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84 phenomena. The instruments required for the study of this previously inaccessible domain are now technically feasible, and specific missions incorporating the required capabilities have been proposed. Their implementation would assure major advances in our understanding of the solar atmosphere and would enhance the value of solar physics in the study of magnetohydrodynamics. m e most important of the new missions proposed for solar astronomy is the Advanced Solar Observatory, which should be devel- oped stepwise in a succession of Shuttle payloads that will ultimately be combined in a long-lived free-flying facility such as a Scientific Space Platform. IX. NEUTRINO ASTRONOMY Every star is a prolific source of low-energy (1- to 15-MeV) neutrinos that are produced as by-products of the nuclear reactions in the star's core and pass freely through its outer layers into space. Every stellar collapse leading to the formation of a neutron star is a source of a burst of intermediate energy (10- to 50-MeV) neutrinos. Collisions between high-energy cosmic-ray nuclei and matter produce high-energy (greater than 50-MeV) neutrinos. And finally, the theory of the big bang implies that we are now immersed in a sea of relic neutrinos with a kinetic temperature of 2 K and a density of 500 cm~3. Thus cosmic neutrinos carry information about a wide variety of high-energy processes. mis information, however, is exceedingly difficult to tap for the very reason that allows neutrinos to escape freely from sources deep inside of stars--their interaction cross section is extremely small at all energies. To obtain a significant rate of detectable interactions a detector must have a very great number of target nuclei in its sensitive volume, which means that it So far, the only positive detection cosmic sources has been of low-energy neutrinos produced in the nearest star, the Sun. Efforts are under way, or under consideration, to detect cosmic neutrinos in the intermediate- and high-energy regimes. At present there seems to be no hope of detecting the relic neutrinos directly even though, according to recent theoretical speculation, they may be the dominant form of mass-energy in the Universe and may comprise the so-called missing mass whose presence is deduced from dynamical analyses of the motions in galaxies and clusters of galaxies. must be very heavy. Of neutrinos from

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85 Progress and prospects for neutrino astronomy in each of the three regimes in which direct detection is or may be possible will be discussed separately. A. Low-Energy Neutrinos 1. Introduction The theory of nuclear reactions as they are believed to occur in standard main-sequence stars has been used to predict the flux at Earth of electron neutrinos produced in the Sun by the processes that result in the fusion of hydrogen into helium. The Sun, being by far the closest star, is by far the brightest source of stellar neutrinos, just as it is by far the brightest source of stellar light. Thus, a crucial test of the theory of stellar energy is provided by a comparison between the predicted and the observed fluxes of solar neutrinos. 2. Progress during the 1970's The history of solar neutrino astronomy is centered on a remarkable radiochemical experiment that employs a detector containing 105 gallons of chlorinated hydro- carbon and operating 1 mile underground in the Homestak gold mine in South Dakota. Neutrinos with energies of several MeV interact with 37C1 to produce radioactive ~ 7 _ . _ e O'er, which undergoes beta decay with a half-life of 34 days. In the experiment, argon gas is extracted from the detector tank every few days and transferred to a tiny low-background proportional counter in which the decays of 37Ar are counted. The measured rate of decay is only about 1.5 per day. Data accumulated from 1970 through 1979 imply that the total flux of solar neutrinos is 2.2 + 0.4 SNUS (1 solar neutrino unit = 10 36 events per 37C1 atoms per see). The current standard theoretical model for the Sun predicts a counting rate of 7.5 SNUS, with a considerable uncertainty due to uncertainties in the nuclear cross sections, the optical opacities, and the conditions in the Sun's core. Nevertheless, to bring the predicted value below 3 SNUS may require nonstandard assumptions such as an inhomgeneous composition of the Sun, repeated deep mixing, a black hole at the center of the Sun, or perhaps even neutrino decay as implied by

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86 recent developments in the Grand Unified meory of the weak, electromagnetic, and strong interactions. 3. Scientific Goals: Present and Future Programs The 37C1 experiment is sensitive primarily to the high- energy "fringe" neutrinos from a relatively unimportant side reaction not to the "pp" neutrinos from the reaction that is believed to yield most of the solar power. Since the flux of fringe neutrinos is extremely sensitive to conditions in the solar core, the 37C1 experiment can never be a conclusive test of the theory of solar power. Therefore it is important to carry out an experiment that tests the predictions for the main-line pp flux. Measure- ment of the energy spectrum of the solar neutrinos will then become the next major objective. m e most promising approach to the measurement of the pp neutrinos is the so-called ~gallium" experiment, which, like the 37C1 experiment, is a radiochemical experiment with 71Ga being the target nucleus and 71Ge being the separable radioactive product nucleus. A prototype detec tor module has been developed at the Brookhaven National Laboratory with 1.4 tons of gallium in an international collaboration involving the Max Planck Institute in Germany, the Weizmann Institute in Israel, the Institute for Advanced Study at Princeton, and the University of Pennsylvania. m e full-scale experiment will require 50 tons of gallium in 35 detectors like the present proto- type. We recommend that it be initiated as soon as possible. Looking further ahead, several other radiochemical experiments have been proposed. For example, one uti- lizing 1l5In measures the pp neutrinos and could provide significant energy resolution. Experimental investiga- tions to determine the feasibility of these alternative approaches should be encouraged. Certain difficult laboratory measurements in nuclear physics are of crucial importance to the calculation of accurate theoretical estimates of solar neutrino fluxes. This is particularly well demonstrated by the impact on these estimates of recent results on the 3He + 4He reac- tion derived from ultra-low-energy experiments. These difficult measurements should be continued with adequate support. -

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87 4. Research in Other Countries The 71Ga experiment currently involves an international collaboration with major contributions from West Germany. Soviet scientists are currently planning a 37C1 detector that is four times larger than the Homestake installation, and they are also contemplating a 71Ga experiment. The feasibility of the 1l5In experiment is also being studied in Japan. B. Intermediate-Energy Neutrinos 1. Introduction The most easily detectable contributions to the neutrino flux in the intermediate-energy range from 10 to 50 MeV are expected to come from gravitational collapses of stars in our Galaxy, which occur with an estimated frequency of one per 10 to 30 years. Although many details of collapse processes are uncertain, there is general agreement on several theoretical conclusions. One is that in the formation of a neutron star the bulk of the approximately 1053 ergs of binding energy is radiated away as neu- trinos. At least 1052 ergs of energy is radiated in electron neutrinos from the process of neutronization. The rest is radiated by thermal processes. The time scale for emission of the neutrinos is dependent on the equation of state of the collapsing matter because neutrinos coming from regions with densities greater than 1011 g/cm3 (the neutrino "photosphere") must diffuse outward by multiple scattering. Measurement of the n light curve" of the neutrinos from a collapse may be the only possible way to obtain direct observational information about the equation of state of hot, dense collapsing matter. 2. Inventory of Present Resources Work in the intermediate-energy regime is proceeding abroad in coincidence with U.S. efforts. Detectors are currently in operation in the Homestake gold mine, in Baksan in the Soviet Union, and in the Ukraine. All are Cerenkov detectors. The Homestake and Baksan detectors have sensitive masses of the order of 300 tons each and should be able to detect a collapse event anywhere in our Galaxy. However, a positive result would probably be

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88 believable only if both detectors are in operation and both record the same event in coincidence. Additional coincidences with signals from smaller detectors would be possible if a collapse event were closer than the Galactic center. Experiments aimed at the detection of proton decays and now under construction will be sensitive to the higher energy neutrinos from collapse events in our Galaxy. In principle these neutrino detectors should provide an early warning system for any collapse event in our Galaxy since the neutrino burst will precede the optical light peak and the x- and/or gamma-ray bursts from the supernova by about 1 week. However, lack of accurate directional information will limit the usefulness of the early warning in preparing for optical and high-energy photon observations. Unfortunately, the detection of the relatively frequent supernovae in galaxies of the Virgo cluster is not within the range of practicality at present. 3. Scientific Goals and Future Programs Existing facilities capable of detecting neutrinos from stellar collapse events in our Galaxy should be adequately maintained and operated for many years. Meanwhile, theo- retical investigations that provide links between astro- physics and particle physics should be encouraged. Theoretical research is needed on the equation of state of matter at ultra-high densities, on supernova mechan- isms, on nucleosynthesis, and on the spectra of neutrinos produced in various astrophysical processes. Related work is required on the theory of neutrino cooling in newly formed neutron stars, an area closely related to x-ray observations of the stellar remnants of supernovae. C. High-Energy Neutrinos 1. Introduction Collisions of high-energy nuclei with matter produce mesons of which the ultimate decay products are elec- trons, gamma-ray photons, and neutrinos. Since high- energy neutrinos are vastly more difficult to detect than high-energy gamma-ray photons, a source of high-energy neutrinos that would be detectable by currently proposed

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89 methods would almost certainly already have been observed as a source of high-energy gamma rays that is much more intense than any known gamma-ray source. An exception would occur if an extremely powerful source of high-energy particles were shielded from Earth by matter or radiation close to the source and so thick that the flux of gamma rays produced in interactions is attenuated to a value below the level of detectability. It has been suggested that such a situation may occur in the vicinity of a pulsar, where magnetically trapped high energy particles strike the surface of the star, or near the core of an active galactic nucleus. It should be noted that a dis- crete source of high-energy gamma rays is not necessarily a neutrino source because high-energy gamma rays can be produced without neutrinos by interactions of high-energy electrons with matter, photons, or magnetic fields. Thus the measurement of the value or upper limit on the neu- trino flux from a bright source of high-energy gamma rays could, in principle, constrain theoretical models of a gamma-ray source and decide the question as to whether the mechanism of gamma-ray production is nuclear col- lisions or electron interactions. 2. Present and Future Programs All experiments aimed at the detection of high-energy cos- mic neutrinos have so far detected only events attribut- able to the background flux of neutrinos produced by collisions of cosmic rays in the atmosphere. Upper limits on the diffuse cosmic neutrino flux have been obtained with detectors located deep underground in gold mines in India, South Africa, and the United States. These limits place significant constraints on the amount of deuterium that was produced after the initial element synthesis in the big bang. In the near future a large Cerenkov detec- tor will be completed for the purpose of detecting proton decays. It will also be sensitive to high-energy neu- trinos, and a by-product of its operation will be improved upper limits on the cosmic neutrino flux. None of these detectors is sensitive enough to detect any cosmic source above the background of atmospheric neutrinos except a discrete source or diffuse background of totally unexpected brightness. Theoretical and experimental studies by the DUMAND (Deep Underwater Muon and Neutrino Detection) group have been carried out to determine the technical feasibility