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See B. B. Rossi, Cosmic Rays (1964); L. I. Dorman, Cosmic Rays (1974); M. W. Friedlander, Cosmic Rays (1989).
cosmic raysHighly energetic particles that move through space at close to the speed of light and that continuously bombard the Earth's atmosphere from all directions. They were discovered by V.F. Hess during a balloon flight in 1912. The energies of the individual particles are immense, ranging from about 108 to over 1019 electronvolts (eV). The intensity (or particle flux) of cosmic rays is very low; it is greatest at low energies – several thousand particles per square meter per second – dropping with increasing energy but apparently leveling off at high energies. Cosmic rays are primarily nuclei of the most abundant elements (mass numbers up to 56) although all known nuclei are represented. Protons (hydrogen nuclei) form the highest proportion; heavy nuclei are very rare. Also present are a small number of electrons, positrons, antiprotons, neutrinos, and gamma-ray photons. All these particles are known collectively as primary cosmic rays.
On entering the Earth's atmosphere the great majority collide violently with atomic nuclei in the atmosphere producing secondary cosmic rays; these consist mainly of elementary particles. The large number of particles produced from one such collision form an extensive air shower (or simply an air shower), in which there is a very rapid, highly complex, but well-defined sequence of reactions. The initial products are principally charged and neutral pions. The neutral pions decay (disintegrate) almost immediately into gamma rays, which give rise to cascades of electrons and positrons by pair production and to more photons by bremsstrahlung radiation from the decelerating electrons and positrons. The charged pions decay into muons, which subsequently decay into neutrinos and electrons. The original primary cosmic ray nucleus, and many of the secondary cosmic rays, undergo more collisions to create further generations of particles. Following the initial collision the number of particles in the shower increases to a maximum at some point high in the atmosphere, where the creation of new particles is balanced by absorption; the number then decreases. Very few primary cosmic rays reach the Earth's surface, the final products including electrons, muons, neutrinos, gamma rays, and some initial products of the primary collision. The maximum number of particles in an air shower depends on the energy of the primary cosmic ray: half a million particles can be produced at maximum development by a primary nucleus of 1015 eV. It is, however, a major problem to distinguish air showers produced by different primary nuclei of the same energy.
Lower-energy primary cosmic rays (<1013 eV), i.e. those with the largest intensity, can be studied directly by sending particle detectors, such as scintillation counters, above the Earth's atmosphere in satellites, spaceprobes, rockets, and balloons. High-energy particles (>1016 eV) have too low a flux for direct measurements and can be studied only through their extensive air showers using arrays of detectors at ground level; particles with energies exceeding 1020 eV have been detected with giant arrays of instruments covering areas greater than one km2. Intermediate energies require sensitive detector arrays on mountaintops or large detectors in satellites.
Most of the lighter primary cosmic-ray nuclei are considered products of collisions of heavier nuclei that occur during their journey from source. These parent nuclei, with other heavy nuclei and most of the primary electrons, are created in some as yet unknown nucleosynthesis process of catastrophic origin. Medium and lower energy particles probably originate in supernova explosions: both the Crab nebula and the supernova remnant of the Vela pulsar are thought to be sources. Shock waves associated with supernovae could provide the acceleration. The extraordinary acceleration processes that could produce the highest energies are not yet identified. There is no evidence as yet for a maximum energy for cosmic rays, although this has been predicted.
It is generally believed that almost all cosmic rays with energies less than about 1018 eV are generated by sources within the Galaxy. It is also thought that these particles are confined to the Galaxy for probably tens of millions of years by the complex and very weak galactic magnetic field. Cosmic-ray particles, being charged, are affected by a magnetic field: they are deflected from their initial paths and become trapped in the galactic field, the lowest energies being most deflected. During their long confinement their directions of travel become almost uniformly scattered: these cosmic rays are nearly isotropic. As cosmic rays enter the Solar System they can be further scattered by magnetic irregularities in interplanetary space: the total intensity at the Earth's orbit is twice as great at sunspot minimum than at sunspot maximum.
Although lower-energy cosmic rays are confined very effectively, higher-energy particles tend to ‘leak out’ of the Galaxy; above a critical energy at which the effect of the magnetic field becomes negligible, they can all escape. Thus the cosmic-ray intensity should decrease with energy. The intensity, however, levels off at the highest energies, and the isotropy decreases; this has been taken as evidence of other sources of cosmic rays outside the Galaxy. Since high-energy particles travel in approximately straight lines (being almost unaffected by magnetic fields) their arrival direction should indicate their source direction: measurements made with the giant detector arrays point to sources well away from the galactic plane, i.e. to possible extragalactic sources. Gamma rays can arise in space from the interaction of primary cosmic rays with interstellar matter. It is hoped that measurements of the distribution of gamma rays in space, especially those of high energies (>1018 eV), will reflect the distribution of cosmic rays and thus reveal the galactic or extragalactic nature of their sources.
the flow of high-energy particles, mainly pro-tons, that is incident on the earth from outer space (primary radiation), as well as the secondary radiation produced by the particles as a result of interaction with atomic nuclei. Virtually all known elementary particles are encountered in secondary radiation.
Cosmic rays are a unique natural source of high-energy and ultrahigh-energy particles, which make possible the study of processes of transformation of elementary particles and of their structure. They also provide the opportunity to detect and study the large-scale astrophysical processes associated with the acceleration and propagation of the particles of cosmic radiation in interplanetary, interstellar, and possibly intergalactic space.
Most particles of primary cosmic radiation have an energy greater than 109 electron volts (eV), or 1 giga electron volt (GeV), and the energy of some particles reaches 1020–1021 eV, and perhaps even higher. Before the development of powerful charged-particle accelerators, cosmic rays were the only source of high-energy particles. Many previously unknown elementary particles were first detected in cosmic rays, and the first data on their decay and interaction with atomic nuclei were obtained from them. Although modern accelerators, particularly colliding-beam accelerators, make possible the careful study of the processes of particle interaction up to energies of 1011-1012 eV, cosmic rays remain the only source of information on particle interactions at still higher energies.
Most primary cosmic rays reach the earth from outside the solar system—from the surrounding Milky Way Galaxy. These are galactic cosmic rays; only a small part of the primary cosmic rays—solar cosmic rays, mainly those of moderate energies (less than 1 GeV)—are associated with the activity of the sun. However, in periods of high solar activity brief, strong upsurges in the flow of solar cosmic rays may take place in interplanetary space. The particles with the highest energy (> 1017 eV) may be of extragalactic origin (from the metagalaxy).
The total energy flux incident on the earth from cosmic rays (of the order of 0.01 erg/cm2per sec) is extremely small in comparison with the flow of solar energy radiated to the earth and is comparable to the energy of the visible radiation of stars. However, the possibility that in the remote past cosmic rays played some role in speeding up the evolution of life on earth has not been excluded.
On the galactic scale the average energy density of cosmic rays is great (∽1 eV/cm3)—of the order of the densities of all other types of energy: the energy of gravitation, the energy of magnetic fields, the kinetic energy of motion of interstellar gas, and the energy of the electromagnetic radiation of stars. Therefore, cosmic rays may have an appreciable impact on the evolution of our galaxy as a whole.
Two main research trends clearly stand out in the physics of cosmic rays: the nuclear-physics trend (the interaction of cosmic rays with matter and the generation, properties, and interactions of elementary particles) and the cosmic-physics trend (the composition and energy spectrum of primary cosmic rays, the generation and propagation of solar and galactic cosmic rays, the change of the intensity of cosmic rays over time, and the interaction of cosmic rays with the earth’s magnetosphere, the solar wind, and shock waves in interplanetary space). As accelerator technology develops, research in the first trend is gradually shifting toward high energies. The increasingly thorough study of near space by direct methods involving satellites and rockets is shifting the center of the second trend toward more remote heavenly bodies. Therefore, the scientific results obtained by using cosmic rays are usually of an exploratory and original nature and are of fundamental importance both for the development of the physics of the microcosm (in the region of characteristic dimensions ≤ 10–13 cm) and for the development of space physics (108–1028cm).
Discovery and main stages of the study of cosmic rays. The existence of cosmic rays was established in 1912 by V. Hess on the basis of ionization of air molecules that they produced. The increase in ionization with altitude proved their extraterrestrial origin. Observations of the tracks of the particles of cosmic rays in a cloud chamber placed in the field of a laboratory magnet (D. V. SkobePtsyn, 1927) and of their deflections in the earth’s magnetic field by means of gas-discharge detectors lifted into the stratosphere by balloons (S. N. Vernov and R. Millikan, 1935–37) proved that primary cosmic rays are a flux of charged particles, mainly protons (the nuclei of hydrogen atoms). The energy of most of the cosmic rays (up to 15 GeV) was measured in this way. The tracks of the nuclei of elements heavier than hydrogen, up to iron nuclei, have been detected by means of nuclear photographic emulsions raised to an altitude of about 30 km (B. Peters and others, 1948).
The detailed study of the charges and masses of particles of secondary cosmic rays led to the discovery of many new elementary particles, in particular the positron, muon, pi-meson, kaon, and A-hyperon (1932–49). In 1932, P. Blackett and G. Occhialini first observed in a cloud chamber groups of genetically related particles of cosmic radiation that were close in direction—what are called showers. In experiments conducted in the period 1945–49 at high-altitude cosmic-ray research stations (by V. I. Veksler, N. A. Dobrotin, and co-workers) and in the stratosphere (by S. N. Vernov and co-workers) it was determined that secondary cosmic radiation is formed as a result of the interaction of primary cosmic rays with the nuclei of atoms in the air. G. T. Zatsepin later showed that the same mechanism, but at higher energies (≥ 1014 eV), explains the development of the extended air showers—fluxes of many millions of particles that cover areas of the order of one sq km or more at sea level—that had been discovered earlier in cosmic rays by P. Auger (1938).
Advances in radio astronomy played a major role in developing a proper approach to the problem of the origin of cosmic rays. The nonthermal cosmic radio-frequency radiation associated with cosmic rays made possible detection of their possible sources. In 1955, on the basis of radio-astronomical observations and energy estimates, V. L. Ginzburg and I. S. Shklovskii gave the first quantitative substantiation of the hypothesis of supernovas as one of the primary galactic sources of cosmic rays.
The extensive worldwide network of cosmic-ray research stations (more than 150) organized in the 1950’s and 1960’s, at which continuous recording of cosmic radiation is conducted, has been the basis for the cosmic-physics trend of research. Many stations are located high in the mountains, and at some stations underground observations are made and balloons bearing devices that automatically record cosmic rays are regularly sent into the stratosphere.
New possibilities for the direct study of primary cosmic rays in a very broad energy range have opened up in connection with the use of recording equipment on artificial earth satellites and unmanned interplanetary probes. In particular, the energy spectrum of primary cosmic rays up to ∽1015 eV was measured directly for the first time by means of an ionization calorimeter carried by satellites of the Proton series (the Soviet physicist N. L. Grigorov and co-workers, 1965–69). Later, prolonged measurements of variations in the composition and intensity of cosmic rays outside the earth’s magnetosphere were made by artificial satellites of the moon and Mars and by the Soviet Lunokhod 1 (1970–71).
Primary galactic cosmic rays; geomagnetic effects. All experimental data agree that the flux of primary cosmic rays reaching the earth from our galaxy is isotropic to a high degree of accuracy (—0.1 percent)—that is, it is independent of direction. Upon entering the earth’s magnetic field, the charged particles of cosmic radiation are deflected from their original direction as a result of the action of the Lorentz force. Therefore, the intensity
and energy spectrum of cosmic rays in near space depend both on the geomagnetic coordinates of the observation site and on the direction of arrival of the cosmic rays. The greater the angle θ between the direction of motion of a particle and the direction of the field’s line of force—that is, the lower the geomagnetic latitude ϕ of the point of observation—the more strongly the deflecting action of the geomagnetic field is manifested. Thus, for a given particle energy the deflection is maximal in equatorial regions and minimal near the magnetic poles. Near the equator the “geomagnetic barrier” does not pass protons with an energy of less than ∽15 GeV and nuclei with an energy of less than ∽7.5 GeV per nucleon (a proton or neutron) that are perpendicular to the surface of the earth. As the geomagnetic latitude increases, the threshold energy of the particles decreases rapidly (∽cos4 ϕ), and in the polar regions the geomagnetic barrier is virtually absent. In addition to the regular latitude dependence, anomalies of the geomagnetic field (especially in the region of the South Atlantic) appreciably affect the intensity of cosmic rays. As a result, the distribution of the intensity of cosmic rays over the earth is very complex (see Figure 1). In the polar regions (ϕ ≧ 60°) the intensity of cosmic rays at the boundary of the atmosphere in years of minimal solar activity is about 0.4 particles/cm2/sec per unit of solid angle.
As the energy of cosmic rays increases, their intensity de-creases, at first slowly and then with increasing rapidity (Figure 2,a). At energies of 1010–1015 eV the flux of particles with an energy greater than some given energy ℰ (the integral spectrum) decreases as—∽ℰ–1.7 (Figure 2,b). In the region of energies greater than 1015 eV, data on extended air showers ( see below) are the only source of information on the energy spectrum of cosmic rays (Figure 2,c). This spectrum cannot be represented by a single exponential law, since it contains some metagalactic cosmic rays.
More than 90 percent of the particles of primary cosmic rays of all energies are protons, approximately 7 percent are alpha particles, and the nuclei of elements heavier than hydrogen and helium account for only a small fraction (∽ 1 percent). Despite this, nuclei with Z > 1 carry about 50 percent of the energy in cosmic rays. The decrease in the abundance of an element progresses more slowly in cosmic rays with an increase in atomic number than for the matter in heavenly bodies throughout the universe. The content of the nuclei of the light elements lithium (Li), beryllium (Be), and boron (B), whose natural abundance is extremely small (≦10–7 percent), is especially great in cosmic rays. There is also an excess of heavy nuclei (Z≧6). From this it follows that the acceleration of heavy nuclei predominates in the sources of cosmic rays, whereas lighter nuclei arise as a result of the disintegration of heavy nuclei (fragmentation) upon interaction with interstellar matter. In the period 1966–71 nuclei much heavier than iron—up to uranium, and perhaps even heavier nuclei—were detected in cosmic rays by using nuclear photographic emulsions and solid-state charged-particle detectors. With an increase in Z, the fluxes of such nuclei attenuate approximately as Z–7–Z–8. In the most thoroughly studied energy region (> 2.5 GeV per nucleon), the nuclear composition of cosmic rays is as follows: protons, about 92 percent; alpha particles, about 7 percent; nuclei with Z = 3–5, about 0.1–0.15 percent; Z = 6–9, about 0.5 percent; Z = 10–15, about 0.1–0.15 percent; Z = 16–25, about 0.04 percent; Z = 26 (iron), 0.025 percent; and Z > 30, ∽10–5 percent.
The average quantity of matter through which cosmic rays pass en route from their sources to the earth has been estimated at 3–5 g/cm2 on the basis of the presence in cosmic rays of Li, Be and B, which are not present in the sources (they burn up quickly as a result of the thermonuclear reactions that take place in stars) and are formed only as a result of fragmentation. Hence, if the average density of matter in the galaxy is known, the distance traveled by cosmic rays in our galaxy and the average lifetime of cosmic rays may be estimated.
Among the components of primary cosmic rays are also electrons and positrons (∽1 percent) and high-energy photons— gamma quanta (∽0.01 percent at energies greater than 100 MeV). Despite their insignificant share in cosmic rays, gamma quanta are of particular interest, since they are not deflected by the magnetic fields of interstellar space and thus make possible detection of certain quasi point sources of cosmic rays. About 20 such sources have been found already. The most interesting of these is the pulsar NP 0532 in the Crab Nebula, which produces a flux of 0.1–0.5 gamma quanta/m2/sec and at the same time is a powerful pulsating X-ray source. In addition, a diffuse flux of gamma quanta from the center of the galaxy, with an intensity of ∽ 1 particle/m2/sec per unit of solid angle, has been detected.
Within the earth’s magnetosphere, at altitudes of ≳1,000 km, much more intense fluxes of protons and electrons that are captured by the geomagnetic field and form the earth’s radiation belt are present in addition to the cosmic-ray flux. The origin of the internal region of the radiation belt is due mainly to the reflux (albedo) of neutrons dislodged by cosmic rays from the nuclei of the atoms of the earth’s atmosphere: the neutrons decay into protons and electrons, which are confined in the natural magnetic trap of the earth’s magnetosphere.
Solar cosmic rays. The strongest increases in the intensity of cosmic rays, in the form of brief irregular outbursts, are associated with chromosphere flares on the sun. Acceleration of the charged particles of the solar plasma by electromagnetic fields (apparently at the boundaries of the sunspots)—that is, the
generation of solar cosmic rays—takes place during such flares. In particular, a highly probable mechanism for the acceleration of particles by the electric fields induced during the rapid convergence of regions of the solar plasma on oppositely oriented magnetic fields has been proposed by the Soviet physicist S. I. Syrovatskii (1965).
During some chromospheric flares the fluxes of solar cosmic rays are hundreds of times more powerful than the fluxes of galactic cosmic rays. For example, during the record flare of Feb. 23, 1956, a 300-fold increase in the flux of cosmic rays with an energy of more than 3 GeV was observed. This could have represented a serious threat to the safety of space flights. Therefore, systematic observations of chromospheric flares, outbursts of radio-frequency and X radiation, and other manifestations of solar activity that, in close conjunction with measurements of the intensity of cosmic rays, make possible the prediction of radiation conditions on the routes of space flights are very important.
On the average, the contribution of solar cosmic rays to the total intensity of cosmic radiation is several percent.
The chemical composition of solar cosmic rays is very close to the composition of the solar atmosphere. In contrast to galactic cosmic rays, they contain no Li, Be, or B nuclei. This shows that the quantity of matter penetrated by solar cosmic rays is extremely small (< 0.1 g/cm2) and that they cannot be generated in the depths of the solar atmosphere, where the density of matter is too great (the acceleration most likely takes place in the outer chromosphere and inner corona of the sun).
The particles of solar cosmic rays have lower energies than those of galactic cosmic rays (their energy spectrum is softer). The energies of protons usually are limited to fractions of a giga electron volt, and protons with energies up to 100 GeV are generated only during rare powerful chromosphere flares. The lower boundary of the energy of the recorded electrons of solar cosmic rays is of the order of tens of kilo electron volts (that is, it is close to the energy of the particles of the solar wind). Low-energy solar cosmic rays have a significant impact on the condition of the earth’s ionosphere at high latitudes, inducing additional ionization of its lower layers. This leads to the attenuation of radio waves and, in some cases, to total loss of shortwave radio communications. Data on the propagation, energy spectrum, and angular anisotropy of solar cosmic rays provide information on the structure of the magnetic field in interplanetary space. The study of spatial and temporal variations in fluxes of solar cosmic rays is helpful in gaining a better understanding of such geophysical phenomena as geomagnetic storms and the aurora polaris.
The nature of the increase in the flux of solar cosmic rays to earth shows that in the initial period after a flare the flux is largely anisotropic, and its maximum is directed at an angle of approximately 45° to the west of the direction of the sun. This was the first direct proof of the curvature of the lines of force of the interplanetary magnetic field in the form of Archimedes spirals (see Figure 3).
Modulation of galactic cosmic rays by the solar wind. Among the periodic variations with time in the intensity of galactic cosmic rays, modulations of intensity that coincide with the 11-year cycle of solar activity play the primary role. The modulations are related to the scattering and “sweeping” of cosmic rays of galactic origin by nonuniformly magnetized regular fluxes of the plasma ejected from the sun at velocities of 300–500 km/sec. Such fluxes, which are called the solar wind, are radiated far beyond the earth’s orbit (to distances of tens of astronomical units [AU]; 1 AU ≈ 150 million km), gradually passing into the turbulent motion of plasma in the boundary layer with an unperturbed galactic magnetic field (Figure 3). According to data on the last two cycles (1948–59 and 1959–70), during peak solar activity the intensity of cosmic rays near the boundary of the earth’s atmosphere declines by 50–60 percent in comparison with the magnitude characteristic of minimum activity. The amplitude of the 11-year variations in cosmic rays proves to be much less at sea level, to which low-energy particles do not penetrate (Figure 4).
There are also other, less marked types of modulation of galactic cosmic rays that are due to various factors. They include, in particular, the 27-day variations connected with the period of the sun’s rotation around its axis, as well as the daily solar variations associated with the earth’s rotation and with the anisotropy of the electromagnetic properties of the medium in which cosmic rays propagate. The aggregate of the information concerning modulation effects leads most investigators to the conclusion that the effective dimensions of the region in which cosmic rays are modulated by the solar wind is 2–5 AU.
Origin and age of galactic cosmic rays. Supernova explosions are considered to be the primary source of cosmic rays. During each such explosion expansion of the shell of the star takes place at tremendous velocity and shock waves that cause acceleration of charged particles to energies of ∽ 1015 eV and higher arise in the plasma. Direct radio-astronomical observation of the partially polarized radio-frequency radiation from the Crab Nebula (1957), which resulted from the explosion in A.D. 1054 of a supernova comparatively close to the solar system, was the main experimental argument in favor of the hypothesis that cosmic rays originate in supernova explosions. The properties of this radiation are such that it should be attributed to synchrotron radiation (magnetic bremsstrahlung)—the radiation of fast electrons in magnetic fields that are “frozen” into fluxes of stellar plasma ejected during the explosion of the supernova. The observation of magnetic bremsstrahlung radio-frequency radiation from other, more distant nebulas produced by supernova explosions became possible subsequently. Further observations showed that the spectrum of the bremsstrahlung electron radiation extends to the optical, X-ray, and even gamma-ray bands and is associated with very high electron energies (up to ∽ 1012 eV).
It is natural that intensive acceleration of heavy charged particles—protons and nuclei (which, however, because of their great mass do not experience significant energy losses to radiation in the magnetic fields)—also takes place in the expanding shells of supernovas, in addition to the acceleration of electrons. Here, the heavier the nucleus the more favorable may be the initial conditions of acceleration (called injection): heavy nuclei may be in a partially ionized state and therefore may be deflected comparatively slightly in magnetic fields. This facilitates the “leakage” of such nuclei out of the dense stellar shell, in which the magnetic field is great. If we take into account the average frequency of supernova explosions in the galaxy as a whole (once every 30–50 years) and the total energy release of each explosion (1051–1052 ergs, or 1063–1064 eV) and assume that approximately 1 percent of the energy is lost to the acceleration of charged particles, then it is possible to explain both the average energy density of cosmic rays (∽ 1 eV/cm3) and the absence of appreciable fluctuations in the cosmic-ray flux.
Methods of radio astronomy have also been used to record more powerful sources of cosmic rays (or, more accurately, their electron component) that are located far beyond the boundaries of our galaxy. These source are, in particular, the intensely radiant small quasi-stellar objects—quasars, the nuclei of some galaxies that experience abrupt explosive-type expansion, and radio galaxies, with their characteristic powerful ejections of matter (accompanied by radio emissions on the scale of entire galaxies).
Heavy charged particles that have been accelerated in galactic sources then propagate along complex trajectories in interstellar space, where the weak (3 X 10–6 to 6 X 10–6 gauss) irregular and nonuniform magnetic fields of the clouds of the interstellar plasma act on them. The charged particles are “lost” in these magnetic fields, whose intensity increases significantly in the regions of the spiral arms of the galaxy, simultaneously with the increase in the concentration of interstellar plasma. Here the motion of cosmic rays has the character of diffusion, during which particles of energies of up to 1017–1018 eV may be confined within our galaxy for tens of millions of years. The diffusion motion of cosmic-ray particles results in almost complete isotropy of the flux of cosmic particles. Only at higher energies do the radii of curvature of the trajectories of the particles (especially protons) become comparable to the dimensions of galaxies and intense “leakage” of cosmic rays into metagalactic space takes place. Despite the high degree of rarefaction of matter, the long voyages of the particles in the metagalaxy lead to energy losses in new processes—photonuclear reactions with the background electromagnetic radiation (called relict radiation) that remains from the early stages of expansion of the once hot universe. The presence of this process greatly reduces the probability that the most energetic part of the cosmic-ray spectrum is due to the metagalactic component.
Fundamentally new possibilities for the experimental study of sources of the most energetic part of the cosmic-ray spectrum (up to energies of 1020–1021 eV) opened up after the detection of unique astrophysical objects, pulsars. According to current concepts, pulsars are small neutron stars (diameter, about 10 km) that formed as a result of the rapid gravitational compression (gravitational collapse) of unstable supernova-type stars. Gravitational collapse leads to a tremendous increase in the density of a star’s matter (to the density of the nucleus and beyond), magnetic field (up to 1013 gauss), and rate of rotation (up to 103 revolutions per sec). All these factors create favorable conditions for the acceleration of heavy charged particles to exceptionally high energies (of the order of 1021 eV) and of electrons to energies of about 1012 eV. Indeed, observations have demonstrated that, in addition to radio-frequency radiation pulsars emit (with the same period) light, X-ray, and sometimes gamma radiation, which may be explained only in terms of the process of magnetic bremsstrahlung radiation of very fast electrons. Thus, the synchrotron radiation of cosmic-ray electrons, which is due to strong magnetic fields localized near unstable “hot” objects—the sources of cosmic rays—makes it possible to answer the question of the origin of cosmic rays by the methods of observational astronomy (radio astronomy, X-ray astronomy, and gamma-ray astronomy).
Research on the nuclear composition of cosmic rays provides important additional information on the sources and age of cosmic rays. From the low relative content of beryllium nuclei in cosmic rays it follows that the radioactive isotope 10Be, whose average lifetime is about 2 million years, is able to decay almost entirely—hence the estimate of 20–50 million years for the outer limit of the age of cosmic rays. Estimates of roughly the same order (10–30 million years) are obtained from the relative con-tent of the group of light nuclei (Li, Be, and B) as a whole and also on the basis of the average time required by cosmic-ray electrons for diffusive propagation from intragalactic sources to the boundaries of the galaxy. Analysis of the superheavy nuclear component (Z > 70) gives an average age of no more than 10 million years for cosmic rays.
There is yet another method of testing various hypotheses of the origin of cosmic rays—measurement of the intensity of cosmic rays in the remote past, especially during periods of known explosions of nearby supernovas (such as the explosion of A. D. 1054). There are two methods that make possible detection of the effects of an increase in the intensity of cosmic rays in the past not only as a result of the explosion of supernovas that are comparatively close to the solar system but also as a result of possible explosive processes of much greater power in the galactic nucleus. These are the radiocarbon method, in which the rate of accumulation in the atmosphere of 14C, which forms as a result of nuclear reactions on exposure to cosmic rays, is determined from the concentration of the isotope 14C in various annual rings of very old trees, and the meteorite method, based on the study of the composition of the stable and radioactive isotopes of meteoritic matter that has undergone prolonged exposure to cosmic rays. These methods indicate that the average intensity of cosmic rays differed comparatively little from the current intensity for tens of thousands and a billion years, respectively. The constancy of the intensity of cosmic rays over a billion years makes improbable the hypothesis that all cosmic rays originated in the process of the explosion of the nucleus of our galaxy, which is considered to be responsible for the formation of the galactic corona (as yet unproven by direct observations).
Interaction of cosmic rays with matter. NUCLEAR-ACTIVE COMPONENT OF COSMIC RAYS AND MULTIPLE PRODUCTION OF PARTICLES. Upon interaction of protons and other nuclei of high-energy primary cosmic rays (of the order of several giga electron volts and higher) with nuclei of the atoms of the earth’s atmosphere, mainly nitrogen and oxygen, the disintegration of nuclei and the formation of several unstable elementary particles (so-called multiple processes), primarily pi-mesons (pions), both charged (π+ and π–) and neutral (π0), with lifetimes of 2.5 X 10–8 sec and 0.8 X 10–16 sec, respectively, take place. Kaons are produced with a much lower probability (by a factor of 5–10), and hyperons and resonances that decay almost instantaneously are produced with a still lower probability. A photograph of the multiple production of particles recorded in a nuclear photoemulsion is presented in Figure 5. The particles emerge from one point in the form of a narrow beam. The average number of secondary particles formed in a single interaction of a proton (or pi-meson) with a light nucleus or with one nucleon of such a nucleus increases with the energy ℰ, at first according to an exponential law close to ℰ⅓ (up to ℰ≈20 GeV) and then (in the region of 2 X 1010–1013 eV) more slowly, in which case it is better described by a logarithmic relation. At the same time, indirect data on extended air showers indicate processes of much greater multiple production at energies of ≧ 1014 eV.
In a broad range of energies of primary and generated particles the angular directivity of the flux of generated particles is such that the momentum component perpendicular to the direction of the primary particle (the transverse momentum) is on the average 300–400 MeV/ c, where c is the speed of light in a vacuum (at very high particle energies ℰ, when the rest energy of the particle, mc2, is negligible in comparison with the kinetic energy, the particle’s momentum is p = ℰ/ c; therefore, in high-energy physics momentum is usually measured in units of MeV/ c).
In a collision primary protons lose an average of about 50 percent of their initial energy (in the process they may undergo charge exchange, becoming neutrons). The secondary nucleons (protons and neutrons) formed upon disintegration of the nuclei and the high-energy charged pions produced in the collisions also will take part in nuclear interactions, together with the primary protons that have lost part of their energy, and cause disintegration of nuclei of atoms of the air and the multiple production of pions. The mean path over which one nuclear interaction is accomplished is usually measured in terms of the specific mass of the penetrated substance; for primary protons it is of the order of 90 g/cm2 of air, or about 9 percent of the atmospheric layer. As the atomic weight A of the substance increases, the mean path gradually lengthens (approximately as A⅓), reaching ∽160 g/cm2 for lead. Pion production takes place primarily at high altitudes (20–30 km) but continues to a lesser extent throughout the entire atmosphere and even at a depth of several meters underground.
Nucleons that escape during nuclear collisions, as well as high-energy charged pions that are not able to decay, form the nuclear-active component of secondary cosmic rays. The frequent repetition of successive cascade interactions of nucleons and charged pions with the nuclei of atoms of air, accompanied by multiple production of new particles (pions) in each interaction, leads to an avalanche-like increase in the number of secondary nuclear-active particles and to a rapid decrease in their average energy. When the energy of an individual particle becomes less than 1 GeV, the production of new particles virtually stops and usually only the processes of partial—and sometimes complete—disintegration of the atomic nucleus, with emergence of comparatively low-energy nucleons, remain. The total particle flux of the nuclear-active component decreases upon further penetration into the atmosphere (Figure 6, curve 1) and at sea level (∽ 1,000 g/cm2) remains less than 1 percent of the nuclear-active particles.
ELECTRON-PHOTON SHOWERS AND THE SOFT COMPONENT OF SECONDARY COSMIC RAYS. The neutral pions that form during interactions between particles of the nuclear-active component and atomic nuclei each decay almost instantaneously (as a result of their very short lifetime) into two photons (γ): π0
→ 2γ. This process gives rise to the electron-photon component of cosmic rays (it is also called the soft, or easily absorbable, component).
In the strong electric fields of atomic nuclei the photons produce electron-positron pairs, e−e+ (γ → e− + e+); the electrons and positrons in turn emit new photons (e± → e± + γ) through bremsstrahlung, and so on. Such processes, which are of the cascade type, lead to an avalanche-like increase in the total number of particles—the formation of an electron-photon shower. The development of an electron-photon shower leads to the rapid division of the energy of π0 into an ever larger number of particles, that is, to a rapid decline in the average energy of each particle in the shower. After maximum development of the soft component, which is achieved at an altitude of about 15 km (—120 g/cm2), gradual attenuation of the component takes place (Figure 6, curve 2). When the energy of each particle becomes less than some critical value (for air, about 100 MeV), energy losses to the ionization of atoms of air and to Compton scattering begin to play a dominant role. The increase in the number of particles in the shower stops, and the individual particles are rapidly absorbed. Virtually total absorption of the electron-photon component takes place in comparatively thin layers of matter (particularly those of great density); under laboratory conditions a lead screen 10–20 cm thick is sufficient, depending on the particle energy.
The change in the number of particles with increased thickness of the penetrated substance—the cascade curve (Figure 7) —is the main characteristic of an electron-photon shower. According to the theory of this process, the number of particles at the maximum of the cascade curve is approximately proportional to the energy of the initial particle. The angles of deflection of the particles from the axis of the shower are defined by the scattering of electrons and positrons, and the average transverse momentum is about 20 MeV/ c.
In addition to the π0-mesons, cosmic rays also contain other sources that form electron-photon showers. These are the high-energy (> 100 MeV) electrons and gamma quanta of the primary cosmic rays, as well as δ-electrons—that is, atomic electrons dislodged by direct electric interaction of the fast charged particles of cosmic rays passing through matter.
At very high energies (≧1014 eV) electron-photon showers in the earth’s atmosphere acquire specific features of extended air showers. The very large number of successive multiplication cascades leads to a marked increase in the total particle flux (counted in the many millions or even billions, depending on the energy) and to extensive spatial divergence (tens or hundreds of meters from the axis of the shower). In extensive atmospheric showers near the surface of the earth there is one particle of the shower for approximately every 2–3 GeV of energy of the primary particle that caused the shower. This makes it possible to estimate, on the basis of the total particle flux in the shower, the energy of the “ancestors” of the showers, which reach the boundary of the earth’s atmosphere. This cannot be done directly because of the extremely low probability that they will directly reach the point of observation.
Because of the great density of the particle flux in an extended air shower, comparatively intense directed electromagnetic radiation is emitted both in the optical portion of the spectrum and in the radio-frequency band. The optical part of the radiation is determined by the process of Cherenkov radiation, since the velocity of most particles exceeds the phase velocity of the propagation of light in the air. The mechanism of radio-frequency radiation is more intricate. In particular, it is connected with the fact that the earth’s magnetic field causes spatial division of fluxes of negatively and positively charged particles. This is equivalent to the appearance of an electric dipole that is variable over time.
COSMIC MUONS AND NEUTRINOS; PENETRATING COMPONENT OF SECONDARY RADIATION. Charged pions that enter the atmosphere under the influence of cosmic rays participate in the development of a nuclear cascade only at sufficiently great energies—until their decay in flight begins to operate. In the upper layers of the atmosphere decay processes become significant at energies of ≲1012 eV.
A charged pion (with an energy of ≲ 1011 eV) decays into a μ± muon (a charged unstable particle with rest mass mμ ≈ 207 me, where me is the mass of an electron, and average lifetime τ ≈ 2 X 10−6 sec) and a neutrino ν (a neutral particle with zero rest mass). The muon in turn decays into a positron (or electron), a neutrino, and an antineutrino. Since the speeds of muons (like those of all other particles in cosmic rays) are very close to the speed of light c, according to the theory of relativity the average time τ before they decay is very great—it is proportional to the total energy ℰ, τ = ℰτ0/ mμc2. In addition, muons, since they are not nuclear-active particles, interact weakly with matter (through electromagnetic interaction) and lose their energy
chiefly to the ionization of atoms (∽2 MeV in a thickness of 1 g/cm2). Therefore the muon flux is the penetrating component of cosmic rays. Even at the comparatively moderate energy of about 10 GeV the muon is able not only to pass through the earth’s entire atmosphere (see Figure 6, curve 3) but also to penetrate the earth to distances of the order of 20 m of soil (Figure 8). The maximum depth at which the highest-energy muons have been recorded is about 8,600 m (converted to the water equivalent). It is precisely because of their great penetrating power that muons form the “skeleton” of extended air showers at great distances (hundreds of meters) from their axis.
Thus, simultaneously with the development of the nuclear cascade described above (which is due to the decay of π0), it is “enveloped” by the electron-photon component and also (by virtue of the decay of π+ and π−) by the penetrating muon component (Figure 9).
High penetrating power, in combination with the coefficient of absorption of the substance at moderate energies (tens and hundreds of GeV), which is directly proportional to the density of the substance, makes the penetrating component of cosmic rays a very convenient tool for underground geophysical and engineering exploration (Figure 10). By measuring the intensity of cosmic rays with a counter telescope in tunnels and by comparing the data with known curves of cosmic-ray absorption in water or soil, it is possible to detect or pinpoint the location of ore beds and cavities and to measure the weight load on the soil from structures standing on it.
In addition to the ionization energy losses of muons, energy losses to the formation of electron-positron pairs, to bremsstrahlung, and to direct interactions with the atomic nuclei of matter become increasingly significant at energies of the order of 1012 eV and higher. As a result of this, at depths ≳ 8 km (water equivalent) at angles ≳ 50° to the vertical the flux of cosmic muons proves to be negligibly small. Experiments conducted in 1964 in mines in India and South Africa with units of extremely large area made it possible to detect at these depths and at angles greater than 50° an additional muon flux whose only source could be interactions of neutrinos with the atomic nuclei of matter. These experiments presented a unique opportunity to study the properties of the most penetrating component of cosmic rays—the neutrino component. Here the study of the interaction of superhigh-energy neutrinos with matter is the most important problem. In particular, the investigation of the increase in the interaction cross section (the decrease of the “transparency” of matter) with increased neutrino energy is of
particular interest for elucidating the structure of elementary particles. This increase in the neutrino interaction cross section has been established in accelerators at energies up to 1010 eV. It is very important to investigate whether this increase in cross section will continue to energies of 1015 eV (which corresponds to the characteristic distance of weak interactions, 6 X 10−17 cm).
Measurements effluxes of solar neutrinos of much lower energies (∽ 1 MeV) will make it possible to move toward a solution to another, cosmic-physics problem of neutrino physics. This is associated with the use of the great penetrating power of neutrinos for indirect measurement of the temperature of the sun’s interior, on which the character of the nuclear reactions transpiring in the sun—the primary source of solar energy—depends.
Problems and prospects. The further study of cosmic rays in laboratories and in space stations is continuing in two directions. In the cosmic-physics trend the nature of the primary processes in which the acceleration of particles to high and superhigh energies (in supernovas and pulsars and, in part, on the sun) is being determined, and the properties of the interplanetary and interstellar medium are being ascertained on the basis of variations in the intensity of cosmic rays and particular features of their composition and angular and energy distribution. Particularly great hopes are placed on research in the field of X-ray and gamma-ray astronomy in close conjunction with radio-astronomical and astronomical observations of possible sources of cosmic rays.
The question of the role of the neutrino as one of the components of primary cosmic rays at energies ≳1020 eV is also interesting. It is difficult to explain the appearance of extensive atmospheric showers of such high energies in terms of charged particles accelerated within our galaxy, and particles of metagalactic origin cannot acquire such energy from collisions with photons from the relict radiation that fills the metagalaxy. Therefore, it is necessary to take into account the possibility that there is a continuous increase in the opacity of matter (and, in particular, of air) to cosmic neutrino fluxes, which in this case could become the “ancestors” of the most powerful extended showers.
Attempts are being made conclusively to resolve the still un-clear problem of the existence of fireballs—hypothetical particles with masses of the order of 3–5 GeV and sometimes much more that decay almost instantaneously into individual particles (mainly pions) according to the laws of statistical physics. Discussion of the degree of usefulness of the description of multiple particle production by hydrodynamic and thermodynamic models, in which the highly excited “hadron matter” (with an undetermined
number of particles) that is formed in nuclear collisions expands until it decays into individual free particles, is far from complete.
REFERENCESGinzburg, V. L., and S. I. Syrovatskii. Proiskhozhdenie kosmicheskikh luchei. Moscow, 1963.
Dorman, L. I. Variatsii kosmicheskikh luchei i issledovanie kosmosa. Moscow, 1963.
Dorman, L. L, and L. I. Miroshnichenko. Solnechnye kosmicheskie luchi. Moscow, 1968.
Dorman, L. I., V. S. Smiraov, and M. I. Tiasto. Kosmicheskie luchi v magnitnom pole Zemli. Moscow, 1971.
Murzin, V. S., and L. I. Sarycheva. Kosmicheskie luchi i ikh vzaimodeistvie. Moscow, 1968.
Bugaev, E. V., lu. D. Kotov, and I. L. RozentaP. Kosmicheskie miuony i neitrino. Moscow, 1970.
Bondarenko, V. M. Ispol’zovanie kosmicheskikh luchei v geologii, Moscow, 1965.
Rossi, B. Kosmicheskie luchi. Moscow, 1966. (Translated from English.)
Dobrotin, N. A. Kosmicheskie luchi. Moscow, 1963.
Zhdanov, G. B. Chastitsy vysokikh energii. Moscow, 1965.
Ginzburg, V. L. Proiskhozhdenie kosmicheskikh luchei. Moscow, 1968.
G. B. ZHDANOV
cosmic rays[′käz·mik ′rāz]
Factual note: Alpha particles cause bit rot, cosmic rays do not (except occasionally in spaceborne computers). Intel could not explain random bit drops in their early chips, and one hypothesis was cosmic rays. So they created the World's Largest Lead Safe, using 25 tons of the stuff, and used two identical boards for testing. One was placed in the safe, one outside. The hypothesis was that if cosmic rays were causing the bit drops, they should see a statistically significant difference between the error rates on the two boards. They did not observe such a difference. Further investigation demonstrated conclusively that the bit drops were due to alpha particle emissions from thorium (and to a much lesser degree uranium) in the encapsulation material. Since it is impossible to eliminate these radioactives (they are uniformly distributed through the earth's crust, with the statistically insignificant exception of uranium lodes) it became obvious that one has to design memories to withstand these hits.