COSMIC RAYS, OBSERVATIONS AND EXPERIMENTS PETER MEYER SHORT HISTORY OF COSMIC RAY OBSERVATIONS The intense research late in the nineteenth century on the phenomenon of radioactivity prepared the way for the discovery of the cosmic radiation. Ionization chambers were developed to detect and measure with ever-increasing sensitivity the ionizing radiations emitted by radioactive substances. It was soon noted that even in the absence of all radioactive materials, a residual amount of radiation passed through the chambers and the question arose whether this radiation may originate from the surface of the Earth, in the atmosphere, or possibly even beyond. To explore this question, a number of manned balloon flights, carrying ionization chambers, were made by the Austrian physicist Victor F. Hess. In 1912, Hess published his result, for which he later received the Nobel Prize: The radiation intensity increases with altitude. The source of the radiation must therefore be sought in the overlying atmosphere or outside the Earth. With no knowledge of its nature, Hess gave it the name Hohenstrahlung ("radiation from above"). It took almost four decades of research to establish that the primary cosmic radiation, before it interacts with the matter of the atmosphere, consists mostly of atomic nuclei of high kinetic energy. These years were extremely rich in discoveries. Elementary particle physics and high-energy physics were born, using the energetic cosmic ray particles as projectiles to study interactions and to produce new particles. Numerous particles were discovered, including positrons, muons, pions, and many more. New detection instruments were invented and used for cosmic ray measurements: Geiger counters, nuclear emulsions, and cloud chambers were among the first with scintillation counters, Cerenkov counters, spark chambers, solid-state detectors, multiwire proportional chambers, transition radiation detectors, and plastic track detectors coming later. With these refined observational tools it was possible to study the cascades of secondary particles that are generated after a high-energy nucleus impinges on the atmosphere. Similar detectors are used to determine the identity of the primary particles. In the 1950s, when large particle accelerators were developed, most of high-energy physics moved to the accelerator laboratory, and research in cosmic rays concentrated largely on the astrophysical questions of their nature, their origin, and their acceleration. The high-energy primary nuclei interact in the first few g cm-2 of the atmosphere, which altogether is about 1000 g cm-2 thick, and produce large numbers of secondary, tertiary, etc. particles and energetic photons. Therefore, the primary radiation could only be studied when it became possible to bring detectors near the top of the atmosphere or above it. A famous balloon flight using Geiger counters first established that the primaries are mostly protons. Cosmic ray particles reach the solar system and the earth from all directions. Galactic magnetic fields deflect the trajectories of the charged particles with the result that their direction of arrival is unrelated to the direction toward their source. To investigate their origin one must rely on other characteristic features: their composition and their energy spectra. Major technical innovations opened the door to modern cosmic ray research. Reliable high-altitude balloons, earth satellites, and deep space probes made it possible to place instruments near the top of the atmosphere or outside the atmosphere and even outside the magnetosphere of the Earth. Equally important was the simultaneous development of modern solid-state electronics, which is ideally suited for space vehicles because of its low mass, low power consumption, and high stability. OBSERVATIONS OF N[JCLEAR ABIlNDANCES Elemental Composition Balloon flights in 1949 carrying nuclear emulsions brought the first evidence that, besides the overwhelmingly abundant protons (almost 90% of the cosmic rays), heavier nuclei are present in the primary cosmic radiation. The relative abundance distribution of the elements in the cosmic radiation, as it is measured today with modern instruments, is shown in Fig. 1. Normalization is at the element Si. The solid circles represent measurements at about 200 MeV per nucleon; the open circles are measurements around 2 GeV per nucleon. The open diamonds are the best estimate of the abundance distribution of the elements in the solar system. This figure represents a major milestone in particle astrophysics. First, it shows the striking similarity between cosmic ray abundances and solar system abundances, leading immediately to the important conclusion that the cosmic ray nuclei, just as the solar system nuclei, were produced by nucleosynthesis in the interiors of stars and only afterwards accelerated to their high energies. A second conclusion follows from the differences between cosmic ray abundances and solar system abundances. These are particularly noticeable in the group of the light elements, Li, Be, and B, and in the subiron group from Sc to Mn. The filling of the abundance valleys in the solar system distribution by the cosmic rays is due to collisions of the source nuclei with the interstellar medium, leading to nuclear disintegration or spallation, with the secondary fragments produced with almost the same velocity as that of the parent nucleus. The ratio of secondary to primary intensity provides a measure of the average amount of material that a cosmic ray particle traverses before being lost from the galaxy. This is called the escape mean free path and amounts to about 7 g cm-2 of material at the energies to which Fig. 1 applies. In recent years it was discovered that the intensity ratio of secondary nuclei to primary nuclei decreases with increasing energy, indicating that the escape mean free path decreases at high energies. The highest energy at which the abundance of individual nuclear species could be measured is about 1 TeV per nucleon (1000 GeV per nucleon) in an experiment carried by the space shuttle. At those energies the escape mean free path is only 1 g cm -2. Several models attempt to describe this behavior. Ultraheavy Nuclei The unique position of Fe (iron) as the stablest of all elements in the periodic system of the elements is manifest by its high relative abundance in the cosmic rays and in the solar system. Beyond Fe the abundances drop rapidly and by many orders of magnitude. In spite of their extreme scarcity, ultraheavy nuclei (UH nuclei) have been discovered in the cosmic radiation, first trough the identification of tracks they had left in natural minerals of meteorites. Shortly afterwards balloon experiments using large-area nuclear emulsions and plastic track detectors provided rough indications of their contemporary flux and composition. More recent satellite experiments with electronic detectors (1978) were able to separate most of the abundant elements with even atomic number. They established the existence of the entire periodic table of the elements in the cosmic rays. Since the UH nuclei are probably produced in nucleosynthesis processes different from those that produced nuclei of lower mass, this knowledge contains significant information on the origin of the elements. Attaining full charge resolution of the UH nuclei is still a task for the future. Isotopes In the past 10-15 years it has become possible to resolve the isotopic abundance distribution for several nuclear species. Differences in isotopic abundances for a given element in the cosmic rays and in the solar system must be due to nuclear phenomena rather than atomic effects. They therefore are important in selection from alternate types of nucleosynthesis processes responsible for the observed sample of isotopes. Although the determination of accurate isotopic abundances is still in its beginning, a number of important results have already been obtained. Cosmic ray Ne and Mg have been observed to have over- abundances of their neutron-rich isotopes **Ne, **Mg, and **Mg when compared to the solar system isotopic abundances. This evidence is of considerable importance in understanding the nature of the cosmic ray sources. The relative abundance of the radioactive isotope **Be has provided a measurement of the average time cosmic rays are contained in the Galaxy. Experiments are planned that will yield much more precise data on isotopic abundances for a wide range of elements. OBSERVATIONS OF ELECTRONS AND POSITRONS The presence of energetic electrons spiraling around galactic magnetic fields was first deducted from the observation of a background of radio frequency synchrotron radiation from the galactic disc. Balloon experiments in 1960 established the presence of a flux of cosmic ray electrons, amounting to about 1% of the flux of protons. Not much later, through the use of a magnet spectrometer in a balloon-borne instrument, it became possible to separate the negatively charged electrons from the positively charged positrons. Negative electrons were found to be about 10 times more abundant than positrons at energies between a few hundred mega-electronvolts and a few giga-electronvolts. Positrons and electrons are expected in about equal amounts if they originate from interstellar collisions of cosmic ray nuclei that produce *** mesons which subsequently decay via ** mesons to electrons or positrons. The fact that negative electrons were found to be more abundant than positrons is proof that part of the electrons must have a different source such as the Crab nebula, or other superova remnants which are known to produce electrons of high energy. The shape of the electron energy spectrum is of particular interest. In contrast to the nuclei, electrons are not lost through collisions with interstellar particles, but lose energy by synchrotron radiation as they spiral about magnetic fields, and by Compton collisions with photons of starlight and the universal blackbody radiation. These energy-loss processes are reflected in the spectral shape. Hence, the electron spectrum is a probe of interstellar electromagnetic fields. OBSERVATIONS OF ANTIPARTICLES Antiprotons of high energy are expected in the galaxy as a product of high-energy nuclear collisions. Measurements with a balloon-borne superconducting magnet spectrometer have established the existence of an antiproton flux. The observed antiproton intensity slightly exceeds that predicted by calculation of their production in nuclear collisions. Speculation on the existence of primary antiprotons, however, is premature until further, more accurate experiments are carried out. Antinuclei with atomic number **** have been searched for but not found in several experiments. The discovery of a single antioxygen or antiiron nucleus would indeed be a spectacular result, because it would signify the existence of macroscopic regions of antimatter. THE ENERGY SPECTRA OF THE NUCLEAR COMPONENTS Spectral measurements of individual nuclear species extend to about 1 TeV per nucleon. The primary nuclei He, C, O, and Mg are observed to follow similar spectral forms to the highest measured energies. These can be described by power laws of the form (*****)*********, having the value of 2.6 to 2.7. Spectra for a few primary elements are shown in Fig. 2. The flattening at low energies is due to the local effect of modulation in the solar wind. Due to the decreasing escape mean free path with energy the spectra of the pure secondary nuclei are all steeper than the primary spectra. It is observed that at high energies their spectra follow power laws (*****)****(***), where **********. For the same reason the spectra at the cosmic ray sources must be flatter than the observed primary spectra, (*****)***** ***(**)*, having an exponent of about 2.1. This number is significant. The most promising and extensively studied mechanism for cosmic ray acceleration is collisionless shocks, produced, for example, in the expanding shell of a supernova remnant. The shock acceleration model predicts a power law for the spectrum of the accelerated particles with an exponent around 2. The energy spectrum of all cosmic rays together has been measured over a much wider range than that of individual species. This is achieved through the analysis of giant air showers where secondaries, tertiaries, and so on propagate all the way down to mountain altitude, or even to sea level. The density and extent of the shower provides a measure of the energy of the incident particle. Figure 3 is a compilation of the all-particle spectrum that reaches to the incredible energy of **** eV, by far the largest energy of any known radiation. Around *****eV the power-law spectrum exhibits a steepening whose origin is not understood. This spectral change may be related to a transition to different cosmic ray sources and composition. At low energies the flux is dominated by protons. Nothing is known about the composition or the acceleration mechanisms at the highest energies. Shock acceleration is not likely to be effective at energies in excess of ********* eV. While the bulk of the cosmic rays is undoubtedly of galactic origin, an extragalactic origin of the highest-energy particles is frequently postulated in the absence of any other explanation. The transition from galactic to extragalactic sources may be the cause for the change in slope of the all-particle spectrum. FUTURE INVESTIGATIONS A substantial observational program in cosmic ray research lies ahead. The elemental composition must be measured at higher energies than has so far been possible, extending into the air-shower region. High resolution composition measurements in the UH regime of nuclei are expected to illuminate several questions of cosmic ray origin. Measurements of isotopic abundances of all elements up to and beyond the iron group are needed to better understand the origin of the elements and the material of which cosmic rays are made. An extension of the isotopic analysis into the regime of the UH nuclei is an important experimental challenge. Finally, detailed measurements of the antiproton and positron spectra, and more sensitive searches for heavier antinuclei, will address fundamental questions of astrophysics and cosmology. Additional Reading Mewaldt, R.A., Stone, E.C., and Wiedenbeck, M.E.(1982). Samples of the Milky Way. Scientific American 247(No. 6) 100. Rossi, B.(1964). Cosmic Rays. McGraw-Hill, New York. Shapiro, M.M., ed.(1983). Composition and Orgin of Cosmic Rays. Reidel, Dordrecht. Simpson, J.A.(1983). Elemental and isotopic composition of the galactic cosmic rays. Ann. Rev. Nucl.Part. Sci. 33 323. Sokolsky, P.(1989). Introduction to Ultrahigh Energy Cosmic Ray Physics. Addison-Wesley, Reading, MA. See also Antimatter in Astrophysics; Cosmic Rays, Space Investigations; Radiation, High-Energy Interaction with Matter.