Sudbury Neutrino Observatory
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The Sudbury Neutrino Observatory (SNO) was a neutrino observatory located 2100 m underground in
Vale A vale is a type of valley. Vale may also refer to: Places Georgia * Vale, Georgia, a town in the Samtskhe-Javakheti region Norway * Våle, a historic municipality Portugal * Vale (Santa Maria da Feira), a former civil parish in the municip ...
's Creighton Mine in Sudbury,
Ontario Ontario is the southernmost Provinces and territories of Canada, province of Canada. Located in Central Canada, Ontario is the Population of Canada by province and territory, country's most populous province. As of the 2021 Canadian census, it ...
, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of
heavy water Heavy water (deuterium oxide, , ) is a form of water (molecule), water in which hydrogen atoms are all deuterium ( or D, also known as ''heavy hydrogen'') rather than the common hydrogen-1 isotope (, also called ''protium'') that makes up most o ...
. The detector was turned on in May 1999, and was turned off on 28 November 2006. The SNO collaboration was active for several years after that analyzing the data taken. The director of the experiment, Art McDonald, was co-awarded the
Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...
in 2015 for the experiment's contribution to the discovery of
neutrino oscillation Neutrino oscillation is a quantum mechanics, quantum mechanical phenomenon in which a neutrino created with a specific lepton lepton number, family number ("lepton flavor": electron, muon, or tau lepton, tau) can later be Quantum measurement, mea ...
. The underground laboratory has been enlarged into a permanent facility and now operates multiple experiments as SNOLAB. The SNO equipment itself was 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 oscillation Neutrino oscillation is a quantum mechanics, quantum mechanical phenomenon in which a neutrino created with a specific lepton lepton number, family number ("lepton flavor": electron, muon, or tau lepton, tau) can later be Quantum measurement, mea ...
s. All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to
electron neutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name ''electron neutrino''. It was first hypothesized by Wolfga ...
s and yielded little to no information on
muon neutrino The muon neutrino is an elementary particle which has the symbol and zero electric charge. Together with the muon it forms the second generation of leptons, hence the name muon neutrino. It was discovered in 1962 by Leon Lederman, Melvin Schwa ...
s 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 reaction sensitive to all neutrino flavours, the other reaction sensitive to only electron neutrino. Thus, such a detector could measure neutrino oscillations directly. A location in Canada was attractive because
Atomic Energy of Canada Limited Atomic Energy of Canada Limited (AECL, Énergie atomique du Canada limitée, EACL) is a Canadian Crown corporation and the largest nuclear science and technology laboratory in Canada. AECL developed the CANDU reactor technology starting in th ...
, which maintains large stockpiles of heavy water to support its CANDU reactor power plants, was willing to lend the necessary amount (worth at market prices) at no cost. The Creighton Mine in Sudbury is among the deepest in the world and, accordingly, experiences a very small background flux of radiation. It was quickly identified as an ideal place for Chen's proposed experiment to be built, and the mine management was willing to make the location available for only incremental costs. A good retrospective on the project. The SNO collaboration held its first meeting in 1984. At the time it competed with
TRIUMF Triumf may refer to: * TRIUMF, Canada's national particle accelerator centre * 14959 TRIUMF, a minor planet * S-400 Triumf, a Russian anti-aircraft weapon system developed in the 1990s * Triumf Riza (1979–2007), Kosovo police officer and member o ...
'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
Cherenkov effect Cherenkov radiation () is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium (such as distilled water) at a speed greater than the phase velocity (speed of Wave propagation, propagat ...
, and it is this light that is directly detected.


Detector description

The SNO detector target consisted of of
heavy water Heavy water (deuterium oxide, , ) is a form of water (molecule), water in which hydrogen atoms are all deuterium ( or D, also known as ''heavy hydrogen'') rather than the common hydrogen-1 isotope (, also called ''protium'') that makes up most o ...
contained in a acrylic vessel. The detector cavity outside the vessel was filled with normal water to provide both
buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...
for the vessel and radiation shielding. The heavy water was viewed by approximately 9,600 photomultiplier tubes (PMTs) mounted on a
geodesic In geometry, a geodesic () is a curve representing in some sense the locally shortest path ( arc) between two points in a surface, or more generally in a Riemannian manifold. The term also has meaning in any differentiable manifold with a conn ...
sphere at a radius of about . The cavity housing the detector was the largest in the world at such a depth, requiring a variety of high-performance rock bolting techniques to prevent rock bursts. The mining was completed in May 1993. The observatory is located at the end of a drift, 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 A cleanroom or clean room is an engineered space that maintains a very low concentration of airborne particulates. It is well-isolated, well-controlled from contamination, and actively cleansed. Such rooms are commonly needed for scientifi ...
setting. Most of the facility is Class 3000 (fewer than 3,000 particles of 1 μm or larger per 1 ft3 of air) but the final cavity containing the detector is an even stricter Class 100.


Charged current interaction

In the charged current interaction, a neutrino converts the
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
in a
deuteron Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium atomic nucleus, nucleus (deuteron) contains one proton and ...
to a
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
. The neutrino is absorbed in the reaction and an electron is produced. Solar neutrinos have energies smaller than the mass of
muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of  ''ħ'', but with a much greater mass. It is classified as a ...
s 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, but when neutrons are captured by a deuterium nucleus, a
gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
(
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
) 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 produced from the dissociated deuterons make their way through the acrylic vessel into the light water jacket surrounding the heavy water, and since light water has a very large cross section for neutron capture, these neutrons are captured very quickly. Gamma rays of roughly 2.2 MeV are produced in this reaction, but because the energy of the photons is less than the detector's energy threshold (meaning they do not trigger the photomultipliers), they are not directly observable. However, when the gamma ray collides with an electron via Compton scattering, the accelerated electron can be detected through Cherenkov radiation.


Electron elastic scattering

In the
elastic scattering Elastic scattering is a form of particle scattering in scattering theory, nuclear physics and particle physics. In this process, the internal states of the Elementary particle, particles involved stay the same. In the non-relativistic case, where ...
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 In particle physics, the W and Z bosons are vector bosons that are together known as the weak bosons or more generally as the intermediate vector bosons. These elementary particles mediate the weak interaction; the respective symbols are , , an ...
. For this reason this interaction is dominated by electron neutrinos, and this is the channel through which the
Super-Kamiokande Super-Kamiokande (abbreviation of Super-Kamioka Neutrino Detection Experiment, also abbreviated to Super-K or SK; ) is a neutrino detector, neutrino observatory located Kamioka Observatory, under Mount Ikeno near the city of Hida, Gifu, Hida, ...
(Super-K) detector can observe solar neutrinos. This interaction is the relativistic equivalent of
billiards Cue sports are a wide variety of games of skill played with a cue stick, which is used to strike billiard balls and thereby cause them to move around a cloth-covered table bounded by elastic bumpers known as . Cue sports, a category of stic ...
, 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

The first scientific results of SNO were published on 18 June 2001, and presented the first clear evidence that neutrinos oscillate (i.e. that they can transmute into one another), 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 theoretical predictions. 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. This was important to the standard solar model. In 2007, the
Franklin Institute The Franklin Institute is a science museum and a center of science education and research in Philadelphia, Pennsylvania. It is named after the American scientist and wikt:statesman, statesman Benjamin Franklin. It houses the Benjamin Franklin ...
awarded the director of SNO Art McDonald with the Benjamin Franklin Medal in Physics. In 2015 the Nobel Prize for Physics was jointly awarded to Arthur B. McDonald, and
Takaaki Kajita is a Japanese physicist, known for neutrino experiments at the Kamioka Observatory – Kamiokande and its successor, Super-Kamiokande. In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur B. McDonald. On 1 O ...
of the University of Tokyo, for the discovery of neutrino oscillations.


Other possible analyses

The SNO detector would have been capable of detecting a
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
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 Super-Kamiokande (abbreviation of Super-Kamioka Neutrino Detection Experiment, also abbreviated to Super-K or SK; ) is a neutrino detector, neutrino observatory located Kamioka Observatory, under Mount Ikeno near the city of Hida, Gifu, Hida, ...
and the Large Volume Detector. No such supernovae have yet been detected. The SNO experiment was also able to observe atmospheric neutrinos produced by
cosmic ray Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the ...
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:


Canada

*
Carleton University Carleton University is an English-language public university, public research university in Ottawa, Ontario, Canada. Founded in 1942 as Carleton College, the institution originally operated as a private, non-denominational evening college to se ...
*
Laurentian University Laurentian University (), officially Laurentian University of Sudbury, is a mid-sized Bilingualism in Canada, bilingual public university in Greater Sudbury, Ontario, Canada, incorporated on March 28, 1960. Laurentian offers a variety of undergr ...
* Queen's University – designed and built many calibration sources and the device for deploying sources *
TRIUMF Triumf may refer to: * TRIUMF, Canada's national particle accelerator centre * 14959 TRIUMF, a minor planet * S-400 Triumf, a Russian anti-aircraft weapon system developed in the 1990s * Triumf Riza (1979–2007), Kosovo police officer and member o ...
*
University of British Columbia The University of British Columbia (UBC) is a Public university, public research university with campuses near University of British Columbia Vancouver, Vancouver and University of British Columbia Okanagan, Kelowna, in British Columbia, Canada ...
*
University of Guelph The University of Guelph (abbreviated U of G) is a comprehensive Public university, public research university in Guelph, Ontario, Canada. It was established in 1964 after the amalgamation of Ontario Agricultural College (1874), the MacDonald I ...
Although no longer a collaborating institution,
Chalk River Laboratories Chalk River Laboratories (; also known as CRL, Chalk River Labs and formerly Chalk River Nuclear Laboratories, CRNL) is a Canadian nuclear research facility in Deep River, about north-west of Ottawa. CRL is a site of significant research and ...
led the construction of the acrylic vessel that holds the heavy water, and
Atomic Energy of Canada Limited Atomic Energy of Canada Limited (AECL, Énergie atomique du Canada limitée, EACL) is a Canadian Crown corporation and the largest nuclear science and technology laboratory in Canada. AECL developed the CANDU reactor technology starting in th ...
was the source of the heavy water.


United Kingdom

*
University of Oxford The University of Oxford is a collegiate university, collegiate research university in Oxford, England. There is evidence of teaching as early as 1096, making it the oldest university in the English-speaking world and the List of oldest un ...
– developed much of the experiment's
Monte Carlo Monte Carlo ( ; ; or colloquially ; , ; ) is an official administrative area of Monaco, specifically the Ward (country subdivision), ward of Monte Carlo/Spélugues, where the Monte Carlo Casino is located. Informally, the name also refers to ...
analysis program (SNOMAN), and maintained the program *
University of Sussex The University of Sussex is a public university, public research university, research university located in Falmer, East Sussex, England. It lies mostly within the city boundaries of Brighton and Hove. Its large campus site is surrounded by the ...
– calibration


United States

*
Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory (LBNL, Berkeley Lab) is a Federally funded research and development centers, federally funded research and development center in the Berkeley Hills, hills of Berkeley, California, United States. Established i ...
(LBNL) – Led the construction of the geodesic structure that holds the PMTs * Pacific Northwest National Laboratory (PNNL) *
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...
(LANL) *
University of Pennsylvania The University of Pennsylvania (Penn or UPenn) is a Private university, private Ivy League research university in Philadelphia, Pennsylvania, United States. One of nine colonial colleges, it was chartered in 1755 through the efforts of f ...
– designed and built the front end electronics and trigger *
University of Washington The University of Washington (UW and informally U-Dub or U Dub) is a public research university in Seattle, Washington, United States. Founded in 1861, the University of Washington is one of the oldest universities on the West Coast of the Uni ...
– designed and built proportional counter tubes for detection of neutrons in the third phase of the experiment *
Brookhaven National Laboratory Brookhaven National Laboratory (BNL) is a United States Department of Energy national laboratories, United States Department of Energy national laboratory located in Upton, New York, a hamlet of the Brookhaven, New York, Town of Brookhaven. It w ...
*
University of Texas at Austin The University of Texas at Austin (UT Austin, UT, or Texas) is a public university, public research university in Austin, Texas, United States. Founded in 1883, it is the flagship institution of the University of Texas System. With 53,082 stud ...
*
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...


Honours and awards

*
Asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
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 Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
. * SNO principal investigator Arthur B. McDonald won the 2015
Nobel Prize in Physics The Nobel Prize in Physics () is an annual award given by the Royal Swedish Academy of Sciences for those who have made the most outstanding contributions to mankind in the field of physics. It is one of the five Nobel Prizes established by the ...
, jointly with
Takaaki Kajita is a Japanese physicist, known for neutrino experiments at the Kamioka Observatory – Kamiokande and its successor, Super-Kamiokande. In 2015, he was awarded the Nobel Prize in Physics jointly with Canadian physicist Arthur B. McDonald. On 1 O ...
of
Super-Kamiokande Super-Kamiokande (abbreviation of Super-Kamioka Neutrino Detection Experiment, also abbreviated to Super-K or SK; ) is a neutrino detector, neutrino observatory located Kamioka Observatory, under Mount Ikeno near the city of Hida, Gifu, Hida, ...
, for the discovery of neutrino oscillation. * SNO was awarded the 2016 Fundamental Physics Prize along with 4 other neutrino experiments.


See also

* DEAP – Dark Matter Experiment using Argon Pulse-shape at SNO location * Homestake experiment – A predecessor experiment conducted 1970–1994 in a mine at Lead, South Dakota * SNO+ – The successor of SNO * SNOLAB – A permanent underground physics laboratory being built around SNO


References


External links

*
SNO's official site


* * * ttps://ewh.ieee.org/reg/7/millennium/neutrino/sno_about.html Showcase of Canadian Engineering Achievement: Sudbury Neutrino Observatory(IEEE Canada). Several articles about the civil engineering of SNO.
SNO experiment
record on
INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1 ...
{{Authority control Neutrino observatories Buildings and structures in Greater Sudbury Research institutes in Canada Nuclear research institutes Particle experiments Underground laboratories Physics beyond the Standard Model Laboratories in Canada