The detector, shown in the artist's conception, was built 6,800 feet under ground, in INCO's Creighton mine near Sudbury, Ontario. Sudbury Neutrino Observatory is a heavy-water Cherenkov detector that is designed to detect neutrinos produced by fusion reactions in the sun. Image and caption courtesy of the Sudbury Neutrino Observatory

Lab researcher to speak Thursday about SNO research findings

Sensitive measurement observes solar neutrinos in a new way

A Laboratory researcher and scientific director of the Sudbury Neutrino Observatory team will talk about some of the team's recent research findings at a Physics and Theoretical divisions colloquium Thursday in the Physics Building Auditorium at Technical Area 3.

Andrew Hime of Neutron Science and Technology (P-23) will speak at 3:45 p.m. "The Sun Shines Underground," is open to Laboratory badge holders.

The research findings were initially presented at the joint American Physical Society/American Astronomical Society meetings in Albuquerque in April.

Using a unique, new experiment, scientists from the Sudbury Neutrino Observatory team measured the total number of all known neutrino types reaching the Earth from the sun and confirmed with high precision that solar electron neutrinos born in the sun change their identity on their journey to Earth. The muon and tau neutrinos, to which electron neutrinos transform, once elusive and uncounted, are now accounted for.

Up until early this year, the number of muon and tau neutrinos could only be inferred by comparing the electron neutrino data from the results of two separate experiments, one taking place at SNO and the other at the Super Kamiokande detector in Japan. For the past year the SNO collaboration has been working on a new analysis of their data that now provides a direct measurement of the total number of solar neutrinos, including the non-electron type.

Neutrinos are particles with no electric charge and very little mass. They are known to exist in three types related to three different charged particles -- the electron, muon and tau. The sun emits electron neutrinos, which are created in thermonuclear reactions in the solar core.

Since the early 1970s, numerous experiments have substantiated the theory that a shower of solar neutrinos was constantly streaming from the sun toward Earth. However, the amount of neutrinos detected by terrestrial neutrino detectors was only a fraction of the number predicted by detailed theories of solar energy production. There seemed to be something wrong with either existing theories or the understanding of neutrinos.

But since the first results from SNO were presented in June 2001, it became apparent that the discrepancy was not caused by problems with any of the models of the sun, but rather that the neutrinos changed types as they traveled from their birthplace in the core of the sun across space toward the Earth.

This year, SNO scientists revealed how they count all types of the neutrinos. "The detector at SNO is the first detector able to measure the total number of neutrinos," said Hime. "A comparison of the total number of neutrinos detected to the number of electron neutrinos detected through a separate interaction in SNO is the key that has resolved the long standing solar neutrino problem."

The neutrino experiments take place 6,800 feet underground in a nickel mine near Sudbury, Ontario. The SNO detector, a 12-meter diameter acrylic plastic heavy-water-filled vessel, uses an array of 9,456 photomultiplier tubes to capture the tiny flashes of Cherenkov light that are created when the roughly 10 solar neutrinos per day that are stopped or scattered in the 1,000 tons of heavy water contained in the SNO detector.

Total neutrino count is determined by measuring the gamma rays generated when a deuteron in the heavy-water — the hydrogen atoms have an extra neutron in their nucleus — is broken releasing a neutron and energy that results in the production of a gamma ray. By counting the gamma rays, the scientists measure the total number of neutrinos coming in.

To obtain a direct measurement of the electron neutrinos, the scientists measure the energy produced through a different reaction when a neutrino reacts with the heavy water in the SNO detector. This reaction results in an inverse beta decay reaction that turns a neutron into a proton with the emission of an electron. The energy produced in these reactions is directly proportional to the energy of the electron neutrinos coming in, thus giving the number of electron neutrinos.

"These new results show in a clear, simple and accurate way that solar neutrinos change their type," said Project Director Art McDonald of Queen's University in Kingston, Ontario. "The total number of neutrinos we observe [also] is in excellent agreement with calculations of the nuclear reactions powering the sun. The SNO team is really excited because these measurements enable neutrino properties such as mass to be specified with much greater certainty for fundamental theories of elementary particles."

The SNO laboratory maintains one of the cleanest environments on Earth, according to Andrew Hamer of P-23 and a member of the SNO team. "In order to make these measurements we had to restrict the radioactivity in the detector to minute levels and determine the background effects very accurately to show clearly that we are observing neutrinos from the sun," he said. "The care taken throughout this experiment to minimize radioactivity, and the careful calibration and analysis of our data, has enabled us to make these neutrino measurements with great accuracy."

The SNO team has submitted these results for publication to Physical Review Letters.

The Los Alamos team has played a lead role in the SNO project since it received funding for full-scale construction 12 years ago. Los Alamos' contributions have spanned essentially all aspects of the SNO experiment, including detector construction, calibration, Monte Carlo simulation, data reduction and analysis.

The Laboratory SNO team members are Mel Anaya, Mark Boulay, Tom Bowles, Steve Brice, Bill Teasdale, Hamer, and Hime of P-23, Mike Dragowsky, Malcolm Fowler, Jerry Wilhelmy of Isotope and Nuclear Chemistry (C-INC), Geoff Miller of Applied Technologies (RRES-AT), Richard Vandewater of Subatomic Physics (P-25) and Jan Wouters of Advanced Information and Business Application Development (IM-8).

For more information go to the SNO Web page at http://www.sno.phy.queensu.ca/ online.

--Shelley Thompson

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