Click for next page ( 144


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 143
17 Opportunities In describing the opportunities for progress in cosmic-ray physics it has been convenient to consider separately those areas requiring measurements above the atmosphere, either on satellites or on strato- spheric balloons, and those areas using earthbound instruments, either on or under the surface. Spaceborne instruments measure directly the charge, energy, and in some cases the mass of individual cosmic-ray particles with energies up to about 10'4 eV. Ground-based instruments infer energy spectra and composition of cosmic rays above about 10'4 eV by measurements of the showers of secondary particles produced by interactions of the primary cosmic rays in the atmosphere. In addition, for particles that penetrate the atmosphere, such as neutrinos and perhaps magnetic monopoles, certain ground-based instruments may detect the primary particle. There also has been a practical, organizational difference between ground-based and spaceborne mea- surements. The spaceborne measurements have been funded princi- pally by the National Aeronautics and Space Administration (NASA), while the ground-based experiments have been funded primarily by the National Science Foundation and the Department of Energy. SPACEBORNE EXPERIMENTS Developments of both spacecraft and instrumentation over the past decade, combined with NASA's plans for a Space Station in the early 143

OCR for page 143
144 COSMIC RA YS l990s, provide us with opportunities for definitive cosmic-ray experi- ments in space. There are a number of important measurements that can be made with a superconducting magnetic spectrometer facility on the Space Station. There are also important observations that can be made with instruments already built or under construction, attached to the Space Shuttle, the Space Station, or the Long Duration Exposure Facility. In addition there are also a few key experiments, using space-proven solid-state detector technology, that require exposure outside the magnetosphere and can be placed there with Shuttle launch and subsequent upper-stage boost. In this section we describe scientific questions that can be answered in the next decade with these existing technologies, and in the re- commendations that follow we again emphasize the next decade and experiments that we now know to be feasible. In a longer-term view there have been suggestions for assembling in space much larger cosmic-ray experiments capable of extending our knowledge even further. Undoubtably these further developments should be studied in the next several years. Isotopes GALACTIC COSMIC-RAY ISOTOPES We now have in hand techniques to measure the mass of individual nuclei and thus determine the isotopic composition of cosmic-ray elements. With the discovery that the heavy stable isotopes of Ne, Mg, and Si are enhanced in galactic cosmic rays, it appears highly likely that the isotopic composition of other less-abundant elements will also differ from that of solar-system material because of different nucleo- synthetic history. The measurements of the neutron-rich isotopes of S. Ar, and Ca, for example, are required to distinguish among models that have been proposed to explain the Ne, Mg, and Si abundance anom- alies. But lower fluxes and larger contributions from interstellar frag- mentation of heavier elements require significantly larger instruments to be able to gather adequate numbers of nuclei to make definitive conclusions. Using well-established techniques of solid-state detectors, it is now possible to construct an instrument with sufficient mass resolution and size that with a few-year exposure outside the magnetosphere the detailed isotopic composition would be determined at energies of a few hundred MeV/amu for all elements in the galactic cosmic rays up to atomic number 30. Such an instrument could be flown on an Explorer- class mission.

OCR for page 143
OPPORTUNITIES 145 With a superconducting magnetic spectrometer on the Space Sta- t~on, isotope measurements of similar precision and sensitivity would be possible at much higher energies several GeV/amu. This extension of isotope measurements to energies where such measurements were previously impossible will permit probing of a variety of cosmic-ray time scales using radioactive isotopes at large Lorentz factors and will probe for energy dependence of sites of cosmic-ray acceleration by comparison of the isotope compositions at various energies. With developments over the past decade in superconducting magnet tech- nology in a wide variety of ground-based applications, and the devel- opments of cryogenic applications in space, such a device appears to be quite feasible for installation on the Space Station in the early 1990s. SOLAR-FLARE ISOTOPES We have only limited direct knowledge of the Sun's elemental composition and almost no direct knowledge of its isotopic composi- tion. Spectroscopic measurements of solar isotopes are difficult to perform; there are observations for only a few of the first 30 elements, and the uncertainties are large. Recently, unexpected isotopic anoma- lies in a number of elements have been discovered in meteorites, giving evidence for the inhomogeneity of the solar system at the time of its formation, and perhaps for nucleosynthesis activity in the solar neigh- borhood immediately before the formation of the solar system. Recent measurements of solar-flare particles with cosmic-ray instru- ments have shown that neon in solar flares has a different isotopic composition than neon in the solar wind but the same isotopic com- position found in some meteoritic components. Other heavy elements for which solar-flare isotope observations have been made show no anomalies at the 30 percent level, but observations with much better statistics are needed to determine composition at the level at which meteorite anomalies are observed a few percent or less. The same spacecraft outside the magnetosphere described above for galactic cosmic-ray measurements at a few hundred MeV/amu can also carry a similar instrument to measure the isotopic composition of solar-flare particles above about 5 MeV/amu. Ultraheavy Elements The quantitative study of ultraheavy (atomic number greater than 30) nuclei has begun with the HEAD-3 satellite. Individual element abun- dances have been measured for elements of even atomic number up to about 60. At higher atomic numbers, resolution and statistics limited

OCR for page 143
146 COSMIC RA YS the quality of the results to relative abundances of charge groups. For the actinide elements, around atomic number 90, the quality of the results is limited by extremely low statistics; a total of only three actinide nuclei were identified in the two experiments. With sufficient improvements in both statistics and charge resolu- tion, significant new results can be expected. For example, with a 2-year exposure of a 100-m2 sr detector one could look for specific elemental tracers of recent reprocess nucleosynthesis such as 93Np, 94Pu, and 96Cm. If the fraction of reprocess material were appreciably greater than 10 percent, one could even estimate the time of the reprocess addition from the relative abundances of these elements. It appears that this next major step in the study of ultraheavy nuclei can be achieved relatively inexpensively using newly developed plastic track detectors on a flight of the Long Duration Exposure Facility (LDEF), a large nearly completely passive spacecraft. The requisite number of nuclei can be detected with this large system, and it appears that sufficient charge resolution can be achieved with proper attention to temperature control and monitoring. Construction of this instrument has begun, in preparation for a launch in 1987. High-Energy Composition and Spectra The energy spectrum of protons, the most abundant cosmic-ray species, has been reasonably well measured on balloons up to energies around 1000 GeV, and measurements of helium nuclei exist up to more than a few hundred GeV/amu. Information on the more abundant of the heavier nuclei (carbon, oxygen, and iron) exists up to about 100 GeV/amu, although the statistical accuracy of the data is still limited. Relative abundances of the secondary nuclei that result from interstel- lar spallation are quite well measured at energies up to about 20 GeV/amu; spectra of these elements define the galactic confinement and propagation of cosmic rays. Various models have been developed for the acceleration of primary cosmic rays and the production of secondaries during propagation. These models have been constructed to agree with the observed data up to about 100 GeV/amu but make different predictions about the spectra at higher energies. Precise observations at very high energies are, therefore, crucial to distinguish among these models. Ground- based measurements have great difficulty in distinguishing individual cosmic-ray elements, so it is necessary to make direct measurements in space, but the low fluxes of these higher-energy cosmic rays require large instruments and long exposures.

OCR for page 143
OPPORTUNITIES 147 A large-area instrument designed to measure the energy spectra of cosmic rays with atomic number 3 through 28 at energies up to a few TeV/amu was successfully flown on the Spacelab for a week in August 1985. The fluxes of very energetic cosmic-ray nuclei are extremely low, so a 1-week exposure gives results that are limited by statistics. Reflight of this instrument on a later Spacelab mission would thus be valuable. Furthermore, attaching this instrument to the Space Station for a year would permit it to extend measurement another decade in energy, approaching the region where inferences from ground-based air-shower detectors suggest a change in the cosmic-ray composition. Complementary observations, with much better energy resolution but at not quite so high energies, up to several hundred GeV/amu, would be possible with a superconducting magnetic spectrometer facility on the Space Station. These observations would permit, for the first time, measurements of fine structure in the cosmic-ray energy spectrum, which might be expected from a superposition of sources with different energy spectra. Positrons, Antiprotons, Deuterium, and 3He Several significant questions about the galactic containment of cosmic rays require the observation of the secondary cosmic rays generated by interstellar collisions of the most abundant cosmic-ray species protons and alpha particles. These observations are best performed with counter telescopes featuring magnetic spectrometers with superconducting magnets flown on the Space Station. Measurements of the positron-to-antiproton ratio can be used to determine the critical energy at which radiative losses dominate escape losses (because both positrons and antiprotons are produced in the same collisions, but the positrons have significant radiative energy losses). This critical energy is related to the root-mean-square trans- verse magnetic field and to the containment time in the storage region. In order to determine this critical energy the positron-to-antiproton ratios must be measured at least up to 100 GeV. Such an exposure is possible on a 5- to 10-day Spacelab mission. Observations of deuterium and 3He are aimed at determining if helium has the same acceleration and propagation history as heavier nuclei and at making detailed measurements of the energy dependence of the confinement time in the galaxy. The first of these objectives can be met with measurements in the l-10 GeV/amu range on l- or 2-day flights of high-altitude balloons or in l-week Spacelab flights. The second objective requires measurements up to about 150 GeV/amu and

OCR for page 143
148 COSMIC RA YS requires longer spacefiight exposures as would be afforded by a superconducting magnetic spectrometer on the Space Station. Antimatter One of the most fundamental questions in cosmology is the symme- try or asymmetry between matter and antimatter in the universe. While current cosmological models favor an asymmetry, experimental limits on extragalactic antimatter are inconclusive. Current limits on the presence of antimatter of heavy nuclei in the cosmic rays are at the level of parts in 104. If distant clusters of galaxies composed entirely of antimatter exist, they may contribute to the cosmic-ray flux in our galaxy at a level of at most 10-7 or 10-6. A search at this improved sensitivity level is therefore meaningful. For an antihelium search, the required number of events could be achieved with an exposure of 0.2 m2 sr day, attainable with a Shuttleborne superconducting spectrome- ter. For anti-iron nuclei, plastic track detectors in combination with plastic scintillators have been proposed, making use of the differences in energy-loss mechanisms in the two kinds of detector, with the differences depending on charge-cubed terms in the collision cross section of nuclei with atomic electrons; here too the necessary expo- sure could be attained with several balloon flights or a 1- to 2-week Shuttle flight. A much more sensitive search, at the level of 10-8, over the full range of abundant elements from helium through iron, would be possible with a superconducting magnetic spectrometer on the Space Station. Nucleus-Nucleus Interactions The study of nucleus-nucleus interactions at high energies has become of great interest in the past few years because of elementary- particle theories that predict new states of matter that can be created only in such collisions, in particular the quark-gluon plasma. In addition information about nucleus-nucleus as well as proton-nucleus collisions is required for interpretations of air-shower data. At present nucleus-nucleus interactions at energies above 4 GeV/amu can be studied only in the cosmic rays. Such studies using emulsion-chamber techniques can also give the composition of the cosmic rays causing these interactions. Balloonborne exposures of such detectors have begun to make significant contributions in this field, and with extended exposures on balloons and on the Space Shuttle we can expect a

OCR for page 143
OPPORTUNITIES 149 significant increase in both the number of interactions studied and the highest energies observed. Solar Modulation of Cosmic Rays The continuous radial flow of coronal plasma and magnetic field outward from the Sun results in a cosmic-ray flux in the inner solar system that is significantly lower than in interstellar space. This effect is significant at energies below several GeV/amu, and the eject increases at lower energies. Indeed interstellar cosmic rays below a few hundred MeV/amu cannot reach the inner solar system at all, at least not near the ecliptic plane. The particles observed near the Earth below this energy had higher energies when they were outside the solar system. The magnitude of this solar modulation varies substantially and rather irregularly during the 22-year solar cycle. The deep-space probes Pioneer 10 and 11 and Voyager 1 and 2, which are leaving the solar system, are providing important data on the extent of the modulating region. The Ulysses spacecraft, which will be launched in 1986 and will fly over the pole of the Sun at a distance of about 1 AU, will provide a direct measurement of modulation effects in a region of the solar system where the interplanetary magnetic field has a configuration different from that near the ecliptic plane. As these probes penetrate uncharted regions of the solar system, it is important to preserve monitors of the magnitude of the solar modulation includ- ing near-Earth spacecraft and ground-level neutron monitors. Neutron monitors provide a precise continuous monitor of the cosmic-ray flux at the Earth by measuring secondary nucleons produced in the atmosphere by nuclear interactions of primary cosmic-ray nuclei. A base of nearly 40 years of continuous observations is available for intercomparison of observations made at different times in the solar cycle. GROUND-BASED EXPERIMENTS Gamma-Ray Astronomy The opportunity to observe directly sources of very energetic particles opens a new frontier in astronomy and astrophysics. The detection of gamma rays of over 10'2 eV from the ground using optical Cerenkov light (10"-10'3 eV) or using extensive air-shower counter arrays (10'3-10'6 eV) is at present one of the most exciting and rapidly

OCR for page 143
150 COSMIC RA YS developing fields in cosmic rays because of the potential for observing high-energy natural accelerators at work. Recent discoveries indicate that a number of binary x-ray sources, such as Cygnus X-3, Vela X-1, and LMC X-4, are sources of very energetic cosmic rays. Indeed, Cygnus X-3 alone may be sufficient to supply all the galactic cosmic rays with energies of 10'6-10'7 eV. New and better measurements are urgently needed to clarify the nature of the signals above l TeV from point sources and to understand their implications. Several experi- ments are currently being developed with this aim, and this effort deserves the strongest possible support. Air-Shower Detectors The only major U.S. program directed toward the study of extensive air showers produced by primary cosmic rays of over 10'7 eV is the Fly's Eye installation in Utah, described earlier. This detector has been expanded by increasing the number of mirrors and phototubes at the second, newer site by a factor of 3. In the future the group has plans to improve resolution and sensitivity by constructing a second-generation system using a larger number of smaller phototubes and to include optical filters to reduce the back- ground from Cerenkov light. There is serious discussion on the development of muon detectors and/or a surface air-shower counter array in conjunction with the Fly's Eye. Detecting the same event with both techniques would provide critical intercalibration of the Fly's Eye data with other surface-array experiments. In addition, data on the lateral spread of cascades determined from the surface array could be correlated with the longitudinal development as seen with the Fly's Eye. The surface array would also collect data during the day. This would add to the global data set on the highest-energy cosmic rays with more conventional surface-array data. As noted earlier, the spectrum, anisotropy, and composition of primary cosmic rays above loft eV are all of significant interest. Although this information is indirect and interpretation of particle physics parameters is complicated by the mixed primary composition, there can be no other access to this extreme energy domain above 10~8 eV through the end of this century. There is an inevitable quest for data beyond our present horizon of about 102 eV. The rates above that energy are so low less than one/100 km2 year) that it is not known at this time whether the spectrum truncates or flattens out above 102 eV.

OCR for page 143
OPPORTUNITIES 151 Neutrino Astronomy The proton decay detectors are able to study neutrinos as a conse- quence of their large detector volumes. Thus far the observed neutrino interactions are from muon- and pion-decay neutrinos, which come in turn from cosmic-ray interactions in the Earth's atmosphere. Neutri- nos, like gamma rays, are unaffected by galactic magnetic fields. Further, they uniquely can penetrate all interstellar environments. Thus, it has been tempting to consider developing a neutrino astron- omy to seek signals from a variety of astronomical sources. It has been proposed to instrument a large volume of seawater with photomulti- pliers to seek Cerenkov signals from such neutrino interactions. Such a system has been christened DUMAND for Deep Underwater Muon and Neutrino Detector. A complete DUMAND installation would contain a three-dimensional matrix of phototubes deployed to observe the Cerenkov light from energetic particles and interactions in 30 million tons of seawater under a shield of 3 to 5 km of ocean. Besides neutrino interactions, cosmic-ray muons would also be observed, and their interactions at energies in excess of those available at current accelerators could be accessible to study. Multiple muon studies, as they bear on primary composition, may also merit attention. The Homestake detector is able to study low-energy neutrinos (a few MeV) and is sensitive to supernova processes that are predicted to produce neutrinos as a consequence of gravitational collapse. The current U. S . proton-decay detectors are primarily concerned with neutrinos expected to result from cosmic-ray interactions in the atmosphere where the observed neutrino energies range from 200 MeV to several GeV. DUMAND would focus on neutrinos of above a few hundred GeV. In its most ambitious manifestation it could have a sensitivity in principle comparable with the sensitivity of the air- shower gamma detectors discussed above. If the observed sources of gammas also produce neutrinos, one will conclude that both come from interactions of very-high-energy protons the gammas from TT() decay and the neutrinos from IT+ decay. If neutrinos are not seen, it would suggest that the gammas arise from electromagnetic processes such as bremsstrahlung and synchrotron radiation. There may also be situations where gammas are absorbed or attenuated near a source while the neutrinos are not. The many technical problems in transforming a large volume of the ocean into a particle detector have been studied for some time. There are currently plans to proceed with a one-dimension test of a design

OCR for page 143
152 COSMIC RA YS concept. This will be a single cable containing several phototube modules along its length. It will be lowered into the ocean and operated to detect cosmic-ray muons. The results of this test will significantly influence future planning in this area. A new proposal for a joint U.S.-Italian experiment in the Gran Sasso Tunnel in Italy has been developed. Dubbed MACRO (Monopole and Cosmic Ray Observatory), its dual objectives are monopole detection beyond the Parker limit and high-energy neutrino astronomy. The Fly's Eye may also serve as a neutrino detector, and the group working with the detector has searched their data for upward-going air showers that would be evidence for energetic neutrino interactions. As running time accumulates, this aspect of the Fly's Eye data could take on astronomical importance. By observing very energetic upward- going showers produced by neutrino interactions in the crust of the Earth, the Fly's Eye may be able to detect neutrinos from primary protons of over 102 eV interacting with the 3-K blackbody radiation. Some perspective on the energy ranges and particle types studied with ground-based cosmic-ray experiments are summarized in the bar chart of Figure 17.1. In general, the lower limits are set by experimen- tal techniques and the upper limits by falling fluxes. NEUTRINOS MAGNETIC MONOPOLES GAMMA RAYS PROTONS AND NUCLEI -AS SEEN BY MUONS -AS SEEN BY AIR SHOWERS '//////\\\\\\\' /////// / / / / \\\\\\\\\ HOMESTAKE I/~\B DUMAND FLY'S EYE //i/ / //// ///// ////////////////// SUPERCONDUCTING COIL DETECTORS SCINTILLATION DETECTORS A///////////\\\\\\ CERENKOV AIR SHOWER DETECTORS ARRAYS '////~0~1111 1 1 1 1 1 PROTON HOMESTAKE DUMAND DECAY DETECTORS SURFACE FLY'S EYE ARRAY loin lol2 ic14 iol6 EN ERGY ( ELECTRON VOLTS ) FIGURE 17.1 The range of energy sensitivity for different particle types of the present and proposed ground-based detectors discussed in this section. The lower limit is generally set by the characteristics of the detector and the upper limit by the falling spectrum of the cosmic-ray flux. The magnetic monopole sensitivities indicated are not related to flux estimates. l 1ol8 lo2o lo6 lo8

OCR for page 143
OPPORTUNITIES 153 Magnetic Monopoles The definitive observation of magnetic monopoles would have a major impact on our understanding of particle physics and astrophys- ics. This particle is so exotic and its discovery would be so important that a significant search effort is warranted. It is possible to design much larger magnetic flux detectors- superconducting coilsthan have been used up to the present. To date coil areas are 100-2000 cm2. Groups that have operated these detectors have developed concepts for coils with a sensitive area of the order of 100 m2, which would permit much more sensitive searches. Scintilla- tion-counter groups have also designed large-area experiments, and at least one is in an advanced stage of construction. In addition, the Homestake detector would be sensitive to monopoles. From Figure 17.1, the interesting astrophysical upper limit to monopole fluxes is 5 x 10-~6 (cm2 sr sky. To detect one event at this limit requires a detector of 1000 m2 operating for a year. Experiments of 1000-10,000 m2 area are possible, and detectors such as MACRO are currently being proposed that are capable of reaching this limit. It appears that physicists will press to extend the search for monopoles until their existence is definitely confirmed or until they are not found in more than one detector of at least 1000 m2 operating for over a year. Nucleon Decay Detectors The nucleon decay detectors present an unusual opportunity for cosmic-ray research. They are large underground detectors sensitive to energetic cosmic-ray muons and to neutrino interactions. The largest nucleon decay detectors have measured for the first time the flux of cosmic-ray neutrinos by directly observing their interactions inside the detector volume in significant numbers. Rates are consistent with expectation, and they offer the possibility of extending the search for neutrino oscillation by comparing fluxes of upward and downward neutrinos. If neutrinos have masses, then electron and muon neutrinos may oscillate into each other (or into other types of neutrinos) over large distances. The diameter of the Earth is so much larger than laboratory scales that this geophysical type of experiment could possibly see effects not accessible in the laboratory; mass difference down to 10-2 eV can be studied, although the limits on the relevant parameter, sin2 0, will be weaker than for laboratory experiments. At the same time, measurement of the neutrino flux and comparison with

OCR for page 143
154 COSMIC RAYS conventional expectations are being used to help calibrate the detec- tors for their primary mission of searching for nucleon decay. All nucleon decay detectors observe multiple muons with varying degrees of spatial and angular resolution. If the energy per nucleon is sufficiently above threshold for production of muons in the atmosphere that can survive to the depth of the detector, then multiple-muon detection rates are in principle sensitive to primary composition because a heavy nucleus is more likely to produce a multiple-muon event than is a proton primary. Detectors now operating have already begun to collect multiple muon events with larger collection areas and a larger range of depths than has been possible previously. Sensitivity to primary cosmic-ray composition may be enhanced significantly by a surface-detector air-shower array in coincidence to estimate the energy of the primary by its accompanying shower. Additionally, data on the lateral spacing of muons the decoherence curve is relevant to the transverse momentum distributions of muons and their parent pions from the primary cosmic-ray interaction in the atmosphere. As the primary cosmic-ray energies explored are within the range of the current generation of pp colliders, these data are of interest principally in the context of the properties of nucleus-nucleus collisions. Solar Neutrinos The importance of the solar neutrino experiment has been correctly emphasized in the report of the Astronomy Survey Committee (As- tronomy and Astrophysic s for the 1 980's, Volume 1, National Acad- emy Press, Washington, D.C., 19821. That report (page 114) ". . . rec- ommends continued, vigorous support for programs to detect and measure the flux of neutrinos from the Sun. Additional facilities are needed to supplement the data currently being obtained by 37CI detectors...." As emphasized in that report, it is feasible to use the inverse beta decay of gallium as a detector of low-energy neutrinos, i.e., the neutrinos from the proton-capture processes in the Sun. Such an experiment, although expensive, would be an independent and more definitive probe of solar nucleosynthesis. Another interesting possibility for solar neutrino study has been discussed. It appears feasible to increase the sensitivity of the water proton-decay detectors through the addition of more and/or larger phototubes to observe solar neutrinos in real time, possibly including directional information. There could be serious backgrounds; never-

OCR for page 143
OPPORTUNITIES 1 55 theless, if this goal could be realized, it would be one of the important opportunities of this decade. . Future Opportunities This field of science has provided opportunities for bold, creative Ideas in the past, and we should be alert to new opportunities presented by new ideas, unexpected results, or developments in related areas of physics. A second-generation Fly's Eye, a surface array at Fly's Eye, developments for neutrino astronomy including solar neutrinos, large- scale monopole detectors, MACRO, new gamma-ray detector systems, and DUMAND are potential candidate programs. Other programs involving international collaborations may also develop in the air- shower field. For example, accessible mountain-top observatories in the Andes and the Himalayas exceed by thousands of feet in elevation (therefore by one or two nuclear interaction mean free paths) any potential U.S. sites. International collaborations at these unique sites involving U.S. participation may evolve in the future. Any of these future possibilities should be regarded as serious candidates for an incremental increase in the support level of ground-based cosmic-ray experiments. THEORY Theoretical calculations are a vital component of cosmic-ray phys- ics. Calculations of stellar and explosive nucleosynthesis form the basis for drawing implications about the relative importance of various astrophysical processes from measurements of cosmic-ray composi- tion; calculation of the neutrino spectrum from gravitational collapse is closely related. Understanding the nearest star depends on modeling of the nuclear reaction cycle in the Sun and other solar calculations, which underlie, for example, interpretations of solar neutrino experi- ments. The processes by which cosmic rays are accelerated to exceed- ingly high, suprathermal energies are intrinsically interesting, and significant theoretical progress is being made in understanding them. Calculations of cosmic-ray propagation lead to understanding of the interstellar environment as well as being fundamental for relating observed composition to composition of cosmic rays in the sources. Simulation studies of extensive air showers provide the basis for interpreting measurements of cascades induced by the highest-energy cosmic rays. Calculations of neutrino fluxes are important to establish

OCR for page 143
156 COSMIC RA YS the background for underground experiments, such as the search for nucleon decay, and to determine the level at which neutrino astronomy may be possible. Another subject of great current interest is the calculation of flux limits on magnetic monopoles from galactic mag- netic fields and neutron star brightness. Even though the computations sometimes require use of large computers, theoretical work in this field is inexpensive relative to the observational. Nevertheless, it is vitally important that it be nurtured and maintained.