.<\/p>\n\n\n\n

Sinclair, who chairs the international nEXO collaboration\u2019s board and is its spokesperson in Canada, is concentrating on the conceptual design for the full-scale detector that could be installed in SNOLAB\u2019s Cryopit. The push for funding \u2014 \u201cat the couple hundred million dollar level,\u201d he says \u2014 is under way. The project, which involves scientists from the United States, Russia, China and Germany, is looking for financial support from U.S. Dept. of Energy, among other potential backers interested in this type of physics. The experiment is so expensive in part because it will need five tonnes of isotopically enriched xenon, which alone is worth about $100 million. To get that amount, 50 tonnes of normal xenon must be put through an isotope separator \u2014 and global production of xenon, a byproduct of liquid oxygen used in the steel industry, is only about 40 tonnes a year, much of it ending up in car headlights, plasma televisions and spacecraft ion drives.<\/p>\n\n\n\n

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<\/p>\n\n\n\n

If the funding for nEXO was secured tomorrow, it would still take about six years to build the experiment and another six years to take data. But to Sinclair, the wait \u2014 and effort \u2014 is worth it. \u201cPeople have been looking for double beta decay for decades, and the discovery that neutrinos have mass means that we now know where to look,\u201d he says. \u201cIt\u2019s a very challenging experiment still, but we know it\u2019s not infinitely challenging, so it has new scientific impetus.<\/p>\n\n\n\n

\u201cWe set up this lab to do experiments that are absolutely fundamental to our understanding of the universe,\u201d he continues. \u201cIt\u2019s a very competitive and challenging field, to come up with experiments that can distinguish the very feeble signals we\u2019re looking for from the backgrounds that normally surround us with radioactivity and cosmic rays and all kinds of other surface noise that we\u2019ve avoided by coming deep underground.<\/p>\n\n\n\n

\u201cIn addition to the scientific legacy we\u2019re building on, there\u2019s a technical one. All the lessons we learned in making a deep underground experiment, in making very pure materials and keeping the whole system very clean, not only did we learn how to do this, but we also convinced the world that it\u2019s possible to do.\u201d<\/p>\n\n\n\n

In the echo of the empty Cryopit, Sinclair explains the significance of nEXO, which can trace its lineage directly to SNO. The breakthrough Higgs boson, which gives mass to all particles, might not work for neutrinos, which are much lighter than any other fundamental particle. It appears that a different mechanism may provide mass to the neutrino. If the neutrino is its own antiparticle, that theory makes sense, and double beta decay can only occur if the neutrino is its own antiparticle. \u201cThe physics would allow a different way to provide mass to such a particle,\u201d says Sinclair. \u201cIt would go a long way toward understanding the structure of the fundamental particles of the universe.<\/p>\n\n\n\n

\u201cBut there\u2019s another motivation, a cosmological mystery that we\u2019ve been struggling with for a long time,\u201d he adds. \u201cWhy is the universe here? Or, at least, why are we in it? We understand the big bang, that the universe started in a state of enormous energy. This energy can produce particles and antiparticles, and particles and antiparticles can annihilate back into energy, and this happened over and over and over again as the universe cooled and expanded. But at the end of that process, we were left with just particles. So there must be a symmetry between matter and antimatter, particles and their antiparticles, and we don\u2019t know what that is.<\/p>\n\n\n\n

\u201cOne of the really surprising outcomes of SNO \u2014 maybe the most significant outcome of SNO \u2014 is that maybe the answer lies in the properties of neutrinos. The arguments are somewhat indirect, but one of the criteria is that we have to understand the question as to whether neutrinos are their own antiparticles. Other aspects of the theory are going to be very challenging to test, but this is one of the windows that we can look at and see if we\u2019re on the right track to understanding this very important fundamental property of the universe \u2014 that we\u2019re here. Because if we didn\u2019t have the asymmetry, by the time the annihilation was over, we would be left with a scattering of particles and antiparticles at such low density that they never interacted. So, we\u2019d never get matter. We\u2019d never get material forming. We\u2019d never form. It\u2019s absolutely critical to understanding the universe \u2014 that, to me, is the main reason we\u2019re here.\u201d<\/p>\n\n\n

\"\"<\/figure>","protected":false},"excerpt":{"rendered":"

David Sinclair stands at the bottom of a cylindrical cavern two kilometres beneath the rolling scrubland of Sudbury, Ont. The walls of the Cryopit at SNOLAB \u2014 a world-class science laboratory accessed via a spur tunnel from an active nickel mine \u2014 are shiny white trowel-smooth shotcrete. The concrete floor is covered with an epoxy finish […]<\/p>\n","protected":false},"author":410,"featured_media":0,"template":"","meta":{"_acf_changed":false,"footnotes":"","_links_to":"","_links_to_target":""},"cu_story_type":[13],"cu_story_tag":[1919,1925],"class_list":["post-7629","cu_story","type-cu_story","status-publish","hentry","cu_story_type-research-discovery","cu_story_tag-faculty-of-science","cu_story_tag-research"],"acf":{"cu_post_thumbnail":false},"_links":{"self":[{"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story\/7629","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story"}],"about":[{"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/types\/cu_story"}],"author":[{"embeddable":true,"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/users\/410"}],"version-history":[{"count":2,"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story\/7629\/revisions"}],"predecessor-version":[{"id":98385,"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story\/7629\/revisions\/98385"}],"wp:attachment":[{"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/media?parent=7629"}],"wp:term":[{"taxonomy":"cu_story_type","embeddable":true,"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story_type?post=7629"},{"taxonomy":"cu_story_tag","embeddable":true,"href":"https:\/\/carleton.ca\/news\/wp-json\/wp\/v2\/cu_story_tag?post=7629"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}