The Sudbury Neutrino Observatory, or SNO for short, differs from other neutrino observatories in that it uses heavy water as its detection material, allowing mu and tau type neutrinos to be detected, while most others, such as Super Kamiokande, can only detect the electron neutrino. This is important, as it allows the theory of neutrino oscillations to be tested. Neutrino oscillations, if they exist, could resolve the Solar Neutrino Problem. Preliminary data from SNO seems to support the theory, but no official papers have been published as of October 19, 2000.

SNO is situated 6800 feet under ground, in the second-to-bottom drift (jokingly referred to as the SNO drift) of INCO's Creighton Mine, near Sudbury, Ontario, Canada. Queen's University is heavily involved in the project.

A more recent note: the results are in. The observatory did indeed detect mu and tau neutrinos, indicating that neutrinos do have mass, do oscillate, and effectively resolving the Solar Neutrino Problem.

Lord Brawl says: "This facility features prominently in Robert Sawyer's Hugo-winner Hominids and sequels." Thanks!

The proposal for the Sudbury Neutrino Observatory was submitted to the Canadian government at the end of 1987. The observatory was to be a research venue for both astrophysics and particle physics, by measuring neutrino radiation from the sun to study solar processes, as well as observing properties of neutrinos. Neutrinos are a class of sub-atomic particles produced in various nuclear reactions. Their existence was first postulated by Wolfgang Pauli in 1931, but because of their weakly interacting nature, were not discovered until 1956 (Neutrino Wiki). It was believed that the fusion reactions that powered the sun should produce neutrinos, and this was confirmed by Ray Davis in 1968 using an underground detector full of carbon tetrachloride, or dry cleaning fluid. (Bahcall 2002, also Neutrino Wiki) However, the results then found, and later confirmed by following experiments, showed a discrepancy between the number of neutrinos expected from the theory, and the number actually measured. Various explanations were attempted, but the problem proved persistent, eventually becoming known as the “Solar Neutrino Problem”.

SNO was created to test the hypothesis that the neutrinos being emitted from the sun were being transformed into other types, or “flavors” of neutrinos, that were previously undetectable. To this end, SNO was conceived as a heavy water detector. The use of heavy water would make SNO able to differentiate between neutrino flavors, and thus resolve the solar neutrino problem. (SNO Collaboration 1987) Because the transformation of neutrinos into other flavors would imply that at least one flavor had mass, such a finding would be proof for neutrino mass and have major implications for particle physics as well. Several other neutrino detector experiments are in operation around the world, such as GALLEX in Italy, Super Kamiokande in Japan, and AMANDA in Antarctica, but SNO is the only one of these to use heavy water, and the only one currently able to differentiate neutrino flavors. (Talcott 2004)

The observatory itself is located more than two thousand meters underground, in a branch of the Creighton mine of INCO Ltd. known as the SNO Drift. The site was selected for several reasons. The nature of the experiment required that it be adequately shielded from cosmic radiation, which would cause errant results, and the two-kilometer depth of the site was deemed sufficient for this. The detector would also, during its construction and operation, require a significant infrastructure to be active for an extended period of time. The Creighton mine was to be in operation for several decades, and the planned site for the detector was separated enough from mining operations to ensure minimal disruption of the experiment. The seismic stability and radioactive suitability of the mine were also considered and found to be adequate for the detector. (SNO Collaboration 1987) The location was very favorable for the experiment, but another uniquely Canadian element was essential to the technical success of the SNO experiment.

Heavy water is identical to normal water, except that it contains deuterium, an isotope of hydrogen. Chemically, its properties are very similar to water, but it has different nuclear properties that make it undergo different reactions with neutrinos. The heavy water used in the detector is what primarily differentiates SNO from other detectors around the world, because it is what allows the different neutrino flavors to be distinguished. Canada was in a unique position to provide the heavy water because of the CANDU nuclear reactors. CANDU reactors were developed in the 1960s, and are the only type of nuclear power generation used in Canada. They use the same basic fission reaction that powers other conventional nuclear power generation plants, but because they use heavy water as a moderator, they are capable of using unenriched uranium fuels, which makes expensive fuel enrichment facilities unnecessary. (CANDU reactor Wiki) Canada produces most of the world’s heavy water, and therefore had a great potential supply for the water that was required for SNO. The initial proposal for the detector called for 1000 tons of heavy water, which was to be loaned by Atomic Energy of Canada Ltd. (AECL) (SNO Collaboration 1987)

Other technical issues to be addressed included the photo-multiplier assembly, which would detect the light that signals neutrinos, and the software to analyze the results coming out of the detector. International partners handled both of these aspects. The United States Department of Energy made an early commitment to SNO in 1989, to provide the photo-multiplier tubes that would surround the heavy water vessel. The United Kingdom, through the University of Oxford provided photo-multiplier reflectors and software for the project. (NRC 1989)

The Three Elements of Big Science Planning in Canada

The scale and characteristics of the project required a highly concerted effort by government, university, and industry, across three countries, but the majority of the construction and operating costs for SNO were to be provided by Canadian sources, with the most coming out of NSERC (NRC 1989). SNO represented a significant investment on the part of the federal government, and was therefore subject to a thorough evaluation process, but the project enjoyed support from the scientific community, and regional and federal governments, which helped ensure its success.

In an article on Canadian astronomy, Richard Jarrell makes three observations about big science planning in Canada. (Jarrell 1995) First, that timing is everything. Jarrell writes that the timing of projects with relation to other events or considerations on a national scale are often important in building support for large projects. Second, he writes that consensus is essential. Consensus of the scientific community as to the feasibility, anticipated benefits, and facility location and specifications sustained over long periods appear to Jarrell to be a necessary condition. Finally, he writes that publicity is crucial. The visibility of projects, and the willingness of scientists to sell the projects are vital. These three points were addressed almost explicitly in a technical and scientific feasibility study report released in 1988. The committee that reported on SNO was composed of many recognized scientists from around the world, including such people as Dr. Masatoshi Koshiba, then at CERN, who went on to win the Nobel Prize for his work with neutrinos. (Scientific Review SNO 1988)

On the timing of the proposal, the committee wrote that SNO was capable of addressing some of the most current and important questions in physics at that time. The solar neutrino problem and the neutrino mass question were both major topics in physics, and this can be seen by the number of other neutrino experiments that were occurring at around the same time. (See Talcott 2004) Moreover, the heavy water that would compose the core of SNO was an available and otherwise unused resource. (Scientific Review SNO 1988, pg 13) In terms of political timing, Dr. C.H. Langford, who chaired the review committee, noted in an interview that federal government support for the project may have been contingent on the willingness of the regional governments to accept SNO in lieu of other federal investment. (C.H. Langford, interview, April 5, 2004) Like for the heavy water, there may have been a limited window of opportunity here.

On the second point of consensus, the scientific review shows a clear scientific consensus in favor of SNO. The main problem that SNO was conceived to solve, namely the solar neutrino problem, was well defined and the physics that would operate the detector were similarly known. In terms of the science, whatever the result, it was known that the result would be significant and important. The review committee was unanimous in its support. (Scientific Review SNO 1988, pg 14) SNO enjoyed a high level of regional support in northern Ontario, as well, because it represented a major investment in the research community for that area. Foreseen at the time by the scientific review committee and others, the area has become known as a major physics research facility, and has recently received even more investment to expand SNO into a permanent institution. (Scientific Review SNO 1988, pg 4. University Affaires 2003)

Jarrell stresses the importance of publicity in order to build support for a project, arguing that civil servants and politicians must be convinced to put up money for a project whose benefits might not be apparent to someone outside the scientific community. This does not immediately appear to be the case however, for SNO. Asked in an interview whether the SNO was ever in jeopardy of being cancelled, the director of the SNO project, director of the SNO Institute Dr. Arthur McDonald replied that, while SNO faced considerable obstacles, it was able to survive because of its scientific merit, implying that political support was dependent on the support of the scientific community. (Charbonneau 2003) de la Mothe and Paquet (1994) call the scientific community in Canada the “Republic of Science”, which resists central planning of science and exercises a great degree of autonomy. Though the extent to which this applies to a big science project like SNO, whose financial needs comprise a large proportion of funding agencies’ budgets, is unclear, Dr. McDonald’s comments suggest that SNO’s acceptance may have been due more to the desires of the scientific community rather than being a conciliatory action by the federal government.

The Response to SNO

As was foreseen by its champions in the physics community during the planning stages, SNO has become a symbol of Canadian excellence in physics, and has furthered the field of physics, as well as raised the international profile of Canadian science. The first results of SNO were announced in July 2001 after several years of preliminary data taking and testing. SNO found that the number of neutrinos transformed into other flavors after leaving the sun correspond to the predictions of standard solar models. (Collins 2001) Other significant results followed, and SNO quickly gained prestige. The SNO results were judged runner-up by Science magazine as the most significant discovery in 2002, and the American Institute of Physics called it one of the top two physics stories of the year. (Charbonneau 2003) The Canadian government and others have honored the director of the observatory Arthur McDonald for his achievements at SNO, but there were other benefits as well. The Sudbury region is quickly becoming a diverse center of academic research with SNO playing an integral role. What was to be a time-limited project will become a permanent underground research facility, with the pledging of $50 million by the Canadian government. (Charbonneau 2003) The observatory continues today to gather neutrino data, and has become an important feature of the scientific landscape.


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A longer version of this writeup was submitted to a class on Canadian Science Policy. Node your homework.

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