SUDBURY NEUTRINO OBSERVATORY | ||
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Background Information |
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The Sudbury Neutrino Observatory is a unique neutrino telescope, the size of a ten-storey building, two kilometers underground in Inco's Creighton Mine near Sudbury, Ontario. It was planned, constructed and operated by a 100-member team of scientists from Canada, the United States and the United Kingdom. Through its use of heavy water, the SNO detector provides unique ways to detect neutrinos from the Sun and other astrophysical objects and measure their properties.
For many years, the number of solar neutrinos measured by other underground detectors had been found to be smaller than expected from theories of energy generation in the sun. This had led scientists to infer that either the understanding of the Sun was incomplete, or that the neutrinos were changing from one type to another in transit from the core of the Sun.
In results presented in June 2001, SNO scientists compared the number of electron-type neutrinos reaching the SNO detector to the number of neutrinos seen by a second reaction, which includes contributions from the other two types of neutrinos, making use of additional data from the Super-Kamiokande detector in Japan. The observed difference in these two numbers showed conclusively that neutrinos change their type enroute to Earth, and arrive as a mixture of electron neutrinos and the other two types.
The results to be announced in April 2002 are based on the SNO detector's ability, through a third type of neutrino reaction, to measure independently the total rate of all of the three known types of neutrinos. The new data could provide independent and more accurate information on the neutrino change in type and on the accuracy of models of the Sun.
The SNO detector consists of 1,000 tonnes of ultrapure heavy water enclosed in a 12-meter diameter acrylic plastic vessel, which in turn is surrounded by ultrapure ordinary water in a giant 22-meter diameter by 34-meter high cavity. Outside the acrylic vessel is a 17-meter diameter geodesic sphere containing 9,600 light sensors or photomultiplier tubes, which detect tiny flashes of light emitted as neutrinos are stopped or scattered in the heavy water. The flashes are recorded and analyzed to extract information about the neutrinos causing them. At a detection rate of about one neutrino per hour, many days of operation are required to provide sufficient data for a complete analysis. The laboratory includes electronics and computer facilities, a control room, and water purification systems for both heavy and regular water.
The construction of the SNO Laboratory began in 1990 and was completed in 1998 at a cost of $80 million (Canadian) with support from the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, the Northern Ontario Heritage Foundation, Industry, Science and Technology Canada, Inco Limited, the United States Department of Energy, and the Particle Physics and Astronomy Research Council of the UK. The heavy water is on loan from Canada's federal agency AECL with the cooperation of Ontario Power Generation, and the unique underground location is provided through the cooperation and support of Inco Limited. Measurements at the SNO Laboratory began in 1999, and the detector has been in almost continuous operation since November 1999 when, after a period of calibration and testing, its operating parameters were set in their final configuration.
In June 2001, the second phase of measurements with the SNO detector was begun, in which ultra pure sodium chloride (salt) was added to the heavy water core of the detector, to enhance signals for some of SNO's neutrino reactions and add further to the accuracy of SNO's neutrino determinations. A third phase of measurements, providing a different experimental sensitivity, is set to begin in 2003.
Further background information can be found on the SNO website: www.sno.phy.queensu.ca .
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(Last revised May 6, 2002) Mail problems/comments to qusno@sno.phy.queensu.ca |