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OCR for page 121
16
Highlights
This chapter discusses the problems being addressed by current and
future cosmic-ray measurements. The general themes are organized by
the history of cosmic-ray matter, starting with its synthesis, proceeding
to its acceleration and propagation through interstellar space, and con-
cluding with its interaction with matter.
To set the stage, we start with a list of the major discoveries of the
last decade.
· New detectors have unambiguously resolved individual isotopes
of neon, magnesium, and silicon. The resulting abundances show
distinct quantitative differences from those found in the condensed
bodies of the solar system, demonstrating conclusively that galactic
cosmic rays are a sample of matter with a nucleosynthetic history that
is different from that of the Sun. At the same time measurements of
solar cosmic rays have provided some of the best measurements of the
isotopic composition of the solar corona.
· Cosmic-ray abundances of individual elements heavier than iron
have been successfully measured, despite the extreme rarity of these
nuclei. The results indicate that the cosmic rays are not dominated by
material recently synthesized in supernova explosions, as data sug-
gested a decade ago, but may well be accelerated interstellar material,
a conclusion that is consistent with the isotope measurements of the
lighter elements.
121
OCR for page 122
122 COSMIC RA YS
· Measurements of the isotopes of the secondary element beryllium'
in particular the abundance ratio of stable 9Be to radioactive '°Be,
demonstrated that the cosmic rays that we observe today were
accelerated on average 10 million to 20 million years ago and have
propagated through interstellar material of mean density lower than the
mean density of the galactic disk.
· The radial gradient of cosmic rays
In the ecliptic plane of the
heliosphere has now been measured. The gradient is less steep than
some earlier models had predicted, and the edge of the modulation
region twhich had earlier been predicted to lie as near as 5 astronomical
units (AU)] has been shown to be beyond 30 AU.
· A low-energy Ltens of millions of electron volts/atomic mass unit
(MeV/amu)] component with highly unusual composition was discov-
ered. This anomalous component is rich in oxygen and nitrogen but
lacks carbon. It suffers modulation with the solar cycle in the same
sense as galactic cosmic rays, so it appears to be either galactic in
origin or to be accelerated in the outer portions of the heliosphere. Its
source and acceleration mechanism is a puzzle.
· Observations of discrete sources of gamma rays with energies to
10~5 eV with ground-based detectors have identified a few cosmic-ray
accelerators of great power.
· At the highest energies, above 10~7 eV, ground-level air-shower
measurements now give clear evidence of anisotropy in arrival direc-
tion; above 10~9 eV this anisotropy suggests that these most energetic
particles in nature may be of extragalactic origin.
· Large new underground detectors designed primarily to search for
nucleon decay have observed and measured the flux of neutrinos from
cosmic-ray interactions in the atmosphere. These detectors are also
being used to study multiple muon events and their relation to the
composition of primary cosmic rays around 10~5 eV.
In addition to these discoveries, a number of other observations also
raise important questions for the future. These include the following:
· Measurements of secondary products of cosmic-ray nuclear inter-
actions in the interstellar medium indicate an energy dependence of the
confinement process at energies from 1 to 100 GeV/amu (1 GeV = a
billion electron volts). Unexpectedly high fluxes of antiprotons suggest
that the cosmic-ray protons that produce them penetrate more matter
before reaching us than do heavier cosmic rays. These data have
altered our picture of the processes by which cosmic rays are confined
to the galaxy and constrain models of cosmic-ray acceleration.
OCR for page 123
HIGHLIGHTS 1 23
· Ground-level observations indicate changes in cosmic-ray compo-
sition at energies just above those reached so far by direct measure-
ments. It appears that between 10'4 and 10'6 eV the cosmic rays are
richer in heavy nuclei relative to protons than they are at lower
energies, while at still higher energies, above 10~7 eV, protons may
again dominate.
· In 1972 measurements of attenuation in air of cosmic-ray protons
up to 50 TeV (1 TeV = 1 trillion electron volts) indicated that the
proton-proton cross section increases with energy. This inference was
subsequently confirmed by direct accelerator measurements. More
recently, results from large air showers suggest that this increase
continues at least another four decades in energy.
· A series of balloon flights of emulsion chambers has observed and
measured the composition and interactions of heavy nuclei of up to 10'4
eV. In some cases the interactions produce up to 1000 secondaries.
· The flux of solar neutrinos observed appears to be significantly
lower than expected from fusion processes in the Sun. This discrep-
ancy has become one of the major unresolved issues of current
astrophysics.
The following sections explore in more detail some of the topics
listed above and their implications for future research.
NUCLEOSYNTHESIS
Measurements of the abundances of elements and isotopes in the
solar system, as observed spectroscopically in the solar photosphere
and directly in terrestrial, meteoritic, and lunar samples, have long
formed the basis of our knowledge of the history of the solar system.
These solar-system abundances have in turn become the benchmark
for studies ranging from stellar structure and nucleosynthesis to the age
and evolution of the galaxy.
Galactic cosmic rays provide a sample of material from outside the
solar system, which can be used to describe the composition of the
Milky Way Galaxy at a time and place far removed from solar-system
formation. The cosmic-ray measurements complement spectroscopic
information derived from optical and millimeter-wave astronomy on
stars and the interstellar medium. Some elements and isotopes that
cannot be measured well spectroscopically are relatively easy to
investigate in the cosmic rays, for example, neon, iron isotopes, and
many of the rare elements heavier than iron.
Abundances of radioactive nuclides and their daughters show that
OCR for page 124
124 COSMIC RA YS
the solar system formed 4.6 billion years ago. Thus, the solar-system
abundances have usually been taken to be representative of the
interstellar medium at that time. However, recent observations of
isotopic abundance anomalies in various meteoritic minerals give
evidence for compositional inhomogeneity of the nebula that formed
the solar system, and these observations give evidence for a significant
`'last minute" infusion into this nebula of products of supernova
nucleosynthesis. Thus the solar-system abundances probably do not
measure the present interstellar medium and may not even be com-
pletely representative of the general interstellar medium 4.6 billion
years ago.
Recent cosmic-ray measurements have resolved clearly the radioac-
tive nuclide '0Be, which has a half-life of 1.6 million years. They
demonstrate that the cosmic-ray nuclei that we observe today were
typically accelerated about 10 million years ago, very recently when
compared with the age of the solar system. Most of them reach us from
distances much greater than a parsec but less than several kiloparsecs.
Thus the cosmic rays sample a region that is large compared with the
probable size of the protosolar nebula but probably does not extend to
the center of the galaxy.
Recent models suggest that the acceleration of the bulk of cosmic
rays occurs in supernova shock waves propagating through the hot
interstellar gas. It thus may be that the cosmic-ray composition is more
representative of the interstellar medium than is the solar-system
composition.
While galactic cosmic rays provide an excellent sample of material
from outside the solar system, energetic particles from the Sun, or
solar cosmic rays, provide in some cases the best solar-system
abundance data available. For example, the solar-system abundances
of noble gas elements and their isotopic compositions, poorly deter-
mined from meteorites or from optical observations of the Sun, can
best be measured in solar cosmic-ray composition studies.
The nucleosynthesis of the elements that make up the solar system
has been understood as the sum of several processes. Primordial
hydrogen and helium are burned in stellar interiors in a series of steps
at increasing temperature and pressure, which release energy as lighter
elements fuse to make heavier ones, building up eventually to elements
in the iron peak. Elements heavier than nickel are principally produced
by neutron capture' either slowly over periods of thousands of years in
evolved stars the (slow) s-process or quickly in seconds during
supernova explosions the (rapid) reprocess. Each nucleosynthesis
process leaves a signature in relative abundances of various nuclides.
OCR for page 125
HIGHLIGHTS 1 25
In the cosmic-ray source composition we look for signatures that
reveal the conditions under which these nuclei were synthesized. We
also test models of nucleosynthesis based on solar-system abundances.
Two points are clear from data already in hand: (1) The material that
is accelerated to form cosmic rays has a composition that is different
from that of the material that formed the solar system. This difference
must reflect a difference in the conditions under which nucleosynthesis
took place, or at least a different mixture of material from the various
nucleosynthesis processes. (2) The composition of the cosmic-ray
source material is distinguished from that of the solar system by subtle
quantitative differences that require precise measurements. These
points are pertinent to the plans for the next generation of experiments.
Isotope Ratios
Quantitative differences between cosmic-ray source and solar-
system composition have been established by isotopic measurements
with excellent mass resolution of the elements Ne, Mg, and Si. The
abundance ratio 22Ne/20Ne is higher in the cosmic-ray source than in
the solar system by a factor of about 4. The four relatively rare
neutron-rich isotopes of Mg and Si are all about 60 percent more
abundant in the cosmic rays (relative to the most abundant isotope of
each element) than in the solar system.
Several mechanisms have been postulated to explain these cosmic-
ray enrichments of the heavier isotopes. These mechanisms involve
nucleosynthesis of cosmic-ray elements under different conditions
from those in the solar system, owing either to spatial inhomogeneities
in the galaxy or to chemical evolution of the galaxy in the time between
formation of the solar system (4.6 billion years ago) and acceleration of
the cosmic rays (only about 10 million years ago). These mechanisms
lead to quantitative predictions for expected isotopic composition of
other cosmic-ray elements so that measurements with much higher
statistical accuracy than are currently available of the elements S. Ar,
and Fe should be able to distinguish among various models.
Abundances of Heavy Elements
In the charge region beyond Fe and Ni, the HEAD-3 experiment has
shown that the cosmic-ray source is not dominated by a single nu-
cleosynthesis process such as the r- or s-process. However, these
results do not rule out an enhancement by a factor of as much as 2 in
either the s-process or the reprocess contribution relative to the solar
OCR for page 126
126 COSMIC RA YS
system. If the solar system were enriched in products of explosive
(supernova) nucleosynthesis due to a nearby supernova shortly before
condensation of the solar nebula, while the cosmic rays were a sample
of "normal" interstellar material, lacking the "last minute" reprocess
enrichment of the solar system, then one would expect the cosmic rays
to appear enriched, by perhaps a factor of 2, in s-process nuclides.
Further measurements of abundance ratios of heavy elements will help
to resolve such questions. Precise decomposition of cosmic rays
heavier than Ni into r- and s-process components will ultimately
require isotope measurements.
Both HEAD-3 and Ariel-6 data demonstrate that the abundance of
actinide elements (Z ~ 90) in the cosmic-ray source is not greatly
enhanced compared with that in the solar nebula, as was suggested by
earlier measurements. In fact, the observed ratio of actinides to
elements in the Z = 80 region is roughly 1 percent. This result already
rules out a classical, actinide-producing reprocess episode of explosive
nucleosynthesis in supernovae as the source of heavy cosmic-ray
nuclei. However, this actinide abundance is so low that its measure-
ment is limited by poor statistics; only one and two actinide nuclei have
been observed by HEAD-3 and Ariel-6, respectively.
Measurements of the relative abundances among individual actinide
elements would show the age of these elements since nucleosynthesis.
Figure 16.1 shows the expected relative abundances of actinide ele-
ments as a function of time since synthesis in an reprocess event. A
synthesis age of the order of 10 million years (the same as the cosmic-ray
propagation time) as indicated by a U/Th ratio of about 5 would, for
example, imply that cosmic-ray acceleration acts on freshly synthesized
material and so would contradict the idea that the cosmic rays are a
sample of today's general interstellar medium. On the other hand, if we
assume that cosmic rays are a sample of today's interstellar medium and
the solar system is a sample from 4.6 billion years ago, the U/Th ratio
in the cosmic rays would provide a measure of the rate of reprocess
nucleosynthesis in the galaxy since the formation of the solar system.
Solar Neutrinos
Recently the capability of detecting neutrinos from the Sun has
opened a new window on stellar nuclear processes. The nuclear fusion
occurring in the Sun is calculated to produce a detectable flux of
electron neutrinos, and accordingly a large-scale experiment has been
operating over the past decade in a South Dakota gold mine. In this
experiment the inverse beta-decay of 37C1 to 37A is detected as
OCR for page 127
HIGHLIGHTS 1 27
loo
10 ~
-
10 2
to
111
_.
_ 10 3
UJ
an
_
10-4 _
_
10-5 _
lo-6
19spt
\ \ Pa
Am\ \
, , , ,1 ,
104 105
Th ~ ~ \ Pu
\ Cm \
\
\
\ Np \ \
_ , ~ 1
1o6 107
\
TIME AFTER r-PROCESS EVENT (yr)
~o8 109
FIGURE 16.1 The relative abundances of the individual actinides as a function of time
after their nucleosynthesis in an reprocess event.
evidence for neutrino capture. The results of this experiment are
enigmatic and important; they suggest a flux of neutrinos less than a
third that calculated. As the neutrinos responsible for this reaction are
of rather high energy, they come from a minor component of the solar
nuclear cycle (boron beta-decay). The reason for the low flux might be
due either to an error in our understanding of the solar cycle or to the
loss of neutrinos through oscillations or other effects in the propagation
from the Sun. In any case this experiment poses an outstanding
challenge to our understanding of the astrophysics of stellar interiors,
of nuclear physics, and of the elementary-particle physics of neutrinos.
ACCELERATION
Recent gamma-ray observations indicate that the bulk of the cosmic
radiation of energy less than 10'3 eV observed near Earth originates in
OCR for page 128
128 COSMIC RA YS
our galaxy. Coupled with the cosmic-ray age since acceleration and an
energy density outside the heliospheric cavity of 1 eV/cm3 or greater,
this suggests an average cosmic-ray luminosity close to 104 ergs/e for
our galaxy. This is at least 10 times greater than the x-ray luminosity of
our galaxy.
Understanding galactic cosmic-ray acceleration is part of a con-
centrated effort to understand all classes of energetic particle accel-
eration in astrophysical settings. Acceleration of particles by the Sun
has been directly observed. The scale of solar acceleration (energy,
time, size) is much smaller than that for galactic cosmic rays. The latter
can be as much as a million times more energetic than solar cosmic rays.
Nevertheless some of the same theoretical approaches are used to
understand both types of process. In addition, we see direct evidence
via electron synchrotron emission that acceleration is also going on in
such diverse objects as supernova remnants, radio galaxies, and
quasars. If our experience with galactic cosmic rays is any guide, these
objects may contain at least 100 times more energy in cosmic-ray nuclei.
The acceleration of energetic particles is apparently a universal phe-
nomenon and deserves a concentrated effort toward its understanding.
Shock Acceleration
Energy requirements suggest supernovae as the cosmic-ray sources,
and early models of cosmic-ray origin assumed these discrete sources.
The power-law spectrum led to later models, which incorporated
diffuse, relatively slow acceleration by random collisions with massive
moving magnetic knots in the interstellar medium. Then a trend back to
discrete sources such as supernovae or pulsars took place because of
the inefficiency of such second-order Fermi acceleration. This evolu-
tion of ideas has been driven by continued improvement of the obser-
vational evidence and development of the theories. The most recent
acceleration models incorporate shock waves generated by supernova
explosions traveling in low-density regions of hot interstellar gas,
which accelerate cosmic rays trapped in the shock front.
Essentially direct observation of acceleration of particles by shock
waves in the solar cavity has stimulated and guided the development of
the theory of shock-wave acceleration generally. Within the solar
system there is enough information to relate the shape of the spectrum
of accelerated particles and its termination to the nature and size of the
accelerating shock. Extending this kind of understanding to galactic
scales is clearly desirable.
OCR for page 129
HIGHLIGHTS 1 29
The most decisive observational constraints to theories of galactic
acceleration will come from measurements of the energy spectra of the
various cosmic-ray components; in particular, the energy dependence
of the secondary/primary ratio at high energies is an important test of
models of cosmic-ray acceleration and confinement. Currently avail-
able data on the composition extend only to about 10'3 eV total energy.
At still higher energies, our information at present is restricted to the
study of showers of secondary particles in the atmosphere, making
possible a determination of the overall energy spectrum of the parent
particles but providing only an estimation of the primary composition.
A better understanding of high-energy composition is essential.
Acceleration Fractionation
There is clear evidence that cosmic-ray elemental abundances after
acceleration differ by factors of 2 to to from one element to another
relative to the standard accepted solar-system abundances (derived
from meteorites and the photosphere). These differences are orga-
nized, at least to first order, by atomic properties of the elements; in
particular Figure 16.2 shows that there is a clear correlation between
the ratio of cosmic-ray source abundance to solar-system abundance
and the first ionization potential of the element. This correlation sug-
gests that the differences are affected by fractionation in the accelera-
tion process or in some process that injects material into the acceler-
ation region.
A similar correlation with first ionization potential has been observed
for the abundances of elements in the solar energetic particles when
compared with the standard solar-system abundances, leading to the
suggestion that similar fractionation effects occur in both solar and
galactic acceleration or injection. An alternate viewpoint suggests that
the standard solar-system abundances are in fact not correctly repre-
sentative of the photosphere or of the interstellar medium. Further
measurements of rare elements in the galactic cosmic rays and in the
solar energetic particles may help to define the role of such fraction-
ation in the acceleration processes.
The striking underabundance of hydrogen in cosmic rays is poorly
understood and does not fit the first ionization correlation. It could
reflect some property of the acceleration mechanism that depends on
the charge/mass ratio (which is unity for hydrogen but less than or
equal to 1/2 for other nuclei). Alternatively, it could reflect a different
origin for protons (and perhaps helium).
OCR for page 130
130 COSMIC RA YS
3.0
~.0
_
:~' 0.3
c:
0.1
0.03
1 1 1 1 1 1 1 1 1
aft _
Get
so ~ o :: 1 T
AN iNe
_ i
He
_ H ~
1 1 1 1 1 1 1 1 1 1
4 6 8 10 12 14 16 18 20 22 24 26
FIRST IONIZATION POTENTIAL (eV)
FIGURE 16.2 The elemental abundances of the cosmic-ray source relative to solar-
system material are roughly ordered by the first ionization potential. However. some of
the remaining differences are well beyond the indicated errors.
Termination of Acceleration Mechanism
Of particular importance in the future will be precise measurements
of the proton spectrum extending to energies between 10'3 and 10'5
eV/nucleon. Here both the time and the size scales of the acceleration
region for nuclei will eventually limit the energy attainable, leading to
a break in the spectrum.
Air-shower observations (which measure the spectrum of the total
energy of cosmic rays their energy/nucleus) indicate that in the region
around 10'5-10'6 eV (where the spectral steepening occurs), the com-
position may become enriched in heavier nuclei. A rigidity-dependent
termination of acceleration, as in the shock mechanism, implies a pro-
gressive enrichment in heavy nuclei with increasing energy per nu-
cleus. It is not yet clear, however, whether this picture is correct in
detail. Direct observations of the composition and spectra between 10'3
and 10'6 eV are required in order to understand galactic cosmic-ray
acceleration and containment models.
OCR for page 131
HIGHLIGHTS 13 1
The acceleration of solar-flare particles (solar cosmic rays) is another
question. While the mean composition, averaged over many flares, is
similar to that of the galactic cosmic-ray sources, including a correla-
tion with first ionization potential, there are dramatic flare-to-flare
variations that remain to be explained. The ratio of heavier elements
(e.g., iron) to lighter elements (e.g., oxygen) varies by an order of
magnitude from flare to flare, and the energy spectra also show wide
variations. In addition some flares have anomalously high fluxes of
3He, thought to be the result of a cyclotron resonance in the acceler-
ation region. Testing models of flare acceleration require correlated
observations of particle spectra and of x rays (from accelerat-
ed electrons) and gamma rays (from accelerated nuclei), as well as
further measurements of the recently observed neutron flux from solar
flares.
High-Energy Gamma Rays
Gamma rays of 10' i-106 eV energy produce electromagnetic cas-
cades in the atmosphere that can be studied from the ground using
atmospheric Cerenkov emission and cosmic-ray air-shower tech-
niques. The Cerenkov light from these air showers is almost parallel to
the shower direction (to approximately 1 degree) so that a telescope
image of this Cerenkov light reveals a fuzzy spot that gives the
direction from which the primary cosmic ray or gamma ray arrived.
Recently, experiments involving surface arrays of particle detectors
have identified gamma rays of up to 10'5 eV from Cygnus X-3 and
possibly from other objects. By tracking the astronomical object of
interest it is possible to separate the point-source gammas from the
isotropic background of air showers produced by charged cosmic rays.
The signal-to-noise ratio may be further aided through the use of
accurate timing and the known timing of the source emissions.
These studies are technically only an extension of astronomy to an
extreme energy of the electromagnetic spectrum. The techniques used
tie this area to other cosmic-ray programs. It is noteworthy that, with
these observations, our study of radiation from the universe spans over
20 orders of magnitude ~ I 020) in wavelengths of electromagnetic
radiation. The results so far have given us the first direct evidence of
discrete astronomical locations of acceleration processes with energies
of 10~5 eV (lOOO TeV). Although this field is only a few years old, the
results are already having a major impact on our understanding of the
. . ,~ .
Origin ot cosmic rays.
OCR for page 132
132 COSMIC RA YS
Anomalous Component
An acceleration process that may have special significance, but
about which very little is known, is responsible for the so-called
anomalous component. Enhanced fluxes of certain nuclei such as He,
N. O. and Ne are observed near the Earth at energies of 10
MeV/nucleon. Why only certain elements are enhanced, how they are
accelerated, and why they appear at the Earth only at certain times are
subjects of much discussion. A solar origin appears to be ruled out. We
may be seeing direct selective acceleration of particles originating in
the local interstellar medium, or we may be seeing particles from
sources nearby in the galaxy with unusual composition. In either case,
measurements of the charge and isotopic composition of these particles
must be made at a new level of accuracy to understand the processes
involved.
GALACTIC COSMIC-RAY TRANSPORT AND THE
INTERSTELLAR MEDIUM
The cosmic-ray flux arriving near the Earth results from a convolu-
tion of source compositions, charge-dependent selection during ac-
celeration, fragmentation from interactions with the interstellar me-
dium en route, and diffusion and scattering processes in the galaxy.
Separating these different physical phenomena is a major task for the
cosmic-ray program in the coming years.
The galactic cosmic rays constitute a highly relativistic gas held in
the galaxy for a time (107 years) that is long compared with the
traversal time for highly relativistic particles across the galaxy (103
years) but short compared with the age of the galaxy (10~0 years). The
physics of containment is poorly understood. We know from measure-
ments of Faraday rotation and the polarization of starlight that the
typical interstellar magnetic field is ~3 FIG. Thus, galactic cosmic rays,
which range in energy from 1 GeV/nucleon to greater than 1 o6
GeV/nucleon, have gyroradii that range from about 0.1 AU to greater
than 1 parsec (pc). However, the distribution of fluctuations in mag-
netic-field magnitude and direction, which are presumably responsible
for scattering the cosmic rays and trapping them in the galaxy, is
unknown. Current estimates suggest that the bulk of the cosmic rays
diffuse to us from distances greater than a parsec but less than several
kiloparsecs.
Key observational parameters for the question of cosmic-ray prop-
OCR for page 133
HIGHLIGHTS 1 33
agation and containment in the galaxy are the abundances of secondary
cosmic rays (produced by interactions with the interstellar gas) relative
to primary particles (accelerated in source regions). Particularly im-
portant are positrons and antiprotons (generated by primary protons);
the light elements Li, Be, and B (fragmentation products principally of
C and O), and certain heavy elements, in particular Sc and V (produced
by spallation of Fe nuclei). Abundances of secondaries, together with
the fragmentation cross sections, give a measure of the average
path-length At traversed by cosmic rays in their lifetime. If the average
density of the medium is known, this can be translated into an average
lifetime.
Energy Dependence of Escape from Galaxy
A fundamental result of measurement of secondary nuclei is that at
higher energies the path length decreases approximately as A<(EJ x
(~/~0) v I, decreasing to At ~ 1 g/cm'at around 100 GeV/nucleon. Only
if such measurements are continued to still higher energies, i.e., well
into the TeV/nucleon region, may one be able to explain the origin of
this energy dependence of A`. For example if it is a consequence of the
diffusion and convection processes by which cosmic rays are trans-
ported out of the galactic confinement volume, then Al is predicted to
continue to decrease as energy increases at a rate that reflects the
spectrum of magnetic inhomogeneities in interstellar space. If, on the
other hand, the effect is due to an energy-dependent escape mechanism
in regions surrounding the acceleration sites, then be would become
independent of energy at a value reflecting the amount of material
traversed by cosmic rays after leaving the source. Several predictions
are shown in Figure 16.3. It is important to emphasize that measure-
ments above 100 GeV/nucleon will not only specify the mode of
propagation of cosmic rays in the galaxy but will also enable us to
deduce the energy spectra at the acceleration site.
The behavior of the escape length as a function of energy below I
GeV/amu is a subject of considerable current interest. There is some
evidence that the distribution of escape lengths is energy dependent
with an energy-dependent deficiency of short path lengths. Such a
path-length distribution could result from a shell of material around the
source regions, in which particles are trapped in such a way that
low-energy particles pass through more material before escaping than
do higher-energy particles. There is also evidence that the mean escape
length becomes independent of energy below about 1 GeV/amu, a
~ ~ I ~ ~ _/1 ~ ~ ~
OCR for page 134
134 COSMIC RA YS
~ .0
0.6
0.4
o
m
<~ 0.2
c:
in
O 0.1
o
m 0.06
0.04
0.02
TIC ~ ,~
lo) Balloon Data
- t H EAO-3
Closed
Galaxy \
-
~t Source
6. Surrounded
by Matter
Typical ~~
Diffusion ~ ~
Model
,, ,,.,., . ,,, ,,,,1 , ,,, ,,.,1
10 100 1000
ENERGY GeV/NUCLEON
FIGURE 16.3 Various models for the containment and propagation of cosmic rays in
the galactic magnetic fields will be tested by measurements in the energy range 1000
GeV/nucleon. Errors quoted for the highest-energy balloon data are much larger than
those that can be obtained from satellite observations of sufficient duration.
feature that is associated in some models with a change from a
high-energy diffusion-dominated transport in the galaxy to a convec-
tion-dominated regime as has been postulated in association with a
galactic wind. This situation would be clarified by extending these
studies to particles whose energy in the interstellar medium is below a
few hundred MeV/amu, which requires direct observations of the
unmodulated cosmic-ray spectra outside the solar system, or possibly
over the solar poles.
The low-energy galactic cosmic rays are also of interest because they
are highly ionizing and couple strongly to the ambient interstellar
medium. The cosmic-ray energy density is comparable with or greater
than that of the interstellar magnetic field and the turbulent motion of
the gas. Cosmic-ray pressure creates bubbles in the interstellar mag-
netic field, puffing it out of the galactic plane, leading to the escape of
cosmic rays. At the same time, gas then flows down the magnetic field,
attracted by the gravitational potential of the galaxy, creating a shock
wave that might trigger stellar condensation. Measurements outside
the heliosphere are required to determine the contribution of these
cosmic rays, most of which have energies below 100 MeV/nucleon.
OCR for page 135
HIGHLIGHTS 135
Correlation Between Anisotropy and Energy
There is a striking correlation between the anisotropy and the flux of
cosmic rays, as shown in Figure 16.4. If the anisotropy reflects
large-scale flow patterns, a simple interpretation would suggest that
there is a single underlying source spectrum of E-2 47 all the way from
10~2 to 10~9 eV, with the remaining observed structure associated with
failure of the containment mechanism. The anisotropy measurements
are made with long-duration, ground-based experiments observa-
tions of muons underground at the lower energies and monitoring
arrival directions of extensive air showers at higher energies. Statistical
uncertainties are large at the higher energies, and measurement of
composition around 10~5 eV is crucial for understanding these intrigu-
ing results.
Secondaries from Light Nuclei
A special role is played by the electron component in the high-energy
cosmic rays. Cosmic-ray electrons, consisting of negatrons mostly
accelerated in source regions plus positrons that are predominantly the
result of interstellar pep collisions, rapidly lose energy through radia-
tive interactions with the interstellar magnetic and photon fields. This
energy loss gives rise to much of the observed nonthermal radio and
UJ
c,
-
o
0.1
~,` 4
-
CL 1
o
'_ 10
o
at)
a:
ENERGY (eV)
~-
1o12 1014 1016
1 1 1
1o1 8 1o2o
1o2
x
104 J
6
103 a
11
-
x
~ a)
FIGURE 16.4 Amplitude of first harmonic as a measure of residence time: anisotropy
(data points) compared with flux (line). Anisotropy has been corrected for solar motion
below 1O'5 eV. [After A. M. Hillas. Annul Rev. Astron. Astrophys. 22, 425 (1984).]
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136 COSMIC RA YS
x-ray background emission of the galaxy. Because of radiative losses,
the lifetime of electrons, and hence the distance they can propagate in
the galaxy before losing a significant fraction of their energy' decreases
rapidly with increasing energy. Thus electrons, observed with an
energy of a few TeV at the Earth, must have been accelerated not
further than a few hundred parsecs from the solar system. Measure-
ment of these high-energy electrons therefore provides the unique
possibility of identifying the distribution of local sources of the cosmic
radiation. In the past 15 years, the total electron flux has been
measured to about 1 TeV.
The observation of the energy spectrum of positrons has a special
importance. It makes possible a direct comparison to the source
spectrum of positrons, which is known through calculations of the pep
production process measured at accelerators. At present the positron
spectrum is known separately only to around 10 GeV. If this measure-
ment could be continued up to a few hundred GeV, it would give direct
information on the deformation of the spectrum due to propagation
effects and radiative energy losses. Such information cannot be unam-
biguously obtained just from observations of electrons since their
energy spectrum at the source is not known a priori. Thus, positron
observations would lead to independent determinations of the confine-
ment time of the electron component in the galaxy together with an
estimate of the magnitude of the magnetic field traversed.
Observations of other kinds of secondaries such as antiprotons, OH
and 3He from interactions of protons, and helium nuclei provide
information on the amount of matter traversed by the most abundant
cosmic-ray constituents. Recent measurements of relatively high
antiproton intensities at around 10 GeV suggest that protons may
traverse 3 to 5 times as much matter as heavier nuclei. A similar
situation seems to exist for helium based on recent observations of a
high 3He/4He ratio. Very-low-energy antiproton measurements are
even more difficult to interpret. More accurate observations of pos-
itrons and antiprotons at different energies and of deuterium and The
should be able to decide the question of whether protons and helium
nuclei have different propagation histories from those of heavier nuclei.
Propagation in Galactic Halo
Observations of the radioactive secondary nucleus "'Be, interpreted
within a simple (leaky-box) propagation model, indicate a cosmic-ray
lifetime of about 10 million to 20 million years. Comparison with the
average path length deduced from the secondary/primary ratio men-
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HIGHLlGHT5 1 37
tioned above implies that the cosmic rays observed at the Earth
propagate in a region with an average density less than that of the
average interstellar medium in the disk This in turn suggests a
containment volume that includes a galactic halo region as well as the
disk. The interrelationship between the matter traversed by the parti-
cles and their age is dependent on the size of the storage volume for
cosmic rays. What is actually measured is the fraction of i°Be that
survives radioactive decay. This depends not only on the mean
cosmic-ray age but also on the distribution of ages, which is exponen-
tial in the leaky-box model but is more complicated in models in which
cosmic rays are stored in a large halo surrounding the galaxy. Further
information about cosmic-ray time scales and hence about the storage
volume will come from measurements of '°Be abundances at higher
energies and of other clock isotopes. These data, in conjunction with
electron and positron measurements, would be able to differentiate
between halo and local storage models and to place constraints on the
distribution of cosmic-ray sources in the galaxy.
Connection with Gamma and Radio Astronomy
The cosmic-ray composition studies discussed above give informa-
tion on the distribution of cosmic rays and matter in the galaxy that is
complementary to that obtained with gamma-ray and radio-astronomy
surveys. Diffuse gamma rays are generated by interactions of cosmic
rays with the interstellar gas; the nonthermal radio emission comes
from cosmic-ray electron synchrotron emission in the galactic mag-
netic fields. By studying this radiation we can also observe the cosmic
rays in localized galactic objects (supernova remnants) and in external
galaxies. These two different perspectives will be helpful in under-
standing the role that cosmic rays and the magnetic fields play in the
evolution and dynamics of astrophysical objects, from supernova
remnants to giant radio galaxies.
HIGH-ENERGY NUCLEAR AND PARTICLE PHYSICS
From the point of view of high-energy physics there are several
reasons to study cosmic rays: (1) to explore particle interactions at
energies much higher than those accessible at accelerators; (2) to study
processes involving neutrinos and high-energy nuclei that are also
inaccessible to present machines; and (3) to look for signals from the
early universe, such as a cutoff of cosmic rays above 102° eV due to the
3-K blackbody radiation, or the presence of antinuclei, which bears on
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138 COSMIC RA YS
the question of whether the universe is baryon symmetric on the largest
scales. In addition there is considerable scope for applying particle
physics to the study of cosmic-ray astrophysics, i.e., to determine the
chemical composition and energy spectra of the primary cosmic rays in
the high-energy region where the flux is too low for direct observation
of the primaries.
Different types of experiments are suited to the different regions of
the primary energy spectrum as determined by the flux. This is
indicated in Figure 15.1(B) in Chapter 15, which shows the integral flux
as a function of primary energy. A scale showing equivalent nucleon-
nucleon center-of-mass energies is superimposed. Note that the region
of the second-generation hadron colliders (one of which is already in
operation) to a large extent overlaps the 10~4-10'6 eV region, which
includes the astrophysically interesting region of the energy spectrum
referred to earlier.
Because of the steeply falling primary spectrum there is a natural
dividing line around 10~5 eV (or somewhat lower) between direct and
indirect experiments. The total flux above this energy is only about 2
particles per (m2 sr week) at the top of the atmosphere. Since the flux
decreases by about 2 orders of magnitude per decade increase in
energy, it will continue to be necessary to explore higher energies with
indirect, ground-based cascade experiments. Because of the antici-
pated direct measurements of primary composition to 10'4-10~5 eV,
coupled with current studies of hadron collisions in the same energy
region, there is now a good prospect for improving significantly our
ability to interpret the cascade measurements at the higher energies.
Nucleon Decay Experiments as Cosmic-Ray Detectors
Motivated by the particle-physics prediction of spontaneous decay
of the free (or bound) proton, large detectors have been designed and
built in this country and abroad that are sensitive to nucleon decay
lifetimes of as great as 1033 years. These large detectors represent a
unique opportunity to collect data on energetic muons and neutrinos
from cosmic rays. The characteristics of the U.S. detectors are noted
here together with specific comments on appropriate cosmic-ray ob-
servations and opportunities.
The largest operating proton-decay experiment employs an 8000-m3
volume of water located at a depth of 600 m, or 1570 m.w.e. (meters
water equivalent), in a salt mine near Cleveland, Ohio. Signals from
Cerenkov light produced by relativistic charged particles are detected
by photomultipliers that line the six surfaces of the tank on a 1-m grid.
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HIGHLIGHTS 1 39
Cosmic-ray neutrino interactions depositing energies of over 200 MeV
are detected at a rate of about one per day. The detector has been in
operation since August 1982.
Two other smaller proton-decay experimental programs have also
been carried out in the United States. At Park City, Utah, a 780-m3
water Cerenkov detector was operated at a depth of about 1700 m.w.e.
A 30-ton detector at the Soudan mine in northern Minnesota is at a
depth of 1800 m.w.e. and consists of a taconite-loaded cement with
proportional chambers as the sensitive elements. Although much
smaller than the other two detectors, its fine-grained tracking capability
has enabled the detector to search for possible sidereal anisotropies of
cosmic-ray multiple-muon events. A larger detector, Soudan 11, is
scheduled to be constructed in the same mine employing the same
general design philosophy.
An unusual experiment has been developed in the Homestake gold
mine in South Dakota. This detector consists of an array of plastic
tanks filled with liquid scintillator, which, when brought into full
operation, will have a sensitive mass of about 300 tons. It is located in
the deep underground cavern occupied by the solar neutrino experi-
ments. The primary objective of the experiment is to search for
neutrino bursts that could be signatures of supernova explosions. This
counter array is, of course, also sensitive to cosmic-ray muons. A
surface array is being added to study the air showers produced by the
same primary events that give rise to the detected muons. Although the
expected number of energetic muons increases with primary energy, at
fixed energy the muon multiplicity is correlated with the atomic weight
of the primary cosmic ray. Consequently, the surface shower data and
underground muon data together provide information concerning the
atomic weight of the primary cosmic-ray nucleus. The Homestake data
will be useful in studies of the mass spectrum of primary cosmic rays
in the energy range 10'4-10'6 eV; these energies are about an order of
magnitude greater than those accessible with the Cleveland proton-
decay detector owing to the greater depth of 1480 m (4200 m.w.e.) of
the Homestake mine.
Nucleus-Nucleus Collisions
The classic cosmic-ray emulsion technique has been modified into a
hybrid emulsion chamber with target material and electromagnetic
calorimeter sections (layers of plastic and lead, respectively, between
photosensitive layers). The first observation of charmed particles was
made over lO years ago with such detectors, and they have been
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140 COSMIC RAYS
adapted for use at accelerators to study charmed-particle spectroscopy
and lifetimes. Scientists are currently collaborating internationally on
the use of such emulsion chambers supplemented by electronic detec-
tors to study primary nuclear composition and properties of nucleus-
nucleus collisions. Several balloon flights have been carried out with
emulsion chamber payloads to explore primary cosmic rays in the
10~2-10~5 eV energy range. This energy range is well beyond that
accessible to current heavy-ion accelerators, and there are fundamen-
tal and novel questions accessible to this kind of cosmic-ray experi-
ment, in particular, the question of whether a new phase of quark-gluon
matter can be achieved in collisions between heavy nuclei at high
energy. Events in which heavy cosmic rays interact to produce nearly
1000 secondary particles have been observed. The energy-density
implied by such multiplicities has been calculated to be above the
threshold for production of a quark-gluon phase. Over 200 interactions
have been analyzed wherein the primary energy exceeds 10~2 eV.
Cross Sections, Spectra, Anisotropies, and Composition of
Primary Cosmic Rays Above 10~7 Electron Volts
Above 10~6 eV cosmic rays remain of interest for high-energy
physicists as well as for astrophysicists, at least until the operation of
a supercollider, which may be completed in the l990s. The goal here is
to determine both cross sections for hadron interactions and the
composition of the primaries. Recent measurements at the CERN pp
collider have confirmed earlier cosmic-ray estimates of the proton
cross section up to 10~4 eV (equivalent to center-of-mass energy of 500
GeV). New air-shower experiments have the potential to measure the
proton cross section and to determine the gross features of the primary
composition as well as in the 10~7-eV to 10~9-eV (center of mass about
100,000 GeV) range, where there may be a transition to extragalactic
cosmic rays.
The most ambitious cosmic-ray air-shower experiment in the United
States is the Fly's Eye experiment being carried out in Utah. This
detector consists of two arrays of photomultipliers deployed 3 km apart
to observe the air scintillation light produced by extensive air showers.
The phototubes are grouped in the focal plane of spherical mirrors, so
that the arrays provide a mosaic image of the sky, with each phototube
sensitive to a hexagonal cone of 5° of the celestial sphere. Timing
information is also available, so that an air shower is recorded as a
series of phototube "hits," with a pulse amplitude and relative time
recorded for each. The data are sufficient to reconstruct completely the
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HIGHLIGHTS 14 1
air shower in space and absolute magnitude. The Fly's Eye data have
two major strengths. First, the Fly's Eye covers or "sees" an effective
area comparable with the largest surface air-shower array; the current
detector is sensitive over an area of almost 100 km2, although data can
only be collected on clear, dark nights. Second, this detector permits
the observation of the longitudinal profile of the shower, hence
providing information on the height of the primary interaction and on
the rate of development of the shower. These data in turn may be
interpreted in terms of the inelastic cross section of protons at very
high energies and in terms of the primary nuclear-mass composition. It
may also be possible to relate the rate of development and shape of the
shower with the secondary-particle multiplicity and other inclusive
parameters of proton interactions.
The Fly's Eye experiment has achieved a major milestone by
directly observing the longitudinal development of individual cascades.
Present results from this experiment and other air-shower experiments
already suggest that the proton-air cross section is larger than 500 mb
at 10~8 eV, as compared with its low-energy value of 280 mb.
Magnetic Monopoles
Most Grand Unified Theories (GUTs) predict the existence of
massive magnetic monopoles, quanta of isolated north or south mag-
netic poles with discrete magnetic-pole strength. Their masses are
predicted to be of the order of 10~6 GeV (or about 0.01 Age, although
some models yield significantly lighter or heavier masses. In the
standard big-bang cosmology, GUT monopoles are produced at an
early stage of the universe. By contrast, in the inflationary-universe
scenario there would be no significant monopole production.
The density of monopoles in the universe today can be bounded by
arguments based on the openness of the universe and the mass of
missing, or dark, matter. Another astrophysical upper limit on the
monopole flux is based on the long-term stability of galactic magnetic
fields. Within these limits, monopoles may exist in the universe with
velocities in the range of lo-4 to 10-3 the velocity of light. At the lower
end of this velocity range, some theorists suggest that they could be
gravitationally bound to the solar system, which might enhance their
local abundances.
If GUT monopoles are able to catalyze proton decay, as suggested
by some current theories, they would produce copious x rays from
neutron stars. Our present failure to observe these x rays can be used
to set more stringent limits to the monopole flux for this specific
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142 COSMIC RA YS
monopole type. The proton-decay catalysis would also be detectable in
proton-decay experiments; thus far this process is not observed.
Searches for monopoles have been conducted with superconducting
coils and with ionization and scintillation detectors. In the former, a
monopole passing through a coil would induce a current step that is
readily detectable with sophisticated instrumentation. This technique
has the advantage that the monopole signal would be almost totally
independent of the monopole velocity. Such coils are limited, however,
in their size. Ionization and scintillation detectors can be made with
larger areas but are calculated to be insensitive to monopoles moving
slower than about 5 x 10-4 the velocity of light.
A signal consistent with a monopole interpretation was reported in
early 1982, using a superconducting coil. However, subsequent
searches by three groups (including the original 1982 author) have
failed to find further evidence for a monopole using the same tech-
nique. These searches have extended the sensitivity by almost a factor
of 100. In addition, data using scintillators have set still more stringent
limits on the flux over the velocity range accessible to them. Although
the 1982 event remains unexplained, the monopole hypothesis for that
event now seems unlikely.
Representative terms from entire chapter:
interstellar medium
