A retired professor from Queen's University in Kingston, Ont. -- the former director of the Sudbury Neutrino Observatory in northern Ontario -- is a co-winner of the 2015 Nobel Prize in Physics for his work on tiny particles known as neutrinos.

Arthur McDonald was roused from sleep at about 5 a.m. on Tuesday by a phone call from the Nobel Prize committee telling him the news.

"I was a little surprised," he said in a telephone interview from Kingston, laughing with joy. "I am overwhelmed, but excited."

The first thing the 72-year-old did as a Nobel Prize winner was hug his wife.

"Thank you," he told her.

McDonald and Japanese scientist Takaaki Kajita were cited for the discovery of neutrino oscillations and their contributions to experiments showing that neutrinos change identities.

"We were also able to determine that neutrinos do have a small mass and that's something that wasn't known before and it helps to place neutrinos in the laws of physics at a very fundamental level," McDonald said.

"So it's very fundamental in terms of understanding how the world works at a very microscopic level."

The Nobel Prize committee was impressed.

"The discovery has changed our understanding of the innermost workings of matter and can prove crucial to our view of the universe," the Royal Swedish Academy of Sciences said in announcing the award early Tuesday.

Even McDonald's colleagues were caught off guard by the announcement. By mid-day Tuesday, Tony Noble, a physicist at Queen's University and an associate director of the neutrino lab, said they were scrambling to put together a party to celebrate the big news.

"This is just incredible and it validates all the incredible work that is being done at the (neutrino lab in Sudbury)," Noble said.

McDonald said being named by the committee is a "very daunting experience, needless to say."

"Fortunately, I have many colleagues as well who share this prize with me."

McDonald said they have put in a "tremendous amount of work" and that he benefited from having a "very friendly collaboration among scientists from Canada, the United States, Britain and Portugal."

He also explained that the work could only have been performed in Canada.

The Sudbury Neutrino Observatory, a collaborative effort by six Canadian universities, is situated two kilometres underground at a working nickel mine in Sudbury, Ont. The group was able to borrow $300 million worth of heavy water -- used in the country's Candu nuclear reactors -- for 10 years for just $1 from Atomic Energy Canada Limited, McDonald explained.

That amount of heavy water, McDonald said, allowed their group the ability to discover different "flavours" of neutrinos in the giant detector they built.

"We were able to observe once per hour a neutrino from the sun because we were able to shield out, in this underground location, all of the other radioactive particles coming from outer space and just observe neutrinos," he said.

McDonald said there was a "eureka moment" when they discovered that neutrinos were able to change from one type to another in travelling from the sun to the Earth.

A native of Sydney, N.S., McDonald studied at Dalhousie University in Halifax in the mid-60s and later at the California Institute of Technology. He was made an officer of the Order of Canada in 2006.

McDonald and Kajita, the director of the Institute for Cosmic Ray Research and professor at the University of Tokyo, will split the prize money, the equivalent of about $1.3 million Cdn.

Kajita seemed flummoxed at a news conference organized by his university. "My mind has gone completely blank," he said after taking the stage. "I don't know what to say."

After getting his composure back, he stressed that many people had contributed to his work, and that there was much work still to do.

"The universe where we live in is still full of unknowns," he said. "A major discovery cannot be achieved in a day or two. It takes a lot of people and a long time. I would like to see young people try to join our pursuit of mystery-solving."

Neutrinos are minuscule particles created in nuclear reactions, such as in the sun and the stars.

For decades the neutrino remained a hypothetical particle until American researchers proved that it was real in 1956.

There are three kinds of neutrinos, and the laureates showed they oscillate from one type to another, dispelling the long-held notion that they were massless. Kajita showed in 1998 that neutrinos captured at the Super-Kamiokande detector in Japan underwent a metamorphosis in the atmosphere, the academy said.

Three years later, in Canada, while working at the Sudbury Neutrino Observatory, McDonald found that neutrinos coming from the sun also switched identities.

"A far-reaching conclusion of the experiments is that the neutrino, for a long time considered to be massless, must have mass," the academy said.

McDonald said that scientists would still like to know the actual masses of the various forms of neutrino. And ongoing experiments are delving into whether there are other types of neutrinos beyond the three clearly observed.

The idea that neutrinos could transform from one type into another was first put forward by the Italian physicist Bruno Pontecorvo in the late 1950s, but scientists' understanding of the process was rather vague until Kajita announced his discovery in 1998, said Antonio Ereditato, director of the Albert Einstein Center for Fundamental Physics at the University of Bern, Switzerland.

"This was a big shock because he proved in a statistically significant manner ... that neutrinos oscillate," said Ereditato. "Then Art McDonald explored another channel using solar neutrinos. It came after Kajita but he also proved neutrino oscillation in another channel. The two deserved this award."

Neutrinos are the second-most abundant particles in the universe after photons, "so any property of neutrinos can have dramatic repercussions on the life of the universe and on its evolution," he said. "This is really one of the milestones in our understanding of nature."

With files from Alan Black and The Associated Press

Sudbury neutrino observatory Nobel Prize