Neutrino astronomy

Neutrinos—nearly massless particles with no charge—are generated in nuclear reactions, such as those deep in the interior of the Sun and other stars or stellar explosions. Neutrinos do not readily interact with matter, which makes their detection difficult, but their detection is important because it provides a way to probe basic energy-generation processes in stars and stellar explosions.

Overview

Neutrinos are uncharged, massless, or nearly massless particles, first proposed in 1930 by physicist Wolfgang Pauli to account for the apparent missing energy in beta decay. Beta decay is the emission of an electron from the nucleus of some radioactive elements. To balance energy but manage to go unobserved in the experiments that had been conducted, Pauli’s hypothetical neutrino had to interact only very weakly with ordinary matter. For more than thirty-five years, particle physicists were unable to confirm the existence of Pauli’s elusive particle experimentally.

Efforts to detect the neutrino rest on observing one of the consequences of “inverse beta decay,” a process in which a neutrino collides with a proton and they are transformed into a neutron plus a positron. Neutrino detectors can function by using a photomultiplier to detect the Cherenkov light radiation emitted by the relativistic positron or by using scintillation counters to detect gamma rays given off by the annihilation of the positron when it encounters an electron. Crude estimates of the likelihood of inverse beta decay reactions suggest that a single neutrino would have to pass through a solid wall many billions of kilometers thick to have, on average, just one such interaction. These estimates indicate that neutrino detectors must be very large to detect a single neutrino interaction event.

In 1956, physicists Clyde L. Cowan Jr. and Frederick Reines, working at the Los Alamos National Laboratory, built the world’s largest scintillation counter, a device to detect the small flash of light given off by the interaction of a neutrino in the apparatus. When placed adjacent to the Savannah River Nuclear Reactor, a high-power reactor that produces about 10¹⁸ neutrinos per second, their apparatus detected only one neutrino interaction every twenty minutes. Nevertheless, Cowan and Reines were able to confirm the existence of Pauli’s elusive particle.

Thermonuclear reactions occurring in the core of the Sun also produce neutrinos. High temperatures and pressures in the Sun’s core permit nuclear reactions in which two hydrogen nuclei combine to form a deuterium nucleus, a positron, and a neutrino. This process, the proton-proton or "p-p" reaction, also releases considerable energy. The resulting deuterium nuclei can then react to form helium and again release energy, causing the core temperature to rise. This process allows the helium nuclei to combine to form an even heavier nucleus. Two of these reactions are essential to producing neutrinos inside the Sun. In the “beryllium-electron capture” reaction, a beryllium-7 nucleus combines with an electron to form lithium-7 and a neutrino. Eventually, this lithium-7 will react with a proton to produce boron-8. The boron-8 can decay into two helium nuclei, a positron and a neutrino.

Eventually, fusion reactions in a star will terminate when the core has been converted into iron since reactions that produce even heavier elements from iron all use energy rather than releasing it. At this stage, if the mass of the star is large enough, the core collapses under its own weight, converting protons into neutrons and emitting a burst of neutrinos. The result is a supernova explosion, accompanied by an intense but short burst of neutrino emission and the formation of a neutron star. Efforts to model the supernova explosion process indicate that, although a bright flash of visible light can be observed for months after the explosion, about ninety-nine percent of the energy is carried away by the neutrinos.

Neutrinos produced in the core of the Sun, because of their low probability of interaction, can pass entirely through the Sun and reach Earth unimpeded. Their number and energies carry information about temperature, pressure, and nuclear reaction processes in the core.

There are three major neutrino-producing reactions in the Sun: the p-p reaction, which produces neutrinos with a continuous range of energies up to about 420,000 electron volts (eV); the beryllium-electron capture reaction, which produces neutrinos with a single energy of 862,000 eV (this reaction produces about thirteen times fewer neutrinos than the p-p reaction); and the boron-decay reaction, which produces neutrinos with a continuous energy distribution up to about fifteen million eV (this reaction produces about one ten-thousandth as many neutrinos as the p-p reaction). Although the boron-decay reaction produces far fewer neutrinos than either of the other two reactions, the high energy of these decay neutrinos makes them much easier to detect on Earth.

Efforts to develop an apparatus for neutrino astronomy have concentrated on detecting the continuous flux of neutrinos emitted by nuclear reactions in the Sun and short bursts from supernova events. Theoretical models of both processes have predicted the number and energies of the neutrinos to be expected from both sources.

At the same time that Cowan and Reines were attempting to detect the neutrino, a chemist at Brookhaven National Laboratory, Raymond Davis Jr., began experiments that eventually led to the detection of neutrinos from the Sun. Rather than employing an electronic detector, which counted individual events as they occurred, Davis used an idea, originally suggested in 1948 by Bruno Pontecorvo, for a chemical detector that could accumulate the chemical by-product of neutrino reactions over several months. Pontecorvo’s idea rested on the fact that a neutrino interacting with a neutron in an atom of chlorine-37 transforms that nucleus into a nucleus of argon-37. It was hoped that argon, being a chemically inert gas, could be extracted easily from large volumes of chlorine-rich liquid. Since argon-37 is radioactive, it could be identified easily by its decay following extraction. Between 1954 and 1956, Davis tested a version of this detector, containing one thousand gallons of chlorine-rich carbon tetrachloride, located near a nuclear reactor at the Brookhaven National Laboratory. In the scientific paper describing the results, Davis suggested that this detector could be scaled up in size to permit the detection of solar neutrinos. The chlorine-37 reaction, however, requires energetic neutrinos, so it is sensitive only to the high-energy neutrinos produced by the boron-decay reaction in the Sun.

After several pilot projects demonstrating the effectiveness of the radiochemical techniques for neutrino detection, Davis constructed a solar neutrino observatory buried about two kilometers below the surface in the Homestake Gold Mine in Lead, South Dakota. The neutrino detector consisted of a large cylindrical tank, six meters in diameter and fifteen meters long, containing 100,000 gallons of perchloroethylene (containing 520 tons of chlorine). 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. The detector began operation in May 1967 and was allowed to sit passively accumulating argon atoms from neutrino interactions for one to three months. Then, Davis would extract the argon produced during the accumulation period and determine the argon-37 abundance by counting its decay. Davis succeeded in observing neutrinos from the boron-decay reaction in the Sun using this apparatus. His detector, however, provided neither directional information nor time resolution.

To sort out the signals from a supernova would require a directional instrument with real-time response. For these efforts, astrophysicists returned to electronic detectors traditionally employed in particle physics but greatly enlarged because of the low probability of neutrino interactions. These detectors consist of large tanks of water serving as targets for neutrino interactions by inverse beta decay. The water tanks are surrounded by photomultiplier detectors that see the tiny flash of Cherenkov radiation emitted by each relativistic positron as it traverses the water. A consortium of physicists from the University of California at Irvine, the University of Michigan, and Brookhaven National Laboratory (IMB) constructed a seven thousand–ton water tank detector in a mine just outside Cleveland, Ohio. The IMB detector, completed in 1981, operated for ten years. A second such detector, the Kamiokande II detector, containing three thousand tons of water, was operated in the 1980s by Japanese physicists of the Kamioka Observatory in the Kamioka Mine, about three hundred kilometers west of Tokyo. Both of these detectors recorded neutrino emissions from Supernova 1987A, providing information on neutrinos' energy distribution and flux for comparison with theoretical models of the explosion process.

Neither the chlorine detector operated by Davis nor the IMB or Kamiokande II water and Cherenkov detector were sensitive to the lowest-energy neutrinos from the p-p reaction. To detect these low-energy neutrinos, astrophysicists employed the chemical detection technique. The target was at first gallium since gallium-71 can interact with a neutrino to produce germanium-71 and an electron. The germanium can be separated from the gallium, providing a measure of the neutrino flux. A neutrino detector consisting of twenty tons of pure gallium was commissioned at the Baksan Neutrino Observatory (founded in 1977) in the Caucasus mountains of Russia in early 1990. Like Davis’s chlorine detector, this detector, as well as a similar one in Gran Sasso, Italy, provided no information on the time or direction of the neutrino. Nevertheless, its sensitivity to low-energy neutrinos permitted the detection of the neutrinos produced by the p-p reaction in the Sun. Given the exorbitant cost of gallium as a reaction mass, larger experiments would turn to cheaper chemical options later.

Applications

Astrophysical models of the Sun prominent in the 1950s and 1960s led Davis to suspect that the neutrino detector would record many monthly events. To describe his results, Davis defined a new unit, the solar neutrino unit (SNU), with one SNU corresponding to producing a single atom of argon-37 in his apparatus every six days. Theoretical calculations in 1963 by astrophysicist John N. Bahcall, using the then-current astrophysical models of the reactions occurring in the Sun's core and the neutrino's accepted physical properties, gave an expected capture rate of fifty SNUs.

Initial results from Davis’s detector, from a forty-day run in May and June 1967 and a 110-day run in June through October 1967, were startling. Davis placed an upper limit of three SNUs, almost one-twentieth the expected amount, on the solar neutrino flux. Still, his results were consistent with detecting no solar neutrinos at all. Initially, astrophysicists suspected that the experiment was flawed, but Davis undertook a series of calibration experiments to establish the efficiency of his detector. He concluded the experiment was not flawed. Bahcall repeated the theoretical calculations in 1969, using revised values for the nuclear reactions in the Sun. The new results lowered the expected flux to eight SNUs, but Bahcall could not explain the much lower results reported by Davis.

Davis improved his argon decay detector in 1970, increasing the experiment's sensitivity. This improvement allowed the first detection of solar neutrinos. Although it lacked directional sensitivity in the detector, the source could only be inferred because the Sun was expected to be the brightest continuous neutrino source in the sky. Davis’s measurements continued almost uninterrupted from 1967 to 1990, giving the same results over the entire period. The long-term average measured over twenty years was only 2.3 solar neutrino units, well below theoretical predictions. The low solar neutrino detection rate observed by Davis was confirmed by the Japanese Kamiokande II neutrino detector, which had begun operation in 1985. Since the Kamiokande II detector was electronic, providing real-time information on the neutrino’s direction, the Japanese physicists could confirm, as had been suspected, that most of the neutrinos detected were from the Sun.

Two general ways have been proposed to explain the large discrepancy between the flux of boron decay neutrinos from the Sun and the number expected from the astrophysical models. Either the physicists’ basic understanding of the neutrino and its interactions was wrong, or the astrophysical models of the interior of the Sun were incorrect. Physicists have suggested that the neutrino might have a small mass, which would alter some of the calculated reaction rates, or that the neutrinos emitted by the Sun might change into a second type of neutrino, not able to participate in the inverse beta-decay reactions on which the detectors rely. Astrophysicists, however, suggested that since the boron-decay reaction is a minor reaction in the core of the Sun, the real test of the solar model would come with the results from the gallium detectors, which are sensitive to the p-p reaction.

A resolution to the solar neutrino problem came from the Sudbury Neutrino Observatory (SNO) in Canada in 2001. That detector incorporated one thousand metric tons of heavy water (water where the hydrogen is replaced by deuterium) and ten thousand photomultiplier tubes in a nickel mine two kilometers underground. The Super-Kamiokonde neutrino detector array verified SNO results. A ramification of neutrinos having even a small mass value is that they can change flavor. Electron neutrinos produced in nuclear reactions deep in the Sun’s core could transform into muon and tau neutrinos before reaching Earth. Previous neutrino detectors were only capable of registering neutrinos of the electron flavor. Although SNO could detect the other flavors, it could not precisely determine the neutrino mass. The mass was estimated to be between 0.05 and 0.18 eV. Apart from solving the solar neutrino problem, another ramification of neutrinos having mass is that they may account for some, but not all, of the missing mass in cosmological models.

Neutrinos from supernovae were expected to be of much higher energy than those from the Sun and clustered very tightly in time. The first extrasolar neutrino event to be observed was Supernova 1987A, an explosion in the Large Magellanic Cloud. The IMB and Kamiokande detectors saw neutrino bursts, with eight neutrinos detected by IMB in a seven-second window and eleven detected by the Kamiokande II detector in a thirteen-second window. Based on the detector efficiency, 3×10¹⁶ neutrinos from Supernova 1987A passed through the IMB detector, resulting in eight interactions.

The number and energies of the neutrinos detected from Supernova 1987A were in general agreement with theoretical predictions. The Kamiokande II investigators indicated that the temperature of the explosion, as inferred from the neutrino energy distribution, was consistent with supernova models. Also, the time between the neutrino burst and the optical brightening was consistent with model estimates for the blue giant progenitor star. The Japanese investigators calculated that neutrinos carried off 8×10⁵² ergs of energy, much more than the 10⁴⁹ estimated to have been carried by electromagnetic radiation. This finding confirmed theoretical models showing that neutrinos carry away most of the gravitational energy liberated in the stellar collapse. If this data had been analyzed in real-time, it would have provided an early warning for the optical astronomers since the neutrino burst was observed three hours before the first optical sighting of the explosion.

Context

Neutrino astronomy has called into question basic ideas about how the Sun generates energy and has confirmed models of the supernova explosion process.

The discrepancy between the minimum number of solar neutrinos expected theoretically and the actual number detected by Davis has been called the solar neutrino problem. Explanations for this discrepancy between theory and experiment focused on two areas—the astrophysical models of the Sun were wrong, or the physicists’ understanding of the neutrino and its interactions was wrong. Modifications to the models for the Sun would require that the core's temperature or helium content be lower than modeled. On the other hand, the neutrinos emitted by the Sun may change type (from electron neutrinos to muon neutrinos) on their way from the Sun to Earth. Since the older detectors were sensitive only to the electron neutrinos, they would miss the muon neutrinos. The Sudbury Neutron Observatory and the Kamiokande neutrino detectors produced data indicating that this solar neutrino problem is resolved due to neutrino oscillation. Earlier detectors could only pick up the electron flavor, but electron-flavor neutrinos transform into the muon and tau flavors. When that oscillation was considered, the validity of solar models was verified. The solar neutrino problem was no longer an issue, as the observed neutrino flux matched theoretical models.

Observation of a neutrino burst from Supernova 1987A was a more successful application of the theory and was consistent with theories of stellar explosions. Measurements demonstrated that most of the energy liberated in the gravitational collapse of a supernova is carried off by neutrinos. The study of these particles is important to the understanding of the supernova process.

The success of the early experiments in neutrino astronomy spurred proposals for other, more advanced instruments. The Japanese group proposed a Super-Kamiokande detector containing 50,000 tons of water, able to register so many solar neutrinos from the boron-decay reaction that it could monitor the temperature of the solar core with an accuracy of one percent over weekly intervals. Also, the Super-Kamiokande detector was built to detect about four thousand neutrinos from each supernova occurring within the Milky Way, including events near the galactic center, which are shielded from optical observation because of the large amount of material along that direction. Such observations provide information on the dynamics of star collapse. Super-Kamiokande began operation in 1996. Then, in 2001, more than 6,600 of its photomultiplier tubes imploded in a chain reaction. Efforts to reengineer the design to preclude such an unfortunate accident were set in motion. By 2006, Super-Kamiokande was back in operation, named Super-Kamiokande-III (post-implosion data, taken before the detector was rebuilt, is referred to as Super-Kamiokande-II).

Other detectors have focused their efforts on observing the low-energy neutrinos from the Sun. Since the neutrinos are able to pass through the Sun unimpeded, they provide the only way to observe directly the nuclear processes occurring in its core.

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