Gravitation, Cosmology, and Cosmic-Ray Physics (1986) / Chapter Skim
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16. Highlights
16. Highlights
Pages 121-142
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From page 121... ...
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.
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From page 122... ...
These include the following: · Measurements of secondary products of cosmic-ray nuclear interactions 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)
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From page 123... ...
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.
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From page 124... ...
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 determined from meteorites or from optical observations of the Sun, can best be measured in solar cosmic-ray composition studies.
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From page 125... ...
(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.
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From page 126... ...
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.
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From page 127... ...
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
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From page 128... ...
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.
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From page 129... ...
An alternate viewpoint suggests that the standard solar-system abundances are in fact not correctly representative 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 fractionation in the acceleration processes.
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From page 130... ...
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.
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From page 131... ...
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.
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From page 132... ...
We know from measurements 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)
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From page 133... ...
For example if it is a consequence of the diffusion and convection processes by which cosmic rays are transported 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.
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From page 134... ...
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.
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From page 135... ...
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 radiative interactions with the interstellar magnetic and photon fields.
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From page 136... ...
Thus, positron observations would lead to independent determinations of the confinement 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.
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From page 137... ...
Connection with Gamma and Radio Astronomy The cosmic-ray composition studies discussed above give information 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 magnetic fields.
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From page 138... ...
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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From page 139... ...
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.
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From page 140... ...
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 fundamental and novel questions accessible to this kind of cosmic-ray experiment, in particular, the question of whether a new phase of quark-gluon matter can be achieved in collisions between heavy nuclei at high energy.
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From page 141... ...
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.
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From page 142... ...
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.
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