Director of the Sudbury Neutrino Observatory. Discovered that neutrinos have mass and that they can change from one type of neutrino to another.
"The research that we are doing is fascinating for the fact that it really bridges the entire universe. We are attempting to understand the most microscopic laws of physics that describe fundamental particles. And, in so doing, we try to understand the most detailed things about the universe, how it was created and evolved, in short the origins of the universe."
Some academics quietly wind down their careers as they near retirement. Not Arthur McDonald.
Particle astrophysics experiments have been the motivation for his research for many years, and he’s not about to leave the exciting work now. In 1984 McDonald became part of an ambitious physics experiment that would hopefully take place in Canada. The project was counterintuitive: they would measure mysterious solar particles from the bottom of a mine.
In 1989, McDonald became the Director of the Sudbury Neutrino Observatory, or SNO as it is more commonly known. The detector was built 2 km underground in INCO’s Creighton mine near Sudbury, and it was completed in 1999 after a decade of construction and financing. After only a few years, it helped particle astrophysicists prove that their model of the Sun was correct. It also showed that neutrinos do have mass, albeit a very, very, tiny one. The discovery changed the laws of physics at a fundamental level.
For McDonald the project harkened back to his graduate research and lessons learned from the Nobel-prize winning physicist William Fowler at Caltech. At that time, researchers in Fowler’s lab were studying the nuclear reactions that power the Sun. However, they were also using the nucleus of the atom to test the laws of physics. “The combination of those two influences, attempting to understand the physics of the sun and other stars and understanding the laws of physics in a detailed way, was extremely powerful,” said McDonald. “Those two interests came together very well in the SNO project and it was at Caltech that I really got the seeds of the interest in the sun and fundamental particle physics.”
The SNO project was also an incredible chance for Canadian scientists to work with colleagues worldwide. According to McDonald there were about 130 scientists from around the world working on the project at any one time. Overall, he said there have been over 270 different authors on SNO-related papers. “This indicates that we have educated a lot of individuals,” said McDonald. “Over a hundred graduate degrees are associated with this project. There is a tremendous team with a wide variety of talents that have contributed to the success of the project; even the people who sweep the floor and keep the facility ultra-clean contribute in a very large way.”
Now, with SNO’s crucial measurements solving what had been called the “Solar Neutrino Problem”, the original experiment has been completed. The last measurement was taken in November 2006, and the last of the heavy water will be returned to the Atomic Energy of Canada Limited by the end of 2007. But that doesn’t mean the lab is being mothballed.
McDonald is working with other colleagues who are leading the development of a new international underground laboratory near the SNO experiment. The new project, called SNOLAB, is the lowest radioactivity laboratory in the world, with an experimental area three times larger than SNO. Experiments in the new SNOLAB will try to answer further fundamental physics questions, such as: What is the origin of all the matter in our Universe? And, what are the dark matter particles that make up nearly a quarter of the Universe’s mass?
Answers to those questions promise to be just as fascinating as the results of the SNO experiment, so you can bet McDonald will be studying the Universe from deep underground for years to come.
Inevitably, the first two questions asked about Art McDonald’s research are: what are neutrinos and how do you measure them?
Neutrinos, simply put, are very small particles that are emitted by the sun and all stars. They are a fundamental particle like quarks or electrons and were described by the Standard Model of elementary particles.
Unfortunately, astrophysicists were uncertain whether their models of the sun were correct or not. Predictions suggested that there should be two to three times more neutrinos than observed. “People had been observing too few neutrinos,” said McDonald. “We didn’t know if there were too few created and our models of the sun were wrong or if something was happening to the solar neutrinos on the way from the Sun to the Earth so existing experiments just couldn’t measure them.” This became known as the Solar Neutrino Problem.
To find out how many neutrinos the Sun was producing, McDonald and an international team of fellow researchers proposed to build a solar observatory 2 km underground. With the leadership of George Ewan and Herb Chen, the project began rolling in 1984, and McDonald took over the reins of the project in 1989.
Seventy million dollars and a decade later, the Sudbury Neutrino Observatory was switched on in 1999. The Observatory contained $330 million worth of heavy water--heavy water's hydrogen atoms have an extra neutron, making it 10% heavier than ordinary water. The heavy water was on loan from Atomic Energy of Canada Limited. A thousand tons of it sat in a 12m diameter acrylic vessel with very elaborate light detectors all around. The heavy water was the most sensitive material available for detecting solar neutrinos and resolving the problem. A couple times an hour a neutrino would hit a heavy water molecule producing a small flash of light that could be seen by the detectors.
SNO was put at the bottom of one of INCO’s mines in Sudbury because rock and earth had to insulate the detectors from radiation from outer space. In fact, underneath all that rock the detector was bombarded by over 100,000 times less radiation than on the surface.
Amazingly after lugging all the equipment for the precise detectors 2 km under the earth, there was only about a gram of mine dust on all the millions of parts in the experiment. That kind of ultra-clean environment allowed McDonald and a host of worldwide physicists to make some tremendous measurements.
What they found was that the standard model of particle physics had to be modified. Neutrinos came in three “flavours”; the regular electron neutrino produced in the sun, but also the muon and tau neutrinos. SNO was able to observe all three of these neutrino flavours for the first time and show that the electron neutrinos had changed flavours before reaching the Earth. Thus, while the standard model of elementary particles had to be slightly modified, the model of the Sun was correct allowing solar astrophysicists to breathe a large sigh of relief.
More intriguing, because neutrinos could change “flavours” this also meant that the extremely small particles had a very small mass. According to McDonald, neutrinos have a mass at least 500,000 times smaller than an electron.
“It is the smallest mass that we know for such particles,” said McDonald. “The determination that it is so small is very important to understanding in a fundamental way the physics at the subatomic level.”
“The research that we are doing is fascinating for the fact that it really bridges the entire universe,” said McDonald. “We are attempting to understand the most microscopic laws of physics that describe fundamental particles. We are trying to understand the most detailed things about the universe: how it was created and evolved. When you go back to study the origins of the universe, the tremendous energy in the Big bang requires that you understand the laws of physics for elementary particles in great detail to understand how the universe evolved.”
By understanding a particle so small that it was almost without mass, McDonald has brought us a step closer to knowing what happened in the earliest moments of the Universe.
The tremendous energy at the Big bang requires that we understand the laws of physics in detail to understand how it evolved, to know the components of the Universe such as dark matter particles which the identity is still unknown, to know how the laws of physics work at the transition from a matter dominated universe. These are very fundamental and wide reaching questions which still need to be answered.
We have the best lab in the world to address these particle astrophysics questions in Canada. It is possible to provide an education at the very frontier of basic physics and basic astrophysics that rivals anything else in the world. Not only is it a fun field but you can also do very well here in Canada.
Sources: image of McDonald from Queens University, images of SNO experiment and neutrino diagram from SNO.
Profile by Graeme Stemp-Morlock.