Sudbury and the Nobel Prize in Physics

Mechanical Ventilator Milano

by Dr. Peter Chow

The Sudbury Neutrino Observatory / Mechanical Ventilator Milano

In 1931, a hypothetical particle is predicted by Wolfgang Pauli, a theoretical physicist and one of the pioneers of Quantum Physics.

Pauli based his prediction on the fact that energy and momentum did not appear to be conserved in certain radioactive decays. Pauli suggested that this missing energy might be carried off, unseen, by a neutral particle which was escaping detection.

In 1934, Enrico Fermi develops a comprehensive theory of radioactive decays, including Pauli’s hypothetical particle, which Fermi coins the neutrino (Italian: “little neutral one”).  With inclusion of the neutrino, Fermi’s theory accurately explains many experimentally observed results.

In 1959, discovery of the neutrino is announced by Clyde Cowan and Fred Reines (recipient of the 1995 Nobel Prize in physics for his contribution to the discovery).

In 1968, the first experiment to detect (electron) neutrinos produced by the sun’s burning (using a liquid Chlorine target deep underground), reports that less than half the expected neutrinos are observed, the origin of the long-standing “solar neutrino problem.”   The possibility that the missing electron neutrinos may have transformed into another type of neutrino (undetectable to this experiment) is suggested.

The Sudbury Neutrino Observatory (SNO) was a neutrino observatory located 2100 m underground in Vale’s Creighton Mine in Sudbury, Ontario, Canada.  The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water.

Neutrinos are one of the 4 fundamental particles which make up the universe. They are also one of the least understood.

The other 3 fundamental particles are the electron, the up-quark and the down-quark.

Inside the atom’s nucleus, each proton is made up of 2 up-quarks and 1 down-quark, while each neutron is made up of 2 down-quarks and 1 up-quark.

Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry an electric charge.   Neutrinos are the lightest of the known subatomic particles, the mass of a neutrino being less than one millionth that of an electron.

Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a “weak” sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances through solid matter, at the speed of light, without being affected by it.

Neutrinos have mass, so they also interact gravitationally with other more massive particles, but their mass is infinitely small (once thought to be zero) and gravity is by far the weakest of the 4 known forces in the universe.

A neutrino can pass through trillions of miles of lead without the slightest effect on its motion.

This should give you significant relief, because as you read this, every second of the day, a hundred trillion solar neutrinos pass through your body – fortunately without effect – passing through your body and the earth as well, as part of their lonely journey through the cosmos, at the speed of light.

Three types of neutrinos are known.  Each type or “flavour” of neutrino is related to a charged particle (which gives the corresponding neutrino its name).

Hence, the “electron-neutrino” is associated with the electron, and two other neutrinos, the muon-neutrino and the tau-neutrino, are associated with heavier versions of the electron called the muon and the tau (elementary particles are labelled with Greek letters, to confuse the layman).

The table below lists the known types of neutrinos (and their electrically charged partners).

Neutrino Ne Nm Nt
Charged Partner electron (e) muon (m) tau (t)


The first measurements of the number of solar neutrinos reaching the Earth were undertaken in 1968, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted.   This deficit became known as the solar neutrino problem.

To explain this, the hypothesis of neutrino oscillations was proposed.

Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino created with a specific flavour (electron, muon, or tau) can transform to have a different flavour.  The probability of measuring a particular flavour for a neutrino varies between three known states, as it flies through space

All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron-neutrinos and so, yielded little to no information on muon-neutrinos and tau-neutrinos.

In 1984, Herb Chen of the University of California at Irvine first pointed out the advantages of using heavy water as a detector for solar neutrinos.   Unlike previous detectors, using heavy water would make the detector sensitive to two reactions, one reaction sensitive to all neutrino flavours, the other reaction sensitive to only electron-neutrino. Thus, such a detector could measure all neutrino oscillations directly.

A location in Canada was attractive because Atomic Energy of Canada Limited, which maintains large stockpiles of heavy water at Chalk River, Ontario, to support its CANDU reactor power plants, was willing to lend the necessary amount (worth C$330,000,000 at market prices at that time) at no cost.

Heavy water (deuterium oxide) is a form of water that contains only deuterium rather than the common hydrogen-1 isotope.  Deuterium is a hydrogen isotope with a nucleus containing a neutron and a proton; the nucleus of a normal hydrogen atom consists of just a proton. The additional neutron makes a deuterium atom roughly twice as heavy as a protium atom.

The Creighton Mine in Sudbury, among the deepest in the world and, accordingly, with very low levels of background radiation and shielded from cosmic rays, was quickly identified as an ideal place for Chen’s proposed experiment to be built.  The mine management was willing to make the location available for only nominal costs.

The SNO detector target consisted of 1,000 tonnes of heavy water contained in a 6-metre-radius (20 ft) acrylic vessel built into the largest man-made underground cavity in the world.

Arthur Bruce McDonald, a Canadian astrophysicist, was the director of the Sudbury Neutrino Observatory.

He was from Sydney, Nova Scotia. He graduated with a M.Sc. in physics in 1965 from Dalhousie University in Halifax and then obtained his Ph.D. in physics in 1969 from Caltech, the California Institute of Technology.   McDonald cited a high school math teacher and his first-year physics professor at Dalhousie as his inspirations for going into the field of physics.

He had worked as a Research Officer at the Chalk River Nuclear Laboratories northwest of Ottawa from 1969 to 1982.  He became professor of physics at Princeton University from 1982 to 1989, leaving Princeton to join Queen’s University where he became Professor from 1989 to 2013.

The detector was turned on in May 1999.  The SNO experiments began to show that solar neutrinos change flavours.  This resolved the solar neutrino problem: the electron-neutrinos produced in the sun had partly changed into other flavours, muon-neutrinos and tau-neutrinos, which previous experiments could not detect.

The SNO experiment could detect all of the neutrino flavours and found no deficit.

In 2013 McDonald became Professor Emeritus of Queen’s University in Kingston, Canada. He continues to be active in basic research in Neutrinos and Dark Matter at the SNO underground Laboratory, now called SNOLAB.

Arthur Bruce McDonald, was awarded the 2015 Nobel Prize in Physics, jointly with Japanese physicist Takaaki Kajita, for the Sudbury Neutrino Observatory’s contribution to the discovery of neutrino oscillation.

Not resting on his Nobel laurels, MacDonald, now age 77, delved into humanitarian work this year.

In the spring of 2020, with the COVID-19 pandemic burning out of control in Italy,  McDonald stepped up to become the leader of a project to mass-produce mechanical ventilators at low cost.

McDonald has stated that he was inspired into action after recognizing the similarities between the requirements of a ventilator and those of equipment for particle physics experiments.

McDonald led the Canadian team, called the Mechanical Ventilator Milano (MVM) consortium, with members from the Chalk River Nuclear Laboratories, SNOLAB and the McDonald Canadian Astroparticle Physics Research Institute.  McDonald, harnessed the broad talents of the team, many of whom would normally have been spending their time working on experiments to solve the mysteries of dark matter.

The design, called the Mechanical Ventilator Milano, is based on the Manley ventilator but uses modern electronics wherever possible.

The open-source design, using readily available off-the-shelf components readily available worldwide from hardware suppliers, was optimized to permit large scale production in a short time and at a limited cost.   This enables quick progress to inexpensive mass production of safe, reliable ventilators for hospitals and patients around the world.

The project received the support of Prime Minister Justin Trudeau who made an initial order of 10,000 to Canadian hospitals, manufactured by Vexos in Markham, Ontario.  A total order of 30,000 MVM ventilators for Canadian hospitals is anticipated.