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IV Cosmic Rays

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Cosmic rays provide our only direct sample of material from outside the solar system. Their composition reflects the nature of the nucleosynthetic processes by which all the elements of the periodic table are being constructed in the galaxy. In addition the cosmic rays are accelerated to relativistic speeds by processes in which nature concentrates vast amounts of energy in relatively few particles. These acceleration processes apparently take place on a wide variety of scales in astrophysical plasmas. Because some cosmic rays have energies higher than man-made beams of particles, they are also of interest for studying interactions of protons and atomic nuclei at ultrahigh energy. Cosmic-ray physics is thus in essence an interdisciplinary field, touching astronomy and high-energy astrophysics, nuclear physics, plasma physics, and elementary-particle physics. It began as the study of energetic particles in the atmosphere, which we now know to be the products of nuclear interactions between the primary cosmic rays and air nuclei. In the past 35 years high-altitude balloons and spacecraft have carried instruments above most of the atmosphere, and the focus of cosmic-ray studies has shifted to the composition and energy spectra of the primary particles themselves, which includes atomic nuclei and electrons. The highest-energy cosmic rays, however, are still accessi- ble only to surface experiments that can overcome the exceedingly low rate of these cosmic rays by exposing detectors of large area for long times. In addition, secondary neutrinos and muons are of great current interest for deep underground experiments, and there is an intense search for magnetic monopoles in the cosmic rays. A major opportunity of the present decade is the ability provided by the Space Shuttle to place large detectors in space and to visit them subsequently for repair. By the early l990s this capability will be supplemented by the Space Station, which will provide a permanent manned presence in space and permit routine maintenance and modi- fication of orbiting instruments as well as assembly of instruments that otherwise would be too large to lift into orbit. The combination of Shuttle and Station will permit us to place new kinds of instruments in 113

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1 14 COSMIC RA YS space, leading to new levels of precision of cosmic-ray instruments and extension of direct observation of the major cosmic-ray components by several orders of magnitude in energy. Ground-based detectors will remain the only source of information in the highest-energy regime, where galactic acceleration and confinement mechanisms probably fail, and one expects a transition to particles from outside our own galaxy. In both space and ground-based observations, instruments are now possible that will be capable of addressing some of the key astrophys- ical questions of processes of nucleosynthesis and particle accelera- tion, as well as questions of the physics of particle interactions at extremely high energies.