Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) is a neutrino observatory located 6,800 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 28 November 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 underground laboratory has been enlarged and continues to operate other experiments at SNOLAB. The SNO equipment itself is currently being refurbished for use in the SNO+ experiment.

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 oscillations. All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and yielded little to no information on muon neutrinos and tau neutrinos.

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 accordingly 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 observed the light produced by relativistic electrons in the water created by neutrino interactions. As relativistic electrons travel through a medium, they lose energy producing a cone of blue light through the Cerenkov effect, and it is this light that is directly detected.

Detector description

The SNO detector target consisted of 1,000 tonnes (1,102 short tons) of heavy water contained in a 6-metre (20 ft) radius acrylic vessel. The detector cavity outside the vessel was filled with normal water to provide both buoyancy for the vessel and radioactive shielding. The heavy water was viewed by approximately 9,600 photomultiplier tubes (PMTs) mounted on a geodesic sphere at a radius of about 850 centimetres (335 in). The cavity housing the detector is the largest man-made 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 1.5-kilometre (0.9 mi) 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 (fewer than 3,000 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 muons and tau leptons, so only electron neutrinos can participate in this reaction. The emitted 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 easily. The electrons produced in this reaction are emitted in all directions, but there is a slight tendency for them to point back in the direction from which the neutrino came.

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 nucleus a gamma ray (photon) with roughly 6 MeV of energy is produced. The direction of the gamma ray is completely uncorrelated with the direction of the neutrino. 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 neutrinos, and this is the channel through which the Super-Kamiokande (Super-K) 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 18 June 2001, the first scientific results of SNO were published, bringing the first clear evidence that neutrinos oscillate (i.e. that they can transmute into one another), as they travel in 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 SNO have since confirmed and improved the precision of the original result.

Although Super-K had beaten SNO to the punch, having published evidence for neutrino oscillation as early as 1998, the Super-K results were not conclusive and did not specifically deal with solar neutrinos. SNO's results were the first to directly demonstrate 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 1,500 times, and two others have been cited over 750 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 (SNEWS) with Super-Kamiokande and the Large Volume Detector. 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.

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:

  • Carleton University
  • Laurentian University
  • Queen's University – designed and built many calibration sources and the device for deploying sources
  • 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, and Atomic Energy of Canada Limited was the source of the heavy water.

United Kingdom
  • University of Oxford – developed much of the experiment's Monte Carlo analysis program (SNOMAN), and maintained the program
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
Honours and awards
  • Asteroid 14724 SNO is named in honour of SNO.
  • 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.