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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 144
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.
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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
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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.
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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
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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
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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
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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.
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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
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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
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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 coils—than 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
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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-
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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
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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.
Representative terms from entire chapter:
space station
