Cosmic rays Electrons and the nuclei of atoms—largely hydrogen—that impinge upon Earth from all directions of space with nearly the speed of light. These nuclei with relativistic speeds are often referred to as primary cosmic rays, to distinguish them from the cascade of secondary particles generated by their impact against air nuclei at the top of the terrestrial atmosphere. The secondary particles shower down through the atmosphere and are found all the way to the ground and below.
The primary cosmic rays provide the only direct sample of matter from outside the solar system. Measurement of their composition can aid in understanding which aspects of the matter making up the solar system are typical of the Milky Way Galaxy as a whole and which may be so atypical as to yield specific clues to the origin of the solar system. Cosmic rays are electrically charged; hence they are deflected by the magnetic fields which are thought to exist throughout the Galaxy, and may be used as probes to determine the nature of these fields far from Earth. Outside the solar system the energy contained in the cosmic rays is comparable to that of the magnetic field, so the cosmic rays probably play a major role in determining the structure of the field.
Collisions between the cosmic rays and the nuclei of the atoms in the tenuous gas which permeates the Galaxy change the cosmic-ray composition in a measurable way and produce gamma rays which can be detected at Earth, giving information on the distribution of this gas.
Cosmic-ray detection. All cosmic-ray detectors are sensitive only to moving electrical charges. Neutral cosmic rays (neutrons, gamma rays, and neutrinos) are studied by observing the charged particles produced in the collision of the neutral primary with some type of target. At low energies the ionization of the matter through which they pass is the principal means of detection. A single measurement of the ionization produced by a particle is usually not sufficient both to identify the particle and to determine its energy. However, since the ionization itself represents a significant energy loss to a low-energy particle, it is possible to design systems of detectors which trace the rate at which the particle slows down and thus to obtain unique identification and energy measurement.
At energies above about 500 MeV per nucleon, almost all cosmic rays will suffer a catastrophic nuclear interaction before they slow appreciably. An ionization measurement is commonly combined with measurements of physical effects which vary in a different way with mass, charge, and energy. Cerenkov detectors and the deflection of the particles in the field of large superconducting magnets or the magnetic field of the Earth itself provide the best means of studying energies up to a few hundred GeV per nucleon. Detectors employing the phenomenon of x-ray transition radiation promise to be useful for measuring composition at energies up to a few thousand GeV per nucleon.
Above about 1012 eV, direct detection of individual particles is no longer possible since they are so rare. Such particles are studied by observing the large showers of secondaries they produce in Earth's atmosphere. These showers are detected either by counting the particles which survive to strike ground-level detectors or by looking at the flashes of light the showers produce in the atmosphere with special telescopes and photomultiplier tubes.
Atmospheric cosmic rays. The primary cosmic-ray particles coming into the top of the terrestrial atmosphere make inelastic collisions with nuclei in the atmosphere. When a high-energy nucleus collides with the nucleus of an air atom, a number of things usually occur. Rapid deceleration of the incoming nucleus leads to production of pions with positive, negative, or neutral charge. A few protons and neutrons (in about equal proportions) may be knocked out with energies up to a few GeV. They are called knock-on protons and neutrons.
All these protons, neutrons, and pions generated by collision of the primary cosmic-ray nuclei with the nuclei of air atoms are the first stage in the development of the secondary cosmic-ray particles observed inside the atmosphere. Since several secondary particles are produced by each collision, the total number of energetic particles of cosmic-ray origin will increase with depth, even while the primary density is decreasing.
The uncharged n0 mesons decay into two gamma rays with a life of about 8 x 10-17 s. The two gamma rays each produce a positron-electron pair. Upon passing sufficiently close to the nucleus of an air atom deeper in the atmosphere, the electrons and positrons convert their energy into bremsstrahlung. The bremsstrahlung in turn create new positron-electron pairs, and so on. This cascade process continues until the energy of the initial n0 has been dispersed into a shower of positrons, electrons, and photons with insufficient individual energies (< 1 MeV) to continue the pair production. The electrons and photons of such showers are referred to as the soft component of the atmospheric (secondary) cosmic rays.
The n ± mesons produced by the primary collisions have a life of about 2.6 x 10-8 s before they decay into muons. Most low-energy n ± decay into muons before they have time to undergo nuclear interactions. Except at very high energy (above 500 GeV), muons interact relatively weakly with nuclei, and are too massive (207 electron masses) to produce bremsstrahlung. They lose energy mainly by the comparatively feeble process of ionizing an occasional air atom as they progress downward through the atmosphere. Because of this ability to penetrate matter, they are called the hard component.
The high-energy nucleons—the knock-on protons and neutrons—produced by the primary-particle collisions and a few pion collisions proceed down into the atmosphere. They produce nuclear interactions of the same kind as the primary nuclei, though of course with diminished energies. This cascade process constitutes the nucleonic component of the secondary cosmic rays.
Solar modulation. The cosmic-ray intensity is lower during the years of high solar activity and sunspot number, which follow an 11-year cycle. This effect has been extensively studied with ground-based and spacecraft instruments.
The primary cause of solar modulation is the solar wind, a highly ionized gas (plasma) which boils off the solar corona and propagates radially from the Sun at a velocity of about 250 mi s (400 km/s). The wind is mostly hydrogen, with typical density of 80 protons per cubic inch (5 protons per cubic centimeter). This density is too low for collisions with cosmic rays to be important. Rather, the high conductivity of the medium traps part of the solar magnetic field and carries it outward.
In addition to the bulk sweeping action, another effect of great importance occurs in the solar wind, adiabatic deceleration. Because the wind is blowing out, only those particles which chance to move upstream fast enough are able to reach Earth. However, because of the expansion of the wind, particles interacting with it lose energy. Thus, particles observed at Earth with energy of 10 MeV per nucleon actually started out with several hundred MeV per nucleon in nearby interstellar space, and those with initial energy of only 100-200 MeV per nucleon probably never reach Earth at all.
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