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Starting a scintillating search: SNO’s second act looks for neutrinos closer to home

Starting a scintillating search

Tim Lougheed
September 24, 2019
Teaser: 

Over the last decade, the Sudbury Neutrino Observatory (SNO) has reinvented itself as SNO+. Led by Mark Chen, the Gordon and Patricia Gray Chair in Particle Astrophysics, the project has taken advantage of a unique piece of research infrastructure and set out on a new mission.

Deck: 

Like a beloved book or movie that you hope has a sequel, the most successful scientific projects cry out for a second act. That is just what has happened to the Sudbury Neutrino Observatory (SNO), which over the last decade has reinvented itself as SNO+, led by Mark Chen, the Gordon and Patricia Gray Chair in Particle Astrophysics, a project that has taken advantage of a unique piece of research infrastructure and set it on a new mission.

[neutrino search - concept illustration]

Difficult detection

The unusual and innovative nature of SNO was apparent even as Queen’s physicist Art McDonald and his colleagues were launching the project in the 1980s. Its goal was to measure the oscillation of solar neutrinos – sub-atomic particles that are part of the broad spectrum of particles and energy emitted by the nuclear reaction that drives our sun. These neutrino interactions promised to shed new light on how our nearest star maintains its massive output, but they would be challenging to observe. In order to do so, scientists had to build a detector in one of the world’s deepest excavations, the Creighton Mine in Sudbury, Ontario, where their equipment would be isolated as thoroughly as possible from the constant bombardment of cosmic radiation raining down on the Earth’s surface.

[concept illustration of a mine]
Illustration by Gary Neill

Although that might sound like a counter-intuitive step, it turns out that the only thing from the sun that can make its way through 2,100 metres of Northern Ontario bedrock is a neutrino. In addition to being much smaller than most other elementary particles, neutrinos also lack any electrical or magnetic properties and so can travel through dense materials that form a barrier to everything else. Unfortunately, this also means they tend to remain invisible to the sophisticated collection of electromagnetic instruments that are usually employed in sub-atomic physics. Instead, neutrinos must be witnessed as simple flashes of light when these particles react with nuclei and atoms inside a large detector.

SNO created a setting for such reactions to occur inside a 12-metre wide sphere filled with about 1,000 tonnes of heavy water, a variation of the familiar liquid where hydrogen atoms contain an extra neutron, used in Canadian-designed CANDU nuclear reactors. This sphere was surrounded by thousands of sensitive light detectors, which could record the flashes as each neutrino revealed its presence. By 1999 this elaborate array was up and running, providing findings that the SNO Collaboration, led by project director McDonald, began to publish over the next few years. This work was the first experimental discovery of oscillations in solar neutrinos, which became a cornerstone observation in our understanding of the sun and earned McDonald the 2015 Nobel Prize in Physics and the 2016 Breakthrough Prize.

Fish-eye photo of the SNO Detector at SNOLAB
Neutrino Detector

Evaluating a buried asset

The technical hurdles that led to this scientific accomplishment are no less remarkable. As the SNO experiment produced their landmark results, a few of the collaborators began to consider what else they could do afterward with this elaborate and ultra-clean scientific facility that they now had available to them. Queen’s University physics professor and the Gordon and Patricia Gray Chair in Particle Astrophysics, Mark Chen, was among those people. As early as 2003, he helped obtain funding from the Canada Foundation for Innovation to expand the SNO facility, turning it into SNOLAB, an expanded underground laboratory at the same level in the mine where the original SNO detector was built.

Chen was pulled into the field of neutrino physics as a graduate student at the California Institute of Technology, where he joined a group looking for these particles around nuclear reactors. “It was a small group so we had to do everything by ourselves,” he recalls. “We built the detectors, trucked them to the reactor, and lowered them in place. I stacked the lead shielding. I coded the software and hooked up the electronics. I mixed the liquid that went into the detection chamber. I did every step of it and I really enjoyed learning about all of the aspects of experimental physics. That got me started with these strange but amazing neutrinos.”

Enjoying the camaraderie of working in smaller collaborations with a tightly-knit body of researchers, he welcomed an opportunity to join SNO and return to Queen’s (he is a Science ’89 graduate) in 2000. Chen became leader of the experiment’s “physics interpretation working group,” which was responsible for comparing SNO’s data with its counterparts, gathered by similar types of underground detectors in Japan and Italy. 

In addition to helping establish the expanded underground lab, Chen conceived and led the development of SNO+, the successor to the original SNO experiment after the heavy water was removed. He took advantage of his experience in California and at Princeton where he spent six years after his doctorate and where neutrinos were detected using a special material known as liquid scintillator. In contrast to a detector filled with heavy water, these devices employ specially developed organic liquids that give off more light from interacting neutrinos than in water.  

Some of the liquids best suited to this task also tend to dissolve acrylic, the light clear plastic that is the preferred building material for such detectors, including the one built at  SNO. In an effort to identify scintillator liquids that could be used in Sudbury, Chen’s physics department work space became filled with as much glassware as any laboratory in the chemistry department. After studying a number of different chemical candidates for the scintillating liquid, he homed in on linear alkylbenzene (LAB), a hydrocarbon chain connected to a benzene ring that lights up when particles like neutrinos interact with it.

Mark Chen with a photomultiplier tube from the detector. Photomultiplier tubes (PMTs) are the eyes of the detector. PMTs are very sensitive light detectors, capable of sensing single light photons and producing an electrical pulse that travels to the data acquisition electronics.
[Mark Chen]

A scintillating turn

As it turned out, LAB is not only compatible with acrylic, but also very transparent, has a high light output, and is less flammable than other scintillator liquids. Above all it is already being mass produced as a feedstock by companies that manufacture surfactants for detergents. These commercial suppliers would have little problem parting with enough LAB to fill the SNO+ detector – about 800 tonnes’ worth –  and at a much more reasonable price than any boutique operation turning out highly-specialized chemicals.

Meanwhile, the scientific task for these detectors has changed significantly. While SNO was dedicated to examining the behaviour of solar neutrinos, SNO+ has begun exploring the nature of neutrino mass and oscillations, and has an interesting new capability – looking for neutrinos generated within our own planet, known as geoneutrinos. 

“The objective is to see the number of neutrinos produced in the deep earth that come from uranium and thorium, because the decays are producing heat and it’s a very significant component of the overall heat output of the earth,” says Chen. “The question then is: how much of that heat output is being generated by radioactivity?” 

An underwater camera mounted in the SNO+ (Sudbury Neutrino Observatory) neutrino detector captures a snapshot image when the 12-metre diameter acrylic sphere is full. Viewed from below, ropes are seen crisscrossing the top of the sphere extending down (foreground), and each of the shiny cells that are visible is a 20-cm diameter super-sensitive light detector. The water-air interface inside and outside the acrylic spherical tank creates visual distortions as light refracts at the optical boundary.
[detector in SNOlab]

While that output is just a tiny fraction of the energy we receive from the sun, geologists say anywhere from 40-100 per cent of the heat generated underground could come from this radioactive decay. Chen and others are therefore hoping to refine that understanding by measuring Earth’s “neutrino glow,” thereby establishing an entirely new perspective on our world’s innermost workings as they witness the behaviour of geoneutrinos.

Further mysteries: Antimatter

[concept illustration of a mine]
Illustration by Gary Neill

At the same time, Chen is looking forward to employing SNO+ for an even more ambitious undertaking, aimed at resolving one of the most fundamental problems in physics: why the universe has essentially no antimatter. This inverse form of matter – made up of particles with opposite charges to the ones that fill our universe – normally annihilates any conventional matter it encounters, but there is no clear reason why processes in our universe should not have produced equal amounts of both at some earlier stage in its history. This symmetry has been observed in all other physical phenomena, but this exception continues to challenge physicists.

Chen is hoping to tackle this mystery with SNO+. He and his colleagues have been tweaking the formulation of the liquid scintillator cocktail. By pouring the rare metallic element tellurium directly into the detector chamber, they intend to create the conditions for a rare form of nuclear decay in which two electrons leave the atom and produce two antimatter versions of the neutrino. If, however, no neutrinos are emitted in this decay it implies the two neutrinos can, in a sense, annihilate each other, thereby proving that neutrinos are their own antimatter particle. This strange property connects back to processes in the early universe that can generate the matter-antimatter asymmetry that allows the universe, as we know it, to exist. While the absence of these particles cannot be directly recorded in SNO+, the energy of the escaping electrons should be different if this strange decay process occurs without neutrinos.

For now, however, Chen and his colleagues must be patient. They have acquired eight tonnes of a tellurium compound, which must be purified to eliminate any trace amount of radioactivity in the material. The SNO+ collaboration is building a large-scale underground chemical process in SNOLAB that will enable the tellurium to be rendered pure enough to add to the ultra-low background neutrino detector. This material, originally mined in Sweden then processed in China, is now “cooling off” underground in Sudbury, where it is shielded from cosmic radiation that can produce trace amounts of radioactive elements from the parent tellurium atoms. In 2019, the purification of this material is scheduled to begin, as this remarkable facility marks yet another milestone in its ongoing scientific and engineering career.

Centres and Institutes

Arthur B. McDonald Canadian Astroparticle Physics Institute

Core research: 

The Arthur B. McDonald Canadian Astroparticle Physics Research Institute is a national hub for astroparticle physics research, uniting researchers, theorists, and technical experts within one organization.

Queen’s University led 13 Canadian institutions in creating the centre’s predecessor organization in 2015. The McDonald Institute, officially launched in 2018, works to enhance Canada’s global leadership in the field, which includes dark matter and neutrino research.

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