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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.
Representative terms from entire chapter:
astronomy survey
