The Sudbury Neutrino Observatory

tte Sudbury Neutrino Observatory, abbreviated SNO and pronounced "snow", is a collaboration of Canadian, American and British scientists, tte detector is located 2 kilometers underground in a working nickel mine near Sudbury, Ontario. Like the previous water detectors, it observes boron-8 solar neutrinos with energies above 7.5 MeV. But unlike Kamiokande or Super-Kamiokande, the SNO detector contains heavy water.

Heavy water is chemically similar to ordinary water, and it doesn't appear or taste any different. In fact, heavy water exits naturally as a constituent of ordinary tap or lake water in a ratio of about one part in 7,000, and expensive chemical and physical processes can separate it.

tte hydrogen in heavy water has a nucleus, called a deuteron, which consists of a proton and a neutron. For ordinary water, the hydrogen is about half as light, with a nucleus that contains only a proton and no neutron. And it is the heavier deuteron that makes the Sudbury Neutrino Observatory sensitive to not just one type of neutrino but to all three known varieties of neutrinos.

One thousand tons, or one million liters, of heavy water, with a value of S300 million, was placed in a central spherical cistern with transparent acrylic walls (Fig. 3.6).

FIG. 3.6 Sudbury neutrino observatory The central spherical flask of this neutrino observatory is 12 meters in diameter, and is surrounded by a geodesic array of thousands of light sensors to detect the flash of light from the interaction of a neutrino with the heavy water. (Courtesy of Kevin Lesko, Lawrence Berkeley National Laboratory.)

FIG. 3.6 Sudbury neutrino observatory The central spherical flask of this neutrino observatory is 12 meters in diameter, and is surrounded by a geodesic array of thousands of light sensors to detect the flash of light from the interaction of a neutrino with the heavy water. (Courtesy of Kevin Lesko, Lawrence Berkeley National Laboratory.)

Since the scientists could not afford its cost, the heavy water was borrowed from Atomic Energy of Canada Limited, which stockpiled it for use in its nuclear power reactors - the heavy water moderates neutrons created by uranium fission in the reactors.

A geodesic array of about 10,000 photo-multiplier tubes surrounds the vessel to detect the flash of light given off by heavy water when it is hit by a neutrino. Both the light sensors and the central tank are enveloped by a 7,800-ton jacket of ordinary water (Fig. 3.7), to shield the heavy water from weak natural radiation, gamma rays and neutrons from the underground rocks. As with the other neutrino detectors, the overlying rock blocks energetic particles generated by cosmic rays.

FIG. 3.7 How Sudbury works Neutrinos from the Sun travel through more than two kilometers of rock, entering an acrylic tank containing 1,000 tons (1 million liters) of heavy water. When one of these neutrinos interacts with a water molecule, it produces a flash of light that is detected by a geodesic array of photo-multiplier tubes. Some 7,800 tons (7.8 million liters) of ordinary water surrounding the acrylic tank blocks radiation from the rock, while the overlying rock blocks energetic particles generated by cosmic rays in our atmosphere. The heavy water is sensitive to all three types of neutrinos.

FIG. 3.7 How Sudbury works Neutrinos from the Sun travel through more than two kilometers of rock, entering an acrylic tank containing 1,000 tons (1 million liters) of heavy water. When one of these neutrinos interacts with a water molecule, it produces a flash of light that is detected by a geodesic array of photo-multiplier tubes. Some 7,800 tons (7.8 million liters) of ordinary water surrounding the acrylic tank blocks radiation from the rock, while the overlying rock blocks energetic particles generated by cosmic rays in our atmosphere. The heavy water is sensitive to all three types of neutrinos.

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