neutrino astronomy


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neutrino astronomy,

study of stars by means of their emission of neutrinosneutrino
[Ital.,=little neutral (particle)], elementary particle with no electric charge and a very small mass emitted during the decay of certain other particles. The neutrino was first postulated in 1930 by Wolfgang Pauli in order to maintain the law of conservation of energy
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, fundamental particles that result from nuclear reactions and are emitted by stars along with light. Approximately 100 billion neutrinos have raced through your body since you began reading this article. The light received from a star is emitted by the surface layers, which in turn absorb the light coming from the interior. Neutrinos, on the other hand, are absorbed only very weakly by matter and, once created by nuclear reactions in the stellar core, pass directly through the outer parts of the star. Thus neutrinos permit astronomers to look directly into the energy-producing core of a star. Their weak tendency to interact with matter also makes them very difficult to detect.

Neutrino "observatories" are located in deep mines or other subsurface locations where hundreds of feet of material shield out the cosmic rays that would completely swamp the tiny effects due to neutrinos. The largest such observatory is in the ice at the South Pole, .87 mi (1.4 km) below the surface; the detector array of the IceCube South Pole Neutrino Dectector forms a cube with edges that are .6 mi (1 km) in length. Neutrinos pass as easily through the overlying material as they pass through the star, but react with the material in the detector. In one form of detector, they react with chlorine to produce a radioactive isotope of argon, which is detectable. Other detectors, such as the IceCube, use a large volume of clear water or ice; when a neutrino interacts with the water or ice to produce a leptonlepton
[Gr.,=light (i.e., lightweight)], class of elementary particles that includes the electron and its antiparticle, the muon and its antiparticle, the tau and its antiparticle, and the neutrino and antineutrino associated with each of these particles.
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 that is moving faster than the speed of light in the water or ice detectable Cherenkov radiationCherenkov radiation
or Cerenkov radiation
[for P. A. Cherenkov], light emitted by a transparent medium when charged particles pass through it at a speed greater than the speed of light in the medium.
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 is emitted.

Because of its proximity, the sun was expected to be by far the most intense source of neutrinos and was the initial object of study. However, several neutrino detectors observed a rush of neutrinos from Supernova 1987A in a nearby galaxy called the Large Magellanic Cloud. Although their journey from the exploding star began at the moment its core collapsed, they did not move quickly at first since the gravity of the core was so strong. When the shock wave from the explosion reached the neutrinos, it freed them to travel between galaxies, and they arrived on earth about three hours before the first visible light of the explosion appeared.

neutrino astronomy

The study of neutrinos produced in various processes, including energy-producing nuclear reactions in the centers of stars, reactions occurring during supernova explosions, and cosmic-ray collisions with matter. The detection of neutrinos from Supernova 1987A confirmed the basic ideas of stellar collapse. Neutrino interaction with other particles of matter is highly improbable, so the great majority escape from their source and travel unhindered through space and any intervening matter. They can thus provide direct information on the processes in which they were created, but their weak interaction with matter makes them difficult to detect.

Solar neutrinos are released by the proton-proton chain, an energy-producing reaction in the Sun's core. The first experiment to detect solar neutrinos was set up by Raymond Davis in 1967 and is still in progress. It is conducted deep underground in the Homestake Gold Mine near Lead, South Dakota. A large quantity of dry-cleaning fluid is exposed for several months to the flux of solar neutrinos. A very small (but calculable) number of neutrinos react with the chlorine-37 atoms in the fluid to produce radioactive argon-37; the amount produced is a measure of the neutrino flux. The average flux detected is 30±14% of that predicted by the standard solar model.

The flux is measured in solar neutrino units (SNU). One SNU equals one neutrino-induced event per 1036 target atoms in a detector. It is calculated from the expected conditions in the solar core and is very temperature dependent. The most widely accepted model predicts a neutrino flux of about 135 SNU. The expected neutrinos in the Homestake detector arise from one of the rarer pathways in the proton-proton chain reaction; they are emitted with high energy, mainly during the decay of boron-8. Another experiment designed to detect these high-energy neutrinos has been conducted since 1987 in the Kamioka mine in Japan. The Kamiokande detector measures the scattering of neutrinos by electrons in ultrapure water. It has measured 46±18% of the predicted flux.

Most solar neutrinos are produced in the initial step of the proton-proton chain, at energies too low to be detected by the Homestake and Kamiokande experiments; the production rate can, however, be more accurately predicted. Since the early 1990s, two international teams have been using large gallium detectors that are sensitive to the low-energy particles: neutrinos reacting with gallium–71 produce germanium–71 plus electrons. SAGE (Soviet–American gallium experiment) is located underground in the Caucasus in Russia. The European–Israeli–American Gallex experiment is in the underground Gran Sasso Laboratory near Rome. The Gallex average flux over two series of observations is 87±16 SNU. The first measurements at SAGE found only 20 SNU but a later run measured 85 SNU, and this has produced an overall figure of 58±14 SNU. The Gallex and SAGE measurements thus agree with each other, within the error limits, giving a combined average of about 60% of the predicted flux.

The discrepancy between the observed and predicted flux of solar neutrinos has not yet been explained. It could imply that the conditions in the Sun's interior and/or the nuclear reactions taking place there are not fully appreciated. Alternatively it could be that the characteristics of the neutrino itself are not fully appreciated and that its possible possession of rest mass could provide an explanation.

Neutrino Astronomy

 

a new branch of observational astronomy that deals with the search for and study of neutrino fluxes from extraterrestrial sources. The neutrino is the only form of radiation that reaches a terrestrial observer from the interior of the sun or a star; the neutrino thus carries information about the internal structure of the sun or star and the processes occurring in it. Present methods of detecting neutrinos permit the observation of neutrino radiation only from the sun and supernovae in our galaxy.

Solar neutrino astronomy. The existence of an intense stream of neutrinos from the sun follows from the modern concept of the sun’s origin and structure; according to this concept, the sun’s luminosity is due entirely to energy from the thermonuclear transformation of hydrogen into helium in the core of the sun. As solar-model computations show (seeSTELLAR MODELS), the principal contribution to energy generation comes from the hydrogen cycle (proton-proton chain), while the contribution provided by the carbon-nitrogen cycle is less than 1 percent. The synthesis of each 4He atom is accompanied by the emission of two electronic neutrinos ve and the total neutrino flux as determined from the sun’s luminosity is about 6.5×1010 neutrinos/(cm2.sec) at the earth’s surface, with the neutrinos carrying away about 3 percent of the energy of the thermonuclear fusion.

The observation of solar neutrinos would be a convincing confirmation of basic ideas about the sun’s thermonuclear evolution. The measurement of neutrino fluxes from the various reactions, using an appropriate set of detectors, constitutes a complete program for studying the internal structure of the sun. Since the solar-neutrino flux undergoes seasonal variations with an amplitude of about 7 percent (which is connected with the eccentricity of the earth’s orbit), observation of these variations would serve as proof that the detected neutrinos were indeed solar neutrinos. Another method of determining the incident direction of the neutrinos consists in measuring the angular distribution of the electrons formed during the capture of the neutrinos in a detector (see below): because of the nonconservation of parity in beta decay, these electrons should be emitted primarily in the direction of the sun.

The first experiments on the observation of solar neutrinos were performed in 1967–68 by the American scientist R. Davis and co-workers with a radiochemical neutrino detector containing 610 tons of liquid perchloroethylene (C2Cl4). The detector was set up underground at a depth of 1,480 m in order to block out the cosmic-ray background. The detection of the neutrinos was based on a method proposed in 1946 by B. M. Pontecorvo. Solar neutrinos with energies greater than 0.814 megaelectron volt (MeV) from radioactive 37 Ar with a half-life of 35 days in the reaction 37C1 + ve → e- + 37 Ar. According to calculations, the principal contribution (76 percent) to the effect must be due to neutrinos of the highest energy (up to 14 MeV) from the decay 8B → 8Be + e+ + ve in the rarest branch of the hydrogen cycle. The flux of these neutrinos varies with the temperature T asT20; thus the chlorine neutrino detector is a unique “thermometer” for measuring the temperature Tc in the sun’s core. Theory has predicted a value Tc ≈ 15×106°K.

In Davis’ experiments, the 37Ar was accumulated in the detector for 100 days, then extracted by bubbling helium through the liquid, adsorbed on activated charcoal at a temperature of 77°K, and placed in a proportional counter, which recorded the number of disintegrations of 37Ar atoms. Measurements taken in 1972, as well as the first measurements in 1967–68, showed that the neutrino effect observed was several times smaller than that predicted by theory and did not exceed the background count of the detector—in the detector, not more than eight atoms of 37Ar were accumulated per experiment instead of the expected 45.

Although solar neutrinos were not detected with certainty, the results of these experiments are an important achievement of neutrino astronomy, since they show that contemporary ideas about solar neutrinos are somehow incorrect. The solution of the solar-neutrino mystery can be sought in three ways. First, it is possible that Tc is less than the theoretical values predicted by standard models of the sun and is about 13×106 °K; that is, the value lies below the sensitivity threshold of the “neutrino thermometer.” This means that the structure of the sun is different from what had been assumed up to now. Second, it may turn out that incorrect values were used for nuclear reaction rates in the calculations of the models; this would imply that the scale of the “neutrino thermometer” was incorrectly calibrated. Third, the “neutrino thermometer” may prove to be entirely “faulty” if something happens to the neutrinos on their way to the earth; for example, the neutrinos may decay if they turn out to be unstable particles, or they may oscillate and be transformed into a state in which they do not interact with chlorine. In order to solve the problem conclusively, it is necessary to increase the sensitivity of the chlorine neutrino detector and conduct additional experiments with detectors sensitive to lower-energy neutrinos, for example, 7Li, 71Ga, 87Rb, and 55Mn.

Another important problem of neutrino astronomy is the observation of solar neutrinos from the reaction2H + p + e-2H + ve (using 37C1 and 7Li detectors), which necessarily accompanies the hydrogen cycle. The detection of these neutrinos would prove the occurrence of the hydrogen cycle on the sun and would lead to the rejection of hypotheses about anomalous properties of neutrinos, thereby confirming the correctness of the conclusion that the carbon-nitrogen cycle does not make a significant contribution to the sun’s energy generation. (If the carbon-nitrogen cycle made the principal contribution, about 300 37Ar atoms would be formed in Davis’ detector.)

Neutrino bursts. Neutrino fluxes from other “quiet” stars, even those that are very near, are extremely small and cannot be detected by contemporary methods. At the same time, the observation of neutrino bursts from a star at the moment of the gravitational collapse of the star is completely feasible. The most likely objects for observation are supernovae in our own galaxy, in which the core collapses just before the burst occurs. A neutrino burst can be detected even when the supernova cannot be observed optically. The duration of such a burst is about 0.01 sec, and the neutrino flux at the earth is 1010-1012 neutrinos/cm2 for each burst. By measuring the delay times of the onset of the burst at detectors at different places on the globe, we can establish the incident direction of the neutrino radiation. The bursts can be detected as a characteristic series of pulses by using several hundred tons of a hydrogen-containing scintillator. Such experiments are planned in the USSR and the USA.

Neutrino astrophysics. The need for studying astrophysical phenomena in which neutrinos participate has created a new branch of astrophysics—neutrino astrophysics. According to current thinking, neutrino emission, which increases rapidly with temperature, has a decisive influence on the final stages of the evolution of stars, when the temperature of the star’s interior reaches 109 °K and higher. This situation is associated with the emission of neutrinos from the hottest innermost regions of a star (since the free length of neutrinos in matter is much greater than a stellar diameter); therefore, the neutrino emission determines the rate of energy loss in these stars. An example is the effect of the hypothetical electron-neutrino interaction predicted by the universal theory of weak interactions (see) on the evolution of the cores of planetary nebulae; by taking this effect into account, we can bring the observed data on the time of evolution into closer agreement with theoretical computations; the improved agreement, in turn, is an argument in favor of this interaction.

When the temperature at the center of a star reaches a value of about 1011 °K, the free length of an electronic neutrino becomes comparable with a stellar diameter; with a further increase in temperature, the star becomes opaque to neutrinos. However, since the neutrino free lengths remain much greater than the photon free lengths, energy in the star is transported by the neutrino gas (neutrino heat conduction), and the loss of energy continues to be determined by the neutrino emission. At temperatures greater than about 2×1011 °K, stars also become opaque to muonic neutrinos vμ. These stages in the life of a star are most puzzling and interesting. It is assumed that neutrino emission plays a decisive role in the mechanism of supernova outbursts.

The development of neutrino astronomy and neutrino astrophysics promises to provide valuable information not only about the structure of celestial bodies but also about the nature of the neutrino itself and the properties of weak interactions.

REFERENCES

Neitrino: Sb. st. Moscow, 1970. (“Contemporary Problems in Physics”.) (Translated from English.)
Bahcall, J. N. “Solnechnye neitrino.” Uspekhi fizicheskikh nauk, 1970, vol. 101, no. 4, pp. 739–53.
Asimov, I. Neitrino—prizrachnaia chastitsa atoma. Moscow, 1969. Pages 92–105. (Translated from English.)

G. T. ZATSEPIN and IU. S. KOPYSOV

neutrino astronomy

[nü¦trē·nō ə′strän·ə·me]
(astronomy)
The observation of neutrinos from the sun and from extrasolar astronomical sources.
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