Sudbury Neutrino Observatory
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The Sudbury Neutrino Observatory (SNO) was located 6800 feet (about 2 km) underground in Inco Limited's Creighton Mine in Greater Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with deuterium nuclei and atomic electrons. The detector turned on in May of 1999, and was turned off on November 28, 2006. While new data is no longer being taken the SNO collaboration will continue to analyze the data taken during that period for the next several years. It is planned to re-use the existing experimental structures for the SNO+ experiment.
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[edit] Experimental motivation
The first measurements of the number of solar neutrinos reaching the earth were taken in the 1960s, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted by the Standard Solar Model. As several experiments confirmed this deficit the effect became known as the solar neutrino problem. Over several decades many ideas were put forward to try to explain the effect, one of which was the hypothesis of neutrino flavour change. All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and not to muon or tau type neutrinos. The SNO experiment was designed to test the neutrino flavour change hypothesis by simultaneously measuring the electron neutrino flux and the total neutrino flux.
[edit] Detector description
The SNO detector target was 1000 tonnes of heavy water contained in a 6 meter radius acrylic vessel. The detector cavity was filled with normal water to provide both buoyancy for the vessel and radioactive shielding. The heavy water was viewed by approximately 9600 photomultiplier tubes (PMTs) mounted on a geodesic sphere at a radius of about 850cm. The experiment did not directly detect neutrinos, but rather observed the light produced by relativistic electrons in the water. As relativistic electrons lose energy they produce a cone of blue light through the Cerenkov effect, and it is this light that is directly detected. The SNO detector was sensitive to three different neutrino reactions, and it was by studying the relative fraction of each reaction that the experiment was able to test for flavour change.
[edit] Charged current interaction
In the charged current interaction a neutrino converts the neutron in a deuteron to a proton. The neutrino is absorbed in the reaction and an electron is produced. Solar neutrinos have energies smaller than the mass of muon and tau particles, so only electron neutrinos can participate in this reaction. The electron carries off most of the neutrino's energy, on the order of 5-15MeV, and is detectable. The proton which is produced does not have enough energy to be detected. The electrons produced in this reaction come off in all directions, but there is a slight tendency for them to point back in the direction the neutrino came from.
[edit] Neutral current interaction
In the neutral current interaction a neutrino dissociates the deuteron, breaking it into its constituent neutron and proton. The neutrino continues on with slightly less energy, and all three neutrino flavours are equally likely to participate in this interaction. Heavy water has a large cross section for neutrons, and when neutrons capture on a deuterium nuclei a gamma ray with roughly 6MeV of energy is produced. The direction of the gamma ray is completely uncorrelated with the direction of the sun. Some of the neutrons wander past the acrylic vessel into the light water, and since light water has a very large cross section for neutron capture these neutrons are captured very quickly. A gamma ray with roughly 2MeV of energy is produced in this reaction, but because this is below the detector's energy threshold they are not observable.
[edit] Electron elastic scattering
In the elastic scattering interaction a neutrino collides with an atomic electron and imparts some of its energy to the electron. All three neutrinos can participate in this interaction through the exchange of the neutral Z boson, and electron neutrinos can also participate with the exchange of a charged W boson. For this reason this interaction is dominated by electron type neutrinos, and this is the channel through which the Super-Kamiokande detector can observe solar neutrinos. This interaction is the relativistic equivalent of billiards, and for this reason the electrons produced usually point in the direction that the neutrino was travelling (away from the sun). Because this interaction takes place on atomic electrons it occurs with the same rate in both the heavy and light water.
[edit] Experimental results
On June 18, 2001, the first scientific results of the Observatory were published [1][2], bringing the first clear evidence that neutrinos change flavour, or oscillate, as they travel through the sun. This oscillation in turn implies that neutrinos have non-zero masses. The total flux of all neutrino flavours measured by SNO agrees well with the theoretical prediction. Further measurements carried out by the Observatory have since confirmed and improved the precision of the original result.
[edit] Other possible analyses
The SNO detector would have been capable of detecting a supernova within our galaxy if one had occurred while the detector was online. As neutrinos emitted by a supernova are released earlier than the photons, it is possible to alert the astronomical community before the supernova is visible. SNO was a founding member of the SuperNova Early Warning System with Super-Kamiokande and LVD. No such supernovas have yet been detected.
The SNO experiment was also able to observe atmospheric neutrinos produced by cosmic ray interactions in the atmosphere. Due to the limited size of the SNO detector in comparison with Super-K the low cosmic ray neutrino signal is not statistically significant at neutrino energies below 1GeV.
[edit] Participating institutions
Large particle physics experiments require large collaborations. With approximately 100 collaborators SNO was a rather small group compared to collider experiments. The participating institutions have included:
[edit] Canada
- Carleton University
- Laurentian University
- Queen's University - designed and built many calibration sources and the device for deploying sources
- TRIUMF
- University of British Columbia
- University of Guelph
Although no longer a collaborating institution, Chalk River Laboratories led the construction of the acrylic vessel that holds the heavy water.
[edit] United Kingdom
- University of Oxford - developed much of the experiment's Monte Carlo analysis program (SNOMAN), and maintained the program
[edit] United States of America
- LBNL - Led the construction of the geodesic structure that holds the PMTs
- LANL
- University of Pennsylvania - designed and built the front end electronics and trigger
- University of Washington - designed and built proportional counter tubes for detection of neutrons in the third phase of the experiment
- Brookhaven National Laboratory
- University of Texas at Austin
- Massachusetts Institute of Technology
[edit] Trivia
- The day after the experiment was officially turned off, an unusually large earthquake occurred at the mine in which SNO is located. [3]. This resulted in damaging the detector to the point of temporary inoperability.
- Asteroid (14724) SNO is named in honour of the Observatory.
- The Sudbury Neutrino Observatory is a major setting in the Neanderthal Parallax trilogy by Canadian science fiction author Robert J. Sawyer.
- Although the observatory itself is not open to the public for tours, a video tour of the facility can be seen at Sudbury's interactive science centre, Science North.
- In November 2006 the entire SNO team was awarded the inaugural John C. Polanyi prize for "a recent outstanding advance in any field of the natural sciences or engineering" conducted in Canada.
- Renowned physicist Stephen Hawking and Nobel prize winners Bertram Brockhouse and Richard Taylor were present at the official opening of the SNO in 1998.
[edit] See also
- Snolab - A permanent underground physics laboratory being built around SNO
Other neutrino observatories
Related Articles
- Particle Physics - General Article on Particle Physics
- Raymond Davis Jr. - Former Director of the Homestake Experiment
- Masatoshi Koshiba - Former Director of Kamioka Observatory
[edit] External links
