Davis Constructs a Solar Neutrino Detector Summary

  • Last updated on November 10, 2022

Raymond Davis, Jr., a chemist at the Brookhaven National Laboratory, constructed a giant neutrino detector that provided the first direct evidence that the Sun is a thermonuclear power. Because so few neutrinos were detected, however, scientists continued to question standard solar models.

Summary of Event

In 1871, Hermann von Helmholtz Helmholtz, Hermann von suggested that no ordinary chemical reaction could be responsible for the enormous energy output of the Sun. By the 1920’s, astrophysicists realized that the energy radiated by the Sun must come from nuclear fusion Nuclear fusion , in which protons or nuclei combine to form larger nuclei and release energy. These reactions were assumed to be taking place deep in the interior of the Sun, where the pressures and temperatures were high enough to allow fusion to proceed. Conventional astronomical observations could record only the particles and light emitted by the much cooler outer layers of the Sun and could not provide evidence for the existence of a thermonuclear furnace in the interior. Of all the particles released in the fusion process, only one type—the neutrino—interacts so infrequently with matter that it can pass through the Sun and reach Earth. These neutrinos provide a way to probe the interior of the Sun and verify directly the hypothesis of thermonuclear energy generation in stars. Neutrinos Astronomy;neutrinos Solar neutrino detectors [kw]Davis Constructs a Solar Neutrino Detector (Early 1967) [kw]Solar Neutrino Detector, Davis Constructs a (Early 1967) [kw]Neutrino Detector, Davis Constructs a Solar (Early 1967) Neutrinos Astronomy;neutrinos Solar neutrino detectors [g]North America;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] [g]United States;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] [c]Astronomy;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] [c]Physics;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] [c]Energy;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] [c]Science and technology;Early 1967: Davis Constructs a Solar Neutrino Detector[09070] Davis, Raymond, Jr. Bahcall, John Norris

The neutrino was “invented” in 1930 by physicist Wolfgang Pauli Pauli, Wolfgang to account for the apparent missing energy in the beta decay, or emission of an electron, from radioactive nuclei. He proposed that an unseen neutrino was also emitted in beta decay, and it carried off the “missing” energy. To balance the energy but not be observed in the decay process, Pauli’s hypothetical particle had to be uncharged, have little or no mass, and interact only very weakly with ordinary matter. Typical neutrinos would have to be able to pass through millions of miles of ordinary matter with only a low probability of undergoing a single collision. Scientists’ detectors, and even the whole Earth or Sun, were essentially transparent to Pauli’s neutrinos.

Because the neutrino is so difficult to detect, it took more than twenty-five years to confirm the existence of the neutrino. In 1956, Clyde Cowan Cowan, Clyde and Frederick Reines Reines, Frederick , both physicists at the Los Alamos National Laboratory, built the world’s largest scintillation counter Scintillation counters , a device to detect the small flash of light given off by the interaction of a neutrino in the apparatus. They placed this scintillation counter adjacent to the Savannah River Nuclear Reactor, a high-power reactor producing about 1 trillion neutrinos every second. Even with this enormous number of neutrinos, only one neutrino interaction was observed in their detector every twenty minutes, but Cowan and Reines were able to confirm the existence of Pauli’s elusive particle.

Neutrinos are able to pass through nearly all forms of matter without interacting with other subatomic particles.

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The task of detecting the solar neutrinos was even more formidable. If an apparatus similar to the Cowan and Reines detector were employed to search for the neutrinos from the Sun, only one interaction would be expected every few thousand years.

At about the same time Cowan and Reines performed their experiment, another type of neutrino detector—this one relying on radiochemical principles—was under development by Raymond Davis, Jr., a chemist at the Brookhaven National Laboratory. Davis employed an idea, originally suggested in 1948 by Bruno Pontecorvo Pontecorvo, Bruno , that when a neutrino interacts with a chlorine-37 nucleus, it produces a nucleus Radioisotopes of argon 37. Since argon is a chemically inert gas, it was hoped that the argon produced by neutrinos could be extracted from large volumes of chlorine-rich liquid by passing helium gas through the liquid. The argon 37 produced is itself radioactive, decaying with a half-life of thirty-five days. Thus, once the argon is extracted from the liquid, it can be detected easily by observing its decay.

Davis tested a version of this neutrino detector, containing about 3,785 liters of carbon tetrachloride liquid, near a nuclear reactor at Brookhaven between 1954 and 1956. In the scientific paper describing these results, Davis suggested this type of neutrino detector could be made large enough to permit detection of the solar neutrinos. As the detector is enlarged, however, the number of events from cosmic-ray interactions goes up, and these events would dominate over the small number of solar neutrino events. To reduce the effect of cosmic rays, Davis decided to bury his apparatus deep underground, using earth as a shield against the cosmic rays.

A pilot solar neutrino detector experiment was assembled by Davis 701 meters below the surface in a limestone mine at Barberton, Ohio. This detector, which operated from 1960 to 1962, consisted of two 1,893-liter tanks of perchloroethylene. Only three argon decays every eighteen days were found, which was consistent with background radioactivity. This initial experiment failed to detect any solar neutrinos, but Davis used it to demonstrate the detection principle and he indicated that a 378,540-liter detector 1,372 meters underground should have the sensitivity required to see the neutrinos from the Sun.

Davis continued his search for solar neutrinos with a much larger detector. This detector was constructed 1,478 meters underground in early 1967 in the Homestake Gold Mine in Lead, South Dakota. The tank was surrounded by water to shield the detector from neutrons emitted by trace quantities of uranium and thorium in the walls of the mine.

To describe his results, Davis coined a new unit, the “solar neutrino unit” Solar neutrino units (SNU), with 1 SNU corresponding to the production of a single atom of argon 37 in his apparatus every six days. Astrophysicist John Norris Bahcall had performed detailed calculations of the number of neutrinos expected in Davis’s detector. Using the best available astronomical models of the nuclear reactions going on in the Sun’s interior as well as the physical properties of the neutrinos, Bahcall had calculated a capture rate of 50 SNUs in 1963.

The first results from Davis’s detector—from a forty-eight-day run in May to June, 1967, and a 110-day run in June to October, 1967—were much lower than expected. Davis was able to place an upper limit of only 3 SNUs on the solar neutrino flux. The discrepancy between the number of neutrinos detected by Davis’s apparatus and the number predicted by the theoreticians left open three possibilities: that the experiment itself was flawed, that the physics of the neutrino was different from that expected, or that the conditions in the interior of the Sun differed from the accepted model. Davis undertook a series of calibration experiments to determine the efficiency of his detector, and concluded that the experiment was not flawed. Bahcall repeated the theoretical calculations in 1969, using more recent information on nuclear reactions, but he could not reduce the theoretical prediction below 8 SNUs, far higher than the upper limit of 3 SNUs established by Davis.

In 1970, the argon decay detector was improved substantially, increasing the sensitivity of the experiment. Davis’s measurements continued almost uninterrupted over subsequent years, giving essentially the same results. The long-term average measured over more than twenty years is only 2.3 SNUs, well below the most recent theoretical predictions.

Significance

The main significance of the detection of solar neutrinos by Davis was the direct confirmation that thermonuclear fusion must be occurring at the center of the Sun, thus confirming the basic model of energy generation in stars. The low number of solar neutrinos Davis detected, however, has called into question some of the fundamental beliefs of astrophysics. As Bahcall explained, “We know more about the sun than about any other star. . . . The sun is also in what is believed to be the best-understood stage of stellar evolution. . . . If we are to have confidence in the many astronomical and cosmological applications of the theory of stellar evolution, it ought at least to give the right answers for the sun.”

For almost two decades, the Homestake detector remained the world’s only solar neutrino detector, so there was no independent way to verify Davis’s results. In July, 1983, however, a group of Japanese scientists commissioned the Kamiokande II detector, which was also sensitive to solar neutrinos. Their measurements provided confirmation of the low solar neutrino flux reported by Davis.

The problem of the “missing” solar neutrinos has occupied the attention of physicists and astronomers since Davis’s initial discovery. Many solutions have been proposed. These solutions can be divided into two broad classes: those that challenge the standard model of the Sun’s interior and those that challenge the understanding of the behavior of the neutrino. Since the neutrino is very difficult to detect, many of its properties are based on physicists’ ideas of how it should behave rather than on direct experimental observation of that behavior.

Davis’s discovery of the low neutrino detection rate at Earth has focused years of attention by many scientists on a better understanding of the how the Sun generates its energy and how the neutrino behaves. New and more elaborate solar neutrino detectors have been proposed and built to resolve these questions. These future experiments are aimed at understanding the details of the process by which stars, including the Sun, shine and the mechanism by which they age and evolve, as well as developing a more complete theory of the physics and behavior of the elusive neutrino. For his work in this area, Davis was awarded the Nobel Prize in Physics Nobel Prize in Physics;Raymond Davis, Jr.[Davis] in 2002. Neutrinos Astronomy;neutrinos Solar neutrino detectors

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Bahcall, John N. “Neutrinos from the Sun.” Scientific American, July, 1969, 28-37. This well-illustrated article by one of the major theoreticians involved in the solar neutrino search provides a detailed description of the apparatus used by Davis as well as an explanation of the processes in the Sun that generate the neutrinos and the mechanism used for their detection.
  • citation-type="booksimple"

    xlink:type="simple">_______. “The Solar-Neutrino Problem.” Scientific American, May, 1989, 54-61. This well-illustrated article, suitable for general readers, describes Davis’s solar neutrino detector, explains how the nuclear processes in the Sun produce neutrinos and how they are detected, and summarizes the results of two decades of measurements by Davis. Discusses the implications of the missing solar neutrinos and describes the various solutions such as neutrino oscillations, variations in the composition of the interior of the Sun, or a lull in solar activity.
  • citation-type="booksimple"

    xlink:type="simple">Davis, Raymond, Jr., et al. “Solar Neutrinos.” Annual Reviews of Nuclear and Particle Science 39 (1989): 467-505. Although intended for professionals, this article provides a detailed, nonmathematical account of the design, operation, and results from the Davis experiment from 1967 through 1988.
  • citation-type="booksimple"

    xlink:type="simple">Ford, Kenneth William. The World of Elementary Particles. New York: Blaisdell, 1963. Written to explain particle physics to general readers, this work describes the properties of the neutrino, explains how they are produced in the Sun and other stars, and describes the difficulties of detecting these particles. Contains a particularly good description of Cowan and Reines’s 1956 experiment and explains the problems associated with solar neutrino detection experiments.
  • citation-type="booksimple"

    xlink:type="simple">Golub, Leon, and Jay M. Pasachoff. Nearest Star: The Surprising Science of Our Sun. Cambridge, Mass.: Harvard University Press, 2001. Provides a nontechnical guide to the science of the Sun, with chapters including “What We See,” “What We Don’t See,” “Eclipses,” “Space Missions,” and “Between Fire and Ice.” Includes illustrations, some in color, and maps.
  • citation-type="booksimple"

    xlink:type="simple">Koshiba, Masa-Toshi. “Observational Neutrino Astrophysics.” Physics Today 40 (December, 1987): 38-42. This well-illustrated, nontechnical article describes the results from the Japanese Kamiokande II neutrino detector, compares those results with those of Davis, discusses the implications of the low solar neutrino flux and describes further solar neutrino experiments planned for the 1990’s around the world.
  • citation-type="booksimple"

    xlink:type="simple">Noyes, Robert W. The Sun, Our Star. Cambridge, Mass.: Harvard University Press 1982. Chapter 3, “Probing the Depths of the Sun,” describes the source of fuel for the Sun, the Davis experiment, and the mystery of the missing solar neutrinos.
  • citation-type="booksimple"

    xlink:type="simple">Wentzel, Donat G. The Restless Sun. Washington, D.C.: Smithsonian Institution Press, 1989. Chapter 2, “Solar Interior: Neutrinos,” focuses on how neutrinos provide an opportunity to “look inside” the Sun and probe its energy generation process. Provides a particularly good, nonmathematical discussion of the various hypotheses to explain the low counting rate of Davis’s apparatus and of the new generation of gallium detectors to search for lower energy neutrinos.
  • citation-type="booksimple"

    xlink:type="simple">Wolfenstein, Lincoln, and Eugene W. Beier. “Neutrino Oscillations and Solar Neutrinos.” Physics Today 42 (July, 1989): 28-36. Describes the various attempts to measure the number of solar neutrinos arriving at Earth and examines the implications of the low flux measured by Davis for the accepted models of the Sun and the neutrino.

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