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Past Winner
2003 Gerhard Herzberg Canada Gold Medal for Science and Engineering

Arthur McDonald

Director, Sudbury Neutrino Observatory Institute

Queen's University

Arthur McDonald
Arthur McDonald

For the past 30 years Dr. Arthur (Art) McDonald has been poking at the Standard Model – physicists' description of the atomic world and its standard operating procedures – looking for cracks. He's smashed atoms, but more often delicately probed their insides looking for the tiniest inconsistencies. What he's found is a world described to near perfection by the Standard Model. Until he set his sights on neutrinos.

"The Sudbury Neutrino Observatory shows clearly a way in which the Standard Model is incomplete. The Standard Model postulates that the three kinds of neutrinos do not change from one type to another and that they do not have a mass greater than zero. In fact, both of these assumptions are wrong," says Dr. Art McDonald, one of three finalists for the 2003 NSERC Gerhard Herzberg Gold Medal. "In addition, with our measurements we could show that the present theories of how the sun burns are very accurate."

These discoveries were the result of one of the major physics experiments of the 20th century, the massive, underground Sudbury Neutrino Observatory (SNO), an international, big science endeavour in the works for 20 years. SNO is a scientific achievement that involves the uniquely Canadian intersection of heavy water, a mine two kilometres underground, and abundant physics expertise and connections. And Dr. McDonald's patient, methodical, and energetic leadership was central to its success.

For Dr. McDonald, the initial SNO results were the culmination of over three decades spent considering the intersection of nuclear, particle and astrophysics, and particularly how to measure sub-atomic phenomena at the very limits of detection.

In the mid-1960s when the Beach Boys were making waves, Dr. McDonald was part of a group in Pasadena, California, that wasn't primarily interested in catching rays on the beach, but in the lab. Dr. William Fowler, head of the group at Caltech where Dr. McDonald studied for his Ph.D., would win the Nobel Prize for his work describing the nuclear reactions that power stars, including the Sun. However, Dr. Fowler's theory, expanded by Dr. John Bahcall, quickly raised a conundrum: While experimental tests were largely consistent with his Standard Solar Model, the detection of neutrinos reaching the Earth, first measured by Nobel Laureate Dr. Raymond Davis in the mid-1960s, was far too low – an anomaly that became known as "The Solar Neutrino Problem."

Neutrinos are extremely abundant particles produced by the nuclear reactions in the sun. They interact only very weakly with matter and therefore leave the sun in seconds and reach the earth at the speed of light.

While major experiments tried and failed to resolve the sun's neutrino riddle, Dr. McDonald honed his Standard Model probing techniques studying the weak interaction, one of the four fundamental forces (along with the strong force that holds the atomic nucleus intact, gravity and electromagnetism). First at Atomic Energy of Canada's (AECL) Chalk River Laboratories in the Ottawa Valley and then at Princeton University, Dr. McDonald developed sophisticated ways to look for parts-per-million differences in the symmetry of nuclear reactions and the role of the weak force in this.

In 1989, after two years as the project's U.S. spokesperson, he was recruited as Director of the SNO project, initiated in 1984 with Herb Chen of the University of California at Irvine and George Ewan of Queen's University as co-spokesmen.

"We knew from the beginning that we could provide significant answers if we could only do the experiment," says Dr. McDonald.

That was no small if.

To build SNO, Dr. McDonald managed the creation of the most sensitive neutrino detector to date. It was a massive engineering project that involved the construction of an ultra-clean, 10-storey-high neutrino detector, containing 1,000 tons of heavy water (worth $300 million, on loan from AECL), two kilometres underground in INCO Ltd.'s Creighton nickel mine in Sudbury. SNO would be the first neutrino detector able to detect all three kinds of neutrinos (electron, muon, and tau) and to be able to distinguish electron neutrinos from the others.

He also assembled "Team Neutrino," a diverse and highly skilled collaboration of more than 130 researchers and technicians from more than a dozen universities and labs in Canada, the U.S. and Britain. The group created a detector from materials that had natural radioactivity levels a billion times lower than tap water, designed special-purpose electronics, and then developed detailed computer models of what to expect.

Since November 1999, SNO has been recording the barely perceptible energy signals of solar neutrinos hitting the detector. Once these thousands of neutrino "hits" were sorted from the nearly half-a-billion "events" recorded by the detector (due to other forms of radioactive energy), the SNO team was able to make a remarkable conclusion: the electron neutrinos produced by the sun were morphing into their sister forms during their journey from the core of the sun to earth, a fact that meant they also have mass. The results also convincingly confirmed the accuracy of Drs. Fowler and Bahcall's detailed solar models.

Once announced, the results created international headlines and sent physicists to their blackboards to consider the implications, including whether these changelings make up all of the Dark Matter in the Universe (a possibility already put into doubt by results from SNO and other measurements).

Future funding from the Canada Foundation for Innovation will enable the creation of SNOLAB, a new international underground laboratory near SNO, aimed at further filtering of the cosmos for consistency with the Standard Model. This time, one of the targets is WIMPS, (Weakly Interacting Massive Particles), enigmatic particles predicted by theories that extend beyond the Standard Model and one of the leading candidates for the 25 per cent of the universe believed to be made from Dark Matter. But after Dr. McDonald and the SNO collaboration's role in the discovery of neutrino flavour change, going after something that no one's ever seen, let alone developed fully into theory, isn't really such a stretch.


Dr. McDonald is a key mentor, coordinator and team builder within the Canadian and international nuclear, particle and astrophysics communities. Since entering academia in 1982, Dr. McDonald has supervised, or supported, the research of more than 100 graduate students, postdoctoral fellows and research associates. The SNO project, and now SNOLAB, are a major training ground for particle, nuclear and astrophysicists, attracting dozens of students from across Canada and around the world, while involving scientists from Britain, the U.S. and Canada. "I'm very conscious of the fact that the reason SNO was such a success is because of the tremendous abilities and teamwork of the more than 160 scientists that have been involved with the project," says Dr. McDonald.

As well as being Director of the SNO project, he is the Chair of the Advisory Committee for the Cosmology and Gravity Program of the Canadian Institute for Advanced Research.

Through his leadership of the SNO project, and emphasis on sharing the project's results, Dr. McDonald has become a key spokesperson for the Canadian physics community. He has provided interviews on SNO to journalists from around the world, and recently co-wrote a 10-page feature on SNO in Scientific American (April 2003 issue). He has been invited to lecture on SNO and neutrino observatories in general at major conferences, and in October he will present the UK-Canada Rutherford Lecture to the Royal Society, London, England.


Arthur McDonald was born in Sydney, Nova Scotia, on August 29, 1943. He graduated from Dalhousie University in Halifax, Nova Scotia, in 1964 with a B.Sc. (Hon. Physics) and 1965 with an M.Sc. (Physics). He continued his studies at the California Institute of Technology in Pasadena, graduating in 1969 with a Ph.D. in Nuclear Physics. From 1969 until 1981 he worked at the Chalk River Nuclear Laboratories of Atomic Energy of Canada, performing fundamental nuclear physics experiments with particle accelerators. In 1981 he began a Professorship in the Physics Department at Princeton University, Princeton, New Jersey, and continued his research program there as Co-Principal Investigator of the Princeton Cyclotron. In 1989 he moved to Queen's University in Kingston, Ontario, as Professor of Physics and Director of the Sudbury Neutrino Observatory (SNO) Institute. In 2002 he was awarded a University Research Chair at Queen's University.