Sudbury Neutrino Observatory
The Sudbury Neutrino Observatory is located 6800 feet underground in
Inco Limited's Creighton Mine in
Greater Sudbury,
Ontario,
Canada. The detector is designed to detect solar
neutrinos through their interactions with
deuterium nuclei and
atomic electrons. The detector turned on in May of 1999, and is scheduled to be turned off at the end of 2006.
Encyclopedia
The
Sudbury Neutrino Observatory is located 6800 feet underground in
Inco Limited's Creighton Mine in
Greater Sudbury,
Ontario,
Canada. The detector is designed to detect solar
neutrinos through their interactions with
deuterium nuclei and
atomic electrons. The detector turned on in May of 1999, and is scheduled to be turned off at the end of 2006.
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.
Detector description
The SNO detector target is 1000 tonnes of heavy water contained in a 6 meter radius
acrylic vessel. The detector cavity is filled with normal water to provide both buoyancy for the vessel and radioactive shielding. The heavy water is viewed by approximately 9600
photomultiplier tubes mounted on a geodesic sphere at a radius of about 850cm. The experiment does not directly detect neutrinos, but rather observes 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 is sensitive to three different neutrino reactions, and it is by studying the relative fraction of each reaction that the experiment can test for flavour change.
Charged current interraction
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.
Neutral current interreaction
In the neutral current interraction a neutrino dissociates the deuteron, breaking it into its constituent neutron and proton. The neutrino continues on with slighly 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.
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 . Because this interaction takes place on atomic electrons it occurs with the same rate in both the heavy and light water.
Experimental results
On June 18, 2001, the first scientific results of the Observatory were published , 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.
Other possible analyses
The SNO detector is capable of detecting a
supernova within our galaxy if one occurs while the detector is 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 is a founding member of the SuperNova Early Warning System with
Super-Kamiokande and LVD. No such supernovas have yet been detected.
It would also be possible for the SNO experiment to observe atmospheric neutrinos produced by
cosmic rays, but because the detector is so much smaller than the Super-K detector SNO's measurement would be extremely statistically limited.
Participating institutions
Large particle physics experiments require large collaborations. With approximately 100 collaborators SNO is a rather small group compared to collider experiments. The participating institutions include:
Canada
...
- designed and built many calibration sources and the device for deploying sources
Although no longer a collaborating institution, Chalk River Laboratories led the construction of the acrylic vessel that holds the heavy water.
United Kingdom
United States of America
Trivia
Asteroid 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.
See also
Other neutrino observatories- Kamioka Observatory
- Baksan Neutrino Observatory
- Laboratori Nazionali del Gran Sasso, Italy
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
External links
