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18 Recommenciations SPACEBORNE EXPERIMENTS We concur with the recommendations of the Astronomy Survey Committee for two moderate programs that are pertinent to spaceborne studies of cosmic rays (see G. B. Field, chairman, Astronomy and Astro- physics for the 1980's, Volume 1, National Academy Press, Washing- ton, D.C., 19821. First, they recommended "an immediate and substan- tial augmentation to the NASA Explorer satellite program,', and they went on to note that "among the scientific areas that at present appear to offer special promise for additional Explorer-class missions are the following, . . . A study of the isotopic and elemental composition of low-energy Galactic cosmic rays and solar energetic particles in the inter- planetary medium." A1SO7 that report said, "The Astronomy Survey Committee recommends a series of cosmic-ray experiments in space, to promote the study of solar and stellar activity, the interstellar medium, the origin of the elements, and violent solar and cosmic processes." The report of the Cosmic-Ray Program Working Group (National Aeronautics and Space Administration, 1982) and the supplement to that report (National Aeronautics and Space Administration, 1985) outline a program that will achieve these objectives of the Astronomy Survey Committee and will take advantage of the opportunities de- scribed in Chapter 17 in the section on Spaceborne Experiments. We recommend implementation of this program as summarized below. 157

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158 COSMIC RA YS This recommended program includes two major new programs: (1) development of a Superconducting Magnetic Spectrometer Facility for the Space Station, which will permit "a series of cosmic-ray experi- ments" as suggested by the second Astronomy Survey Committee recommendation above, and (2) a Cosmic-Ray Composition Explorer that is essentially the Explorer described in the first Astronomy Survey Committee recommendation above. This program also includes other recommendations that are important for the vitality of cosmic-ray research. We note that there are a few active research groups in other countries carrying on balloonborne and spaceflight cosmic-ray exper- iments. In the past there has been international cooperation, with complementary experiments from different countries on the same spacecraft or cooperative international development of a single exper- iment. We would expect this cooperation to continue in the future, particularly with the development of the Superconducting Magnetic Spectrometer Facility. Major New Programs As our highest priority, we recommend the development of a Superconducting Magnetic Spectrometer Facility for the Space Station capable of conducting a wide variety of measurements on the energetic galactic particles above 1 GeV. The heart of the facility would be a superconducting magnet and trajectory-defining detectors that would have a maximum detectable rigidity of several thousand GV. Above and below the magnet would be a variety of Cerenkov counters and energy-loss detectors, with the individual ancillary detectors being changed from time to time in order to optimize the detector configu- ration for various scientific objectives. This magnet facility would permit a series of significant cosmic-ray observations. A search for antinuclei heavier than antiprotons would be possible with the unprecedented sensitivity of 10-~; the detection of even a small flux of heavy antinuclei would have a profound influence on cosmology. The spectrum of antiprotons would be measured up to about 1000 GeV, giving important information about cosmic-ray con- finement in the Galaxy and conceivably displaying the signature of exotic processes such as the annihilation of photinos. A significant contribution of this facility would be measurement of isotopic compo- sition with excellent statistics and mass resolution over an energy range previously inaccessible to isotope resolution; these measure- ments would provide important signatures of the nucleosynthesis of

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RECOMMENDA TIONS 159 cosmic rays and other matter and would give us radioactive clocks at high Lorentz factor for probing time scales of cosmic-ray acceleration and galactic confinement. The facility would permit measurements of electron and positron spectra to about 1000 GeV, providing unique clues concerning the distribution in the galaxy of sites of cosmic-ray acceleration. The excellent momentum resolution of the magnet facility would make possible the measurement of energy spectra of cosmic-ray nuclei over a very wide energy region, from a few GeV/amu to several hundred GeV/amu with unprecedented resolution, making possible a sensitive search for spectral or temporal changes that could carry the signature of individual sources of cosmic rays. Also as a high priority we recommend an Explorer-class mission on a spacecraft outside the magnetosphere to carry high-resolution exper- iments to resolve the individual isotopes and elements of galactic cosmic rays, solar energetic particles, and anomalous cosmic rays in the energy region below 1 GeV/amu. Using established techniques, these experiments would have sufficient mass resolution and collecting power to determine the detailed isotopic composition and the energy spectra of all elements up through atomic number 30, with exploratory measurements of heavier nuclei. This Explorer mission would provide a detailed comparison of the elemental and isotopic structure of solar matter (from solar energetic particles), local interstellar matter (which is believed to be the source of the anomalous cosmic rays), and more distant galactic matter (which is the source of the galactic cosmic rays), thereby adding new dimen- sions to studies of the nucleosynthesis and subsequent evolution of both galactic and solar-system matter. In addition it would allow particle injection and acceleration processes to be studied on scales ranging from in situ observations of interplanetary shock acceleration, to flare acceleration on the Sun, to cosmic-ray acceleration in the galaxy. Continuing Programs An essential prerequisite for the major new programs described above is the availability of frequent, relatively low-cost opportunities for exposing new instruments to space. High-altitude balloons have provided these opportunities for many years and are likely to continue to be the best way to test new detector concepts, make modest scientific advances, and educate graduate students. Similarly, if low- cost, relatively fast turn-around opportunities can be developed for attached instruments on the Space Shuttle, those will also prove valuable.

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160 COSMIC RA YS Continued tracking of the Pioneer and Voyager spacecraft and near-earth IMP-8 will provide otherwise unattainable information about the modulation of cosmic rays in the heliosphere. The cosmic- ray experiment on Ulysses (formerly called the international Solar Polar Mission) and the cosmic-ray experiment that has been selected for the WIND spacecraft in the International Solar Terrestrial Program will be valuable additions to this network and will make valuable advances in our knowledge of cosmic-ray isotopes. The Cosmic Ray Nuclei Experiment, which was successfully flown on Spacelab-2 in August 1985, made important measurements of cosmic-ray composition to a few TeV. Its upper energy is principally limited by the low statistics imposed by its short (less than a week) exposure. We endorse the NASA decision to fly this experiment again on another Spacelab flight, and we strongly recommend placing this instrument on the Space Station for at least a year. With such an extended exposure it will be able to measure directly the cosmic-ray composition at energies where ground-based observations suggest a change of composition. Such a change is expected from some models of cosmic-ray acceleration, and measurement of the composition at these energies is important for testing these models. We endorse the NASA decision to develop the Heavy Nuclei Collector, a very-large-area plastic-track detector to be launched in 1987 on the Long Duration Exposure Facility. This experiment will be capable of measuring actinide nuclei in the cosmic rays with high enough resolution and statistics to use these radioactive elements to measure the time scale since nucleosynthesis of the heavy cosmic rays. Interpretation of measurements of cosmic-ray composition depends critically on knowledge of partial cross sections for spallation of heavy nuclei in collision with the interstellar gas. A continued program of measurement of such cross sections using the Bevalac heavy-ion accelerator is essential. We recommend continued support of theoretical investigations re- lated to-particle astrophysics, including studies of shock acceleration and of the interrelated problems of injection-acceleration-confinement ~ . Or cosmic rays. Studies for the Future A number of important measurements have been proposed in addition to those for which we have given high-priority recommenda- tions above. Several of those deserve further study for possible im-

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RECOMMENDATIONS 161 plementation during the last few years of this century and the beginning of the next. The Space Station will make possible assembly in space of very large instruments. We can identify three such devices whose feasibility should be studied: a high-energy array capable of measuring cosmic rays to 10'6 eV, a large electronic detector capable of detecting hundreds of the rarest actinide nuclei and determining their energy spectra, and a spaceborne down-looking detector capable of observing the atmospheric scintillation from air showers of the highest-energy cosmic rays. A study should be made of sending a new advanced set of instru- mentation out of the heliosphere to measure a wide variety of interstel- lar parameters at distances of at least 100 AU. Polar-orbiting platforms are part of the plans for the Space Station. Planning for these polar platforms should take into account the value of high-inclination orbits for studies of cosmic rays at moderate energy. GROUND-BASED EXPERIMENTS Ground-based experiments are supported by the the National Sci- ence Foundation (NSF) and the Department of Energy (DOE) and consequently have not received attention by NASA panels and work- ing groups. Major NSF programs such as the Fly's Eye have been reviewed by the National Science Board as well as the normal referee procedures and the NSF physics advisory committee. The DOE- supported programs have been administered through the Division of High Energy Physics. In 1982-1983 the DOE convened an ad hoc advisory panel to advise it on experiments related to elementary- particle physics not using high-energy particle accelerators. All the DOE-supported programs have been reviewed by the Experimental Technical Assessment Panel (ETAP). The recommendations articu- lated here concur with the conclusions of ETAP and of the NSF advisory structure in every instance where the questions have been addressed. In response to increased activity in the field, DOE set up in late 1985 a standing High Energy Physics Advisory Panel (HEPAP) Subpanel on Non-Accelerator Particle Physics. Most of the ground-based cosmic-ray experiments involve a group of physicists, an equipment inventory, and a budget on the scale of a typical experiment in particle physics in the external beam of a particle accelerator; and many are carried out by high-energy physicists. The total U.S. effort in the ground-based cosmic-ray experiments is much less than 1 percent of the particle-physics budget.

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162 COSMIC RA YS This contrasts with programs directed toward similar physics ques- tions abroad. The Soviet Union and Japan spend relatively a much larger fraction of their effort here; and nations without high-energy particle accelerators, such as India, Australia, and Brazil, also have a relative commitment much greater than that of the United States. In spite of their modest scale, the U.S. programs remain internationally preeminent. It is appropriate that we continue to choose carefully the experimental efforts in this area and to support them vigorously. It may be noted that Japan, China, the Soviet Union, and South America have quite extensive programs involving emulsion chambers and calorime- ters at mountain observatories. In particular, the Soviet Union is building a very ambitious mountain-top experiment in Armenia the ANI. We do not recommend similar programs for the United States at this time. Gamma-Ray Astronomy The observation of gamma rays of energies above 10~' eV through ground-based observation of the Cerenkov light from air showers and direct detection of electrons from larger air showers provides the strongest evidence of discrete astronomical sources of acceleration processes extending beyond 10'5 eV. Careful measurement of the direction and time structure of such showers has revealed several such sources, and the promise of further significant discoveries is very high. In order to exploit this recently developed field an expanded effort in utilizing existing detectors and in building new detectors is occurring. We recommend programs in gamma-ray astronomy as our highest priority ground-based cosmic-ray observation. Highest-Energy Cosmic Rays and Extensive Air Showers The Fly's Eye Program is unique among experiments around the world for studying cascades of 10~8 eV and higher energies, and it serves as a focus for cosmic-ray research at the highest energies in the United States. We recommend also as a very high priority continued support of Fly's Eye and its improvements. We endorse studies of possible complementary surface detectors such as muon counters and scintillation-counter air-shower detectors.

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RECOMMENDATIONS 163 These could expand the value of the Fly's Eye observations and could lead to new approaches to this energy region in the future. High-Energy Neutrino Astronomy As with gamma rays, neutrinos can provide in principle a line-of- sight signal from energetic astronomical sources of cosmic rays, penetrating regions that might be opaque to all electromagnetic radia- tion. Unfortunately, this very penetration is also related to the signif- icant difficulty and cost in detecting such neutrinos. Neutrino detectors discussed, planned, and proposed include the MACRO detector in Italy and DUMAND of Hawaii. The full-scale DUMAND detector would be more expensive than any single ground-based experiment discussed here, and there are serious enough questions concerning that proposal to reserve judgment concerning its construction pending results from the prototype. At the same time, the discoveries that would result from a serious look at neutrinos of 10~2 eV and above from astronomical sources would be exciting. We recommend that funding should be sought for neutrino astron- omy detectors if their feasibility and cost-effectiveness can be clearly established. Magnetic Monopoles The search for these theoretically predicted, elusive, but fundamen- tally significant particles should be continued and extended. ~ We suppport the construction of scintillation- or proportional- counter detectors capable of at least reaching the Parker bound, corresponding to at least a 1000-m' area. Larger flux-loop detectors of areas of the order of 100 m' should be built, and searches for monopoles trapped in meteorites or magne- tite should be extended. Large Underground Detectors The upgrading and expanded exploitation of these detectors should be encouraged and supported. Justified and built to search for proton decay, these detectors are also valuable for cosmic-ray studies. These include the following:

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164 COSMIC RA YS The study of neutrinos from the interaction of cosmic rays in the atmosphere and the search for neutrino oscillations as evidence for finite neutrino masses. The search for neutrino bursts from gravitational collapse of supernovae and other astronomical sources of neutrinos below 1 TeV. The study of muons underground, especially when coupled with surface air-shower arrays, in order to better understand primary composition in the 10'3-10'6 eV energy region. Solar Neutrinos This problem, addressed by astronomers and particle physicists as well, merits continued serious effort. We support the following: Construction of detectors for neutrinos of lower energy through inverse beta decay, such as the proposed gallium experiment. Exploration of the feasibility of electronic detection of v-e scat- tering in large underground detectors or other devices. THEORY . The theoretical calculation and modeling of various processes are vitally important to continued progress in understanding cosmic-ray physics. The major theoretical activities concern the following: Stellar and explosive processes leading to generation of cosmic- ray nuclei and related photons and neutrinos. Acceleration mechanisms and propagation. Interactions and cascading of cosmic rays in the atmosphere and in the interstellar medium. New concepts and the synthesis of ideas can lead to breakthroughs In our understanding quite out of proportion to the investment.