A unique telescope has seen its "first light" - created when elusive particles known as neutrinos interact in 1000 tonnes of heavy water.

The Sudbury Neutrino Observatory (SNO) has begun to detect neutrinos created in the Sun and the Earth's atmosphere. Among its first images are stunning examples of the pools of light formed by the interactions of neutrinos that began life in the sun or in the atmosphere on the opposite side of the Earth.

Says Professor Art McDonald, from Queen's University, Ontario, Canada and SNO Institute Director:

"This is tremendously exciting. It is 15 years since the start of the SNO project, and to see such clear examples of neutrino interactions within days of finally turning on the full detector was a real triumph for the entire SNO team."

SNO has been built by a collaboration of nearly 100 scientists from 11 universities and laboraties in Canada, the US and the UK. Located 2000 metres underground in a nickel mine in Sudbury, Ontario, Canada, it is shielded from cosmic rays and other sources of unwanted "background". It is designed to detect neutrinos from sources beyond the Earth, in particular from the Sun, which bathes each square centimetre of the Earth's surface with billions of neutrinos a second.

Although neutrinos are the commonest particles in the Universe - at least as common as the photons of light - some of their basic properties, such as how much they "weigh", are still not known for sure. This is mainly because neutrinos are not at all easy to study as they interact rarely with other matter and so are very difficult to "see".

SNO has been built to help resolve some of the mysteries that continue to surround neutrinos, in particular the puzzle of why previous experiments do not detect as many neutrinos from the Sun as expected. The results will help to answer questions about the nature of matter at the smallest scales, as well as provide insight into the structure of the stars and the Universe as a whole.

You can learn more about SNO at its official web site: www.sno.phy.queensu.ca

The Sudbury Neutrino Observatory (SNO) is a major international scientific project sited 2000 meters underground near Sudbury, Ontario, Canada. The observatory uses heavy water as a detection medium for neutrinos from the sun and other astrophysical sources. The SNO detector sees neutrinos - tiny ghost-like particles which are fundamental building blocks of matter - through the faint flashes of light that are produced as they are stopped or scattered in 1000 tonnes of heavy water at the heart of the detector.

The SNO project has the potential to provide a measurement of the number of electron neutrinos reaching the earth from the Sun, as well as the total numbers of all neutrino types in a similar energy range. With these two measurements, SNO could determine unambiguously whether the neutrinos produced in the core of the Sun are changing to another type in transit to the earth. This will occur if neutrinos have a rest mass greater than zero. This result will provide valuable information about the basic properties of neutrinos and the energy generation processes in the Sun. A measurement by SNO of a change from electron neutrinos to another type could define the difference in mass between the neutrino types, of fundamental importance for particle physics and cosmology.

Since the water filling of the detector was completed at the end of April, 1999 several weeks of preliminary observations have been carried out. Although analyses of the signals seen in this period are still in progress, an initial review indicates that both signals and backgrounds appear to be near the design targets for the detector. A detector event is recorded whenever more than about 30 of SNO's 9600 ultra-sensitive light sensors see at least one light photon at the same time. SNO's events recorded thus far include neutrino signals (producing a cone of light) and background signals from cosmic ray particles and from the detector itself. Preliminary measurements of the detector's background and residual radioactivity is a promising indication of the success of SNO's ultrapure materials selection, water purification processes and cleanliness measures during construction. Final sealing of the detector has been completed this week and it is now scheduled to go into operation on a continuous basis.

The past year, since the completion of the construction phase in April 1998, has been a very busy one for SNO scientists and supporting workers. Included in this work was the final assembly and commissioning of the electronics for the light sensors and SNO's computer data handling system and the operating of systems for purifying, cooling and recirculating both heavy and light water for the detector. Measurements were made for the empty and partly-filled detector to check on light sensor operations and to calibrate the systems. A number of different calibration sources were installed and tested during this period as well. A variety of technical problems encountered were also studied and remedied during this commissioning phase.

Since only 10-20 solar-neutrino events are expected per day, and careful analysis is required for neutrinos and background events, the Sudbury Neutrino Observatory has a multi-year measurement program planned. It is expected that the results from SNO will contribute unique information on neutrinos and their properties in the near future. The world's only heavy water based neutrino detector, SNO has the potential to make a major contribution to the world-wide research effort on the properties of neutrinos and their role in the universe.

The SNO project has been supported by the federal and provincial governments in Canada (NSERC, Industry Canada, National Research Council of Canada, FEDNOR, Northern Ontario Heritage Fund, Province of Ontario), by the U.S. Department of Energy and the U.K. Particle Physics and Astronomy Research Council. The Observatory is sited in INCO's Creighton mine through the extensive cooperation of INCO Limited. The heavy water from Canada's reserves is on loan from AECL with the cooperation of Ontario Hydro.

Canadian A&A Network    Ed Kennedy    Tables of Contents