Observing the Universe from Deep Underground

	.
Above: A 3D view of the SNO Laboratory.  The ramp to the cavity base was used during construction and is now sealed. (SNO)

How can an observatory examine the sun and stars from deep underground? The Sudbury Neutrino Observatory will use 1,000 tonnes of heavy water to detect elusive particles called neutrinos emitted from the centre of the sun and from remote exploding stars. Neutrinos are the only particles that emerge directly from the thermonuclear furnaces at the cores of stars. By detecting these particles scientists gain direct information about processes that are otherwise concealed. Measurements will also add to our knowledge of crucial, little understood properties of the neutrinos themselves, such as their mass and whether they can change into other types of neutrinos.

The detector must be housed deep underground to protect it from natural cosmic rays from space which interfere with observation of neutrinos. All materials used in the detector itself have been selected to have minimal radioactivity. Thus when completed, this new laboratory will be the least radioactive site in the world.

The Sudbury Neutrino Observatory is housed in a cavern as large as a 10-storey building, in the deepest section of Inco Limited's Creighton Mine. As the western world's largest producer of nickel, Inco is a major participant in the project and has applied its advanced mining technology in making the huge excavation required. In the cavern, an enormous acrylic bottle will hold 1000 tonnes of radiation-free heavy water valued at $300 million that is on loan from Atomic Energy of Canada Ltd, an agency of the Canadian government. Sensors mounted on a sphere surrounding this bottle, in pure regular water, will detect the tiny amounts of Cerenkov light emitted by neutrinos interacting with the heavy water.

The Elusive Neutrino

Neutrinos are one of the fundamental building blocks of nature, along with other subatomic particles called quarks, electrons, and muon and tau particles. Small numbers of neutrinos are produced by some naturally occurring radioactive materials, but vast quantities of them are liberated in the energy-generating fusion reactions in the cores of the sun and other stars. Two hundred trillion trillion trillion neutrinos are created at the sun's core every second.

Neutrinos are the only subatomic particles that can penetrate virtually all types of matter. Most neutrinos would emerge unscathed after travelling through a wall, of lead one light-year thick! Neutrinos stream out from the sun's core and into space at the speed of light unaffected by the sun. Because they reach Earth in minutes compared to the thousands of years it takes other particles to escape from the sun, they are light speed couriers bearing the news of what's happening today inside the sun. However, observing neutrinos has proven to be notoriously difficult and very little is known about their detailed properties. We do know that there are three neutrino types or flavours - electron, muon and tau-type neutrinos. Electron type of neutrinos are produced in the sun's core, but a new theory suggests that they change from one type to another as they travel. SNO, with its sensitivity to detect all three types, will check on this theory. We do not know whether they have a mass or whether they are little bundles of energy with no mass, always moving at the speed of light. This question must be answered before neutrinos can be placed within the framework of the basic forces and building blocks of nature - the so-called Grand Unified Theories - which attempt to describe all of nature in terms of a single comprehensive superforce. Steven Weinberg, winner of the Nobel Prize for Physics in 1979, has said that the confirmation of neutrino mass, which the Sudbury Neutrino Observatory is fully capable of doing, "would illuminate some of the deepest questions of particle physics."

A neutrino observatory must have a very large receiving device if it is to have a reasonable opportunity of trapping just a few specimens by direct strikes on atomic nuclei. It should also be able to capture the three known types of neutrinos. Finally, the receiver must be interfaced to instruments and computers that can record what happens so that scientists can study the interactions in detail. A huge vat of transparent liquid fits these requirements the best, which means a neutrino 'telescope' looks more like a swimming pool than a precision scientific instrument. But looks can be deceiving. Neutrino observatories are at the very cutting edge of research in physics and astronomy. The key to the Sudbury observatory is the availability of pure heavy water. No other country in the world has available reserves of large amounts of heavy water sufficient for a neutrino observatory. The ultra-pure heavy water that will be used in the Sudbury Neutrino Observatory is currently stockpiled for eventual use as the moderating agent in CANDU reactors. It is not radioactive and is the same as ordinary water except that it has a neutron, in addition to a proton in the nucleus of each hydrogen atom in the water.

Heavy water is the ideal medium for detecting neutrinos because all three types of neutrinos react with it. About 7,000 neutrinos a year are expected to interact with the nuclei of atoms in the 1000 tonnes of heavy water in the observatory's giant acrylic container. This is 50 times the rate that neutrinos can be observed in the best existing detector.

Analysis of the photomultiplier readings will permit determination of the energy, direction of travel and time of arrival of each neutrino that interacts. Researchers will then be able to identify what type and how many neutrinos are from the sun or elsewhere in the universe and will measure other important attributes of these phantom-like particles.

Although neutrinos from the sun are being studied at four other neutrino observatories in the United States, Japan, Russia and Italy, these experiments are not sensitive enough to provide definitive answers to key scientific questions. The Sudbury Neutrino Observatory, more powerful than any existing detector, will provide a tremendous amount of new information.

More than two kilometres below the earth's surface, deep in the rock of the Canadian Shield near Sudbury, Ontario, a 60 member team of scientists from Canada, the United States and Britain, is completing the world's most powerful observatory to study the energy generation processes inside the sun and distant stars.

The $70 million project was funded in January 1990 and construction is taking place over a seven year period. Excavation of the underground site began in February 1990, and the detector will be completed in March 1998. The Sudbury Neutrino Observatory Institute has been formed to build and operate the laboratory. An engineering and project management company, Monenco/Agra Limited, prepared the detailed design and supervised much of the construction of the observatory. Most of the research and development work needed to finalize the design and materials used, was carried out at the participating institutions. Several steps in the complex final assembly process for the detector are shown in the accompanying figures.

Low Backgrounds and Cleanliness

.
	Above: A technician follows the clean room procedures at the SNO laboratory. (SNO)

To clearly see neutrino events, it is imperative that scientists eliminate as much of the surrounding natural radioactivity as possible in the SNO detector and laboratory. Added events in the detector from this radioactivity would be difficult to separate from neutrino detection. Site location, construction materials and particle content of the air in the installation were important factors in radioactivity elimination. Sudbury was the ideal location for the detector since the Creighton mine site allowed the observatory to be located two kilometers underground, beneath a pre-Cambrian rock shield. The relatively low natural radioactivity of the norite rock helps to achieve the required conditions. At this depth, cosmic rays are essentially negligible, but neutrinos pass through the rock shield to the observatory almost unscathed. Desired environmental conditions were partly achieved by careful monitoring and selected of materials and components, ensuring minimal traces of Uranium and Thorium (radioactive elements). Ultra pure ordinary water and a thick plastic wall of coating were incorporated into the design of the detector to shield the center from radioactivity and the radon gas from coming from the surrounding rock.

In order to maintain low background radiation, clean room procedures had to be incorporated into the everyday activities at the SNO observatory. Since mine dust can also produce interference, it is crucial that clean room conditions be preserved throughout the assembly and operation phases of the detector. As the final components are installed, the target of less than one gram of dust deposited on the inner surfaces is being met by maintaining air quality at better than class 10,000 conditions (fewer than 10,000 half micron sized particles per cubic foot).

The SNO scientific collaboration has developed a series of unique materials, process technology and equipment which will have a number of 'spin-offs'. The neutrino observatory site will also provide a Canadian laboratory for underground science, not only for particle physics and astrophysics, but for a variety of other industrial and basic scientific applications that require a clean, ultra low radiation environment deep underground. The excavation of the 22 metre diameter by 30 metre high cavity in the strong, undisturbed norite rock of the Sudbury Basin, was a unique process. A rock mechanics study by a geotechnical engineering team associated with SNO has added to our knowledge of how rock stresses are altered during excavation. This international team of scientists participating in the project has established Canada as a major participant in underground science - the exciting new frontier of the 1990's and beyond.

The Sudbury Neutrino Observatory will open a new window on the universe when it begins operations in 1998. The project is a unique blend of Canada's world-class mining expertise, its heavy-water production resources, and the talents of marry scientists and engineers. The international research centre will be a training ground for scientists and engineers and a focus for research into the fundamental building blocks of matter.

	.
Above: A photograph of the completed light sensor sphere (18 m in diameter) for the SNO detector. (Bob Chambers, SNO)

Supporting Agencies

Natural Sciences and Engineering Research Council Canada
U.S. Department of Energy
National Research Council Canada
Northern Ontario Heritage Fund
Industry Canada
U.K. Particle Physics & Astronomy Research Council

University of British Columbia
Brookhaven National Laboratory
Centre for Research in Particle Physics, Carleton University
University of Guelph
Laurentian University
Lawrence Berkeley Laboratory
Los Alamos National Laboratory
Oxford University
University of Pennsylvania
Queen's University
University of Washington, Seattle

The Sudbury Neutrino Observatory's findings could profoundly affect our current understanding of the long-term future of the universe, the energy generation processes of the sun, and the framework of the basic forces of Nature. Neutrinos hold the key to all three issues and the Sudbury Neutrino Observatory can attack all of them.

If neutrinos have mass, the type of neutrinos produced in the centre of the sun, called electron neutrinos, could change before reaching Earth into one of the other two types, called muon and tau neutrinos. They would then have escaped detection at the other neutrino observatories since those detectors are only sensitive to electron neutrinos. So far, electron neutrinos have been recorded in abundance's two times lower than expected. This result - the solar neutrino problem - is a major unsolved discrepancy in astrophysics today.

The Sudbury Neutrino Observatory can detect separately the number of electron neutrinos and the total number of all neutrino types. This will show clearly whether electron neutrinos change. If they do, that means neutrinos have mass. If they don't, calculations of solar energy production are incorrect. Either result will be a major scientific step forward.

The sun in not the only source of neutrinos. When a massive star explodes as a supernova, the blast unleashes times more neutrinos than the sun will produce in its 10 billion-year lifetime. The Sudbury Neutrino Observatory will provide Canadian and international astronomers with a new way to study supernovas by directly observing neutrinos from the exploding star. The 1987 supernova produced a burst of neutrinos which were detected at two underground neutrino detectors simultaneously.

Although most supernova sightings are quite rare, astronomers suspect that most go unobserved because their light for visual detection is blocked by dust clouds in our galaxy. Neutrinos, of course, pass through dust clouds with impunity so the Sudbury Observatory will be the most effective supernova monitor on Earth. Accurate timing of a burst of neutrinos from a supernova permits scientists to measure their mass.

The detection of neutrino mass could have major implications for the future of the universe. Neutrinos are thought to be the most abundant particle in the universe. They were produced in enormous quantities in the Big Bang when the universe was born. Even if they each have a tiny mass, together they could easily 'outweigh' the combined total of all other known forms of matter in the universe. Protons, neutrons and electrons - the stuff of rocks, people, planets and stars - could be a minority component of a universe dominated by neutrinos.

There could be enough gravitational attraction caused by neutrinos to slow the expansion of the universe and cause a collapse into a 'Big Crunch' - the opposite of the Big Bang - billions of years in the future. The Sudbury Neutrino Observatory will add new information to this question of the influence that neutrinos will have on the fate of the universe.