The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located 6800 feet (about 2 km) underground in Vale Inco's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water. The detector turned on in May 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. The equipment is currently being refurbished for use in the SNO+ experiment.
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
In 1984 Herb Chen of the University of California at Irvine first pointed out the advantages of using heavy water as a detector for solar neutrinos. Unlike previous detectors, using heavy water would make the detector sensitive to two reactions, one sensitive to all neutrino flavours, which would allow a detector to measure neutrino oscillations directly. The Creighton mine in Sudbury, among the deepest in the world and blessed with low background radiation, was quickly identified as an ideal place for Chen’s proposed experiment to be built.
The SNO collaboration held its first meeting in 1984. At the time it competed with TRIUMF’s “KAON Factory” proposal for federal funding, and the wide variety of universities backing SNO quickly led to it being selected for development. The official go-ahead was given in 1990.
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 target consisted of of heavy water
contained in a 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 . The cavity housing the detector is the largest underground cavity in the world, requiring a variety of high-performance rock bolting techniques to prevent rock bursts.
The observatory is located at the end of a long drift, whimsically named the “SNO drift”, isolating it from other mining operations. Along the drift are a number of operations and equipment rooms, all held in a clean room setting. Most of the facility is Class 3000 (less than 3000 particles of 1 μm or larger per 1 m³ of air) but the final cavity containing the detector is Class 1000.
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 5-15 MeV
, 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 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 small cross section
for neutrons, and when neutrons capture on a deuterium nuclei a gamma ray with roughly 6 MeV 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 2 MeV 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 (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.
Experimental results and impact
On June 18
, the first scientific results of the Observatory were published, bringing the first clear evidence that neutrinos change flavour, or oscillate
, as they travel from 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.
Although Super-Kamiokande had beaten SNO to the punch, having published similar results as early as 1998, the SuperK results were not conclusive and did not specifically deal with solar neutrinos. SNO’s results were much more “direct”, and directly demonstrated oscillations in solar neutrinos. The results of the experiment had a major impact on the field, as evidenced by the fact that two of the SNO papers have been cited over 1300 times, and two others have been cited over 500 times. In 2007, the Franklin Institute awarded the director of SNO Art McDonald with the Benjamin Franklin Medal in Physics.
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
. 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 1 GeV.
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:
Although no longer a collaborating institution, Chalk River Laboratories led the construction of the acrylic vessel that holds the heavy water.
United States of America
- The day after the experiment was officially turned off, an unusually large earthquake occurred at the mine in which SNO is located. This damaged 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 Award 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.
- SNOLAB - A permanent underground physics laboratory being built around SNO
Other neutrino observatories