The invention:Adevice that provided the first direct evidence that the Sun runs on thermonuclear power and challenged existing models of the Sun. The people behind the invention: Raymond Davis, Jr. (1914- ), an American chemist John Norris Bahcall (1934- ), an American astrophysicist Missing Energy In 1871, Hermann von Helmholtz, the German physicist, anatomist, and physiologist, suggested that no ordinary chemical reaction could be responsible for the enormous energy output of the Sun. By the 1920’s, astrophysicists had realized that the energy radiated by the Sun must come from 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, in an immense thermonuclear furnace, where the pressures and temperatures were high enough to allow fusion to proceed. Conventional astronomical observations could record only the particles of 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. Then scientists realized that the neutrino might be used to prove that this huge furnace existed. 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 the earth. These neutrinos provide a way to verify directly the hypothesis of thermonuclear energy generated in stars. The neutrino was “invented” in 1930 by the American physicist Wolfgang Pauli to account for the apparent missing energy in the beta decay, or emission of an electron, from radioactive nuclei. He proposed that an unseen nuclear particle, which he called a neutrino, was also emitted in beta decay, and that it carried off the “missing” energy. To balance the energy but not be observed in the decay process, Pauli’s hypothetical particle had to have no electrical charge, 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 in order to reach the earth. Scientists’ detectors, and even the whole earth or Sun, were essentially transparent as far as Pauli’s neutrinos were concerned. Because the neutrino is so difficult to detect, it took more than twenty-five years to confirm its existence. In 1956, Clyde Cowan and Frederick Reines, both physicists at the Los Alamos National Laboratory, built the world’s largest scintillation counter, a device to detect the small flash of light given off when the neutrino strikes (“interacts” with) a certain substance in the apparatus. They placed this scintillation counter near the Savannah River Nuclear Reactor, which was producing about 1 trillion neutrinos every second. Although only one neutrino interaction was observed in their detector every twenty minutes, Cowan and Reines were able to confirm the existence of Pauli’s elusive particle. 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 could be expected every few thousand years. Missing Neutrinos At about the same time that Cowan and Reines performed their experiment, another type of neutrino detector was under development by Raymond Davis, Jr., a chemist at the Brookhaven National Laboratory. Davis employed an idea, originally suggested in 1948 by the nuclear physicist Bruno Pontecorvo, that when a neutrino interacts with a chlorine-37 nucleus, it produces a nucleus of argon 37. Any argon so produced could then be extracted from large volumes of chlorine-rich liquid by passing helium gas through the liquid. Since argon 37 is radioactive, it is relatively easy to detect. Davis tested a version of this neutrino detector, containing about 3,785 liters of carbon tetrachloride liquid, near a nuclear reactor at the Brookhaven National Laboratory from 1954 to 1956. In the scientific paper describing his results, Davis suggested that this type of neutrino detector could be made large enough to permit detection of solar neutrinos.Although Davis’s first attempt to detect solar neutrinos from a limestone mine at Barberton, Ohio, failed, he continued his search with a much larger detector 1,478 meters underground in the Homestake Gold Mine in Lead, South Dakota. The cylindrical tank (6.1 meters in diameter, 16 meters long, and containing 378,540 liters of perchloroethylene) was surrounded by water to shield the detector from neutrons emitted by trace quantities of uranium and thorium in the walls of the mine. The experiment was conducted underground to shield it from cosmic radiation. To describe his results, Davis coined a new unit, the “solar neutrino unit” (SNU), with 1 SNU indicating the production of one atom of argon 37 every six days. Astrophysicist John Norris Bahcall, 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, had predicted a capture rate of 50 SNUs in 1963. The 1967 results from Davis’s detector, however, had an upper limit of only 3 SNUs.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. 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 about the Sun.” Many solutions to the problem of the “missing” solar neutrinos have been proposed. Most of these solutions can be divided into two broad classes: those that challenge the model of the sun’s interior and those that challenge the understanding of the behavior of the neutrino. Since the number of neutrinos produced is very sensitive to the temperature of the sun’s interior, some astrophysicists have suggested that the true solar temperature may be lower than expected. Others suggest that the sun’s outer layer may absorb more neutrinos than expected. Some physicists, however, believe neutrinos may occur in several different forms, only one of which can be detected by the chlorine detectors.Davis’s discovery of the low number of neutrinos reaching Earth has focused years of attention on a better understanding of how the Sun generates its energy and how the neutrino behaves. New and more elaborate solar neutrino detectors have been built with the aim of understanding stars, including the Sun, as well as the physics and behavior of the elusive neutrino.