Cosmic-ray astronomy

Type of physical science: Astronomy; Astrophysics

Field of study: Observational techniques

Cosmic rays, ionizing radiation from space, provide a way to investigate the energy-generation mechanisms in exploding stars and allowed the development of the field of particle physics.

Overview

At the beginning of the twentieth century, physicists were actively studying the properties of x-rays, discovered in 1895 by Wilhelm Conrad Rontgen, and radioactivity, discovered in 1896 by Antoine-Henri Becquerel. Many of these investigations employed the gold-leaf electroscope as a detector of ionizing radiation. When an electroscope is given an electric charge, the two gold leafs repel each other, causing them to move apart. Before Becquerel's discovery of radioactivity, it had been known that no matter how well insulated the electroscope was, the initial charge would leak away eventually. With the discovery of radioactivity, this leakage was attributed to the ionizing radiation given off by decay of radioactive elements in the earth and the air.

Scientists began careful studies of the strength of this ionizing radiation. Suspecting that its intensity should decrease with increasing distance from Earth, Father Thomas Wulf took an electroscope about 275 meters up the Eiffel Tower in Paris in 1910. Wulf observed a small decrease in the leakage rate, but not as much as expected, and he suggested that ionizing radiation was also coming down from the sky. In 1912, Victor Franz Hess, an Austrian physicist, carried an electroscope 5,300 meters above the earth in a balloon. Hess demonstrated that at low altitudes, the intensity of ionizing radiation decreased as the balloon ascended, but above about 1,500 meters, it began to increase again, ultimately reaching an intensity several times that at the surface of the earth. Hess suggested there was an extraterrestrial source of ionizing radiation. By the mid-1920s, Hess's explanation had gained general acceptance, and this ionizing radiation from space was given the name "cosmic rays." Since then, astronomers and physicists have attempted to determine the energy distribution and composition of this radiation and have searched for its sources. Hess was awarded the 1936 Nobel Prize in Physics (with Carl David Anderson) for his pioneering efforts.

Most early cosmic-ray researchers believed that cosmic rays were γ (gamma) rays, or photons of very high energy. It was suspected that these high-energy gamma rays were produced in space by the reactions in which hydrogen atoms combined to produce helium, and from similar combinations of other abundant atoms, such as nitrogen and oxygen. In this model, the cosmic rays signaled the creation of new elements in space.

The early cosmic-ray researchers, using electroscopes as their cosmic-ray detectors, could see only the average effects of the cosmic rays over a period of seconds or minutes as their electroscopes discharged. To investigate the energy distribution and composition of the cosmic rays, it was necessary to develop instruments that could detect individual cosmic-ray events. The Geiger-Müller counter, invented in 1929, was one such instrument.

Studies by German physicists Walther Bothe and Werner Kolhörster using Geiger-Müller counters produced results suggesting that the cosmic rays were high-energy charged particles, such as protons, α (alpha) particles, or the nuclei of heavier atoms, rather than gamma rays. These results, however, were not conclusive.

The magnetic field of the earth provided a way to distinguish between uncharged photons, such as gamma rays of x-rays, and charged particles, such as the alpha particles emitted in certain radioactive decays. In 1897, Sir Joseph John Thomson had shown that the paths of charged particles moving in a magnetic field are bent into circular arcs. In a given magnetic field, the lower the energy of the particle and the higher its charge, the smaller the circular path. Thus, if the cosmic rays from space were charged, they would be bent by the earth's magnetic field. Because of the shape and strength of the earth's field, this would result in fewer cosmic rays arriving near the equator than at the poles. If, on the other hand, the cosmic rays were uncharged, they would not be affected by the earth's magnetic field. Thus, the variation of cosmic-ray intensity with latitude would provide a way to distinguish charged cosmic rays from uncharged cosmic rays. In addition, if the cosmic rays were positively charged, they should arrive in greater numbers from the western sky than from the east, and vice versa if their charge were negative. This phenomenon, dubbed the east-west effect, provided an opportunity to determine the charge of the cosmic rays. To search for answers, cosmic-ray researchers undertook scientific expeditions to various parts of the world.

The early results were ambiguous. In 1927, Jacob Clay, a Dutch physicist, measured the cosmic-ray intensities at various points along a journey from Leiden, Netherlands, to Java, Indonesia. He reported that the cosmic-ray intensity was at a minimum in the vicinity of the Suez Canal, near the equator, and increased by about 10 percent farther north. In 1928, Robert Andrews Millikan, an American physicist, and his colleague G. Harvey Cameron reported no difference between the cosmic-ray intensities at the California Institute of Technology in Pasadena, California, and those in Bolivia; in 1930, Millikan found no difference between the cosmic-ray intensities in Pasadena and those in Churchill, Manitoba.

The most ambitious survey expedition was undertaken beginning in 1930 by American physicist Arthur Holly Compton. He carried out a worldwide survey of cosmic-ray intensities at sea level and on mountaintops, using an instrument known as an ionization chamber. By 1933, Compton's surveys showed the first significant evidence for the existence of a latitude effect, confirming that the majority of the cosmic rays were electrically charged.

Although the latitude effect was established by moving a detector from site to site, detection of the east-west effect required measurement of the arrival directions of numerous cosmic rays at a single site. To perform these measurements, the first crude cosmic-ray telescopes were developed. These telescopes consisted of two Geiger-Müller counters separated by a small distance. The detectors were connected to electronic devices to record "coincidences," that is, the simultaneous triggering of both detectors. These coincidences correspond to particles traveling along the line passing through the two detectors. Cosmic rays arriving along other paths could pass through one or the other detectors, but not both. Measurements by Luis W. Alvarez, working in collaboration with Compton, showed a pronounced excess of cosmic rays arriving at their Mexico City site from the west. A few months later, Italian physicists Bruno Rossi and Sergio De Benedetti, working at Asmara, Eritrea, found that the cosmic rays arriving from the west exceeded those arriving from the east by 26 percent. This work demonstrated that a significant portion of the cosmic rays consisted of positively charged particles, most likely protons and other heavier atomic nuclei.

The cosmic-ray telescopes developed to investigate the east-west effect provided no information on the actual direction of the sources of these cosmic rays. Because the path of a charged particle is bent every time it encounters a magnetic field, each cosmic ray arriving at Earth has had its path bent many times as it traveled through space. Thus, the search for sources of cosmic rays has focused on looking for sites capable of accelerating charged particles to the high energies observed on Earth.

Since the discovery of cosmic rays, a variety of sources and acceleration mechanisms have been proposed. In the 1920s, before it was established that cosmic rays are predominantly positively charged nuclei, it was suggested that they were gamma rays produced in the formation of heavier elements from hydrogen. In the 1940s, Enrico Fermi developed a model for the acceleration of cosmic rays by interactions with interstellar clouds. In Fermi's model, the cosmic rays interacted with magnetic fields in the interstellar clouds, which scattered them back in the direction from which they came. This process, which is analogous to the collision of a ball with a wall, results in an energy increase for the particle if the interstellar cloud (the wall) is moving toward the incoming particle. Although the Fermi acceleration mechanism worked to explain the lower-energy cosmic rays, the intensity of the magnetic fields in the interstellar clouds was found to be insufficient to explain the higher-energy cosmic rays.

By the mid-1950s, astronomical observations of the Crab Nebula, a remnant of the supernova of 1054, had established that much of its visible light was produced by synchrotron radiation emitted by electrons with energies up to one hundred billion electronvolts. This indicates that the combination of magnetic and electric fields required to accelerate charged particles still exists within the Crab Nebula. The explosion itself is also believed to have provided the necessary conditions for particle acceleration. As the shock wave from a supernova interacts with the gas in the surrounding interstellar medium, particle acceleration occurs along the shock front, thus creating higher-energy cosmic rays. Astrophysicists generally agree that the frequency of supernova events in Earth's galaxy is sufficient to account for the number and energy distribution of the observed cosmic rays.

In 2012, while looking for sources of low-energy cosmic rays outside Earth's solar system, researchers from France's National Center for Scientific Research (CNRS) and Alternative Energies and Atomic Energy Commission (CEA) discovered a previously unknown source of cosmic rays: the Arches Cluster, located about one hundred light-years from the center of the Milky Way. The densest known star cluster in the galaxy, the Arches Cluster consists of around 150 young, massive stars and thousands of smaller stars traveling through space at approximately seven hundred thousand kilometers per hour, fast enough to create a shock wave. The researchers identified low-energy cosmic rays emanating from the cluster, most likely created when it collided with a gas cloud. The Arches Cluster is the first major source of low-energy cosmic rays discovered outside the solar system and the first known confirmation that cosmic rays can be created in outer space by mechanisms other than supernovas.

Applications

Cosmic rays have been used as a tool to investigate a wide variety of problems in astrophysics, solar physics, cosmology, high-energy nuclear physics, geophysics, archaeology, and geology. The discovery that most cosmic rays are high-energy protons led to the new field of particle physics, which studies, among other things, the collision of high-energy particles with various targets.

From the 1930s until 1953—when the Cosmotron, a high-energy particle accelerator, was completed at Brookhaven National Laboratory—cosmic rays were investigators' only source of high-energy particles. Targets and particle detectors were taken to mountaintops or flown in high-altitude balloons to record rare collision events. The construction of high-energy particle accelerators has turned modern particle physics into a laboratory science, but many of the early discoveries are attributable to the cosmic-ray research of the 1930s.

In 1929, Paul Adrien Maurice Dirac, a theoretical physicist, proposed the existence of antimatter, particles with the same mass but the opposite charge of their normal matter counterparts. Dirac proposed that these antiparticles could be created in pairs with their complementary normal particles and that when an antiparticle collided with its normal particle, the two would annihilate each other, giving rise to a burst of energy. During both the creation of a particle-antiparticle pair and the subsequent annihilation, energy and mass would be converted to each other, in accordance with Albert Einstein's equation E = mc².

The first detection of an antiparticle occurred in 1932, when Carl David Anderson exposed a cloud chamber to cosmic radiation. Anderson saw evidence for the positron, or antielectron, a particle with the mass of an electron but a positive electric charge. For his discovery, Anderson shared the 1936 Nobel Prize in Physics with Hess.

Cosmic-ray experiments also succeeded in detecting the particle responsible for holding the atoms of the nucleus together. In 1935, Hideki Yukawa, a Japanese physicist, predicted that this nuclear binding force came about through the exchange of a particle called a π-meson (pi meson, or pion for short) between the protons and neutrons in the nucleus. Yukawa calculated that the pion should be two hundred to three hundred times more massive than the electron, but still six to nine times lighter than a proton or neutron. In 1947, the pion was discovered in cosmic rays by C. M. G. Lattes, Giuseppe Occhialini, and Cecil Frank Powell, a year before the first pion was detected in a collision of particles from an accelerator beam.

Context

The discovery of cosmic rays evolved from the efforts of scientists to understand why the charge on electroscopes leaked away no matter how well they insulated these devices. What began as a scientific curiosity led eventually to the birth of a new field of physics. Particle physicists designed experiments to examine the interaction of individual cosmic rays with targets and to study the products of these interactions. The first antiparticle, the positron, and the first mesons, which give rise to the force holding atomic nuclei together, were discovered in cosmic rays.

With the construction of the Cosmotron, a high-energy proton accelerator at Brookhaven National Laboratory, particle physicists moved into accelerator laboratories, leaving the cosmic rays to the astronomers and astrophysicists. At this point, interest shifted from using cosmic rays as a tool to probe the world of subatomic particles to using the rays as a tool to understand the high-energy processes occurring in stars and in space. Balloon and satellite measurements were designed to detect the energy distribution and composition of cosmic rays, which were then used to place constraints on the age, or duration of travel from the source to the earth, of the cosmic rays. Constraints on the acceleration mechanisms were also established.

Since the highest-energy cosmic rays far exceed the maximum energy of even the highest-energy particle accelerators, particle physicists may once again begin to use cosmic rays to observe particle interactions at the highest energy range. Since these high-energy particles interact in the earth's atmosphere, such experiments will need to be performed on orbiting satellites or on the lunar surface.

Cosmic rays also constitute a hazard for astronauts and electronic devices, especially when they journey beyond the shielding of the earth's magnetosphere. Astronauts can receive significant radiation doses while in space. During their week-long journey to the moon, one of the Apollo crews received a radiation dose exceeding the annual dose permitted for nuclear reactor plant workers. Concern has also been expressed about the cosmic-ray dose received by airplane flight crews, who spend many hours per year flying at high altitudes where cosmic radiation is greater than at the earth's surface.

On Earth, the development of smaller computer-memory devices has increased their sensitivity to cosmic rays. As a cosmic ray passes through an individual memory element, the ionization produced can be sufficient to change the information state of that element. Cosmic-ray interactions may eventually limit the minimum size and maximum speed of computer memory chips.

Not much is known about how low-energy cosmic rays behave in space, as they are typically prevented from entering the solar system due to solar wind, although they are believed to influence star formation by ionizing and heating interstellar clouds. The 2012 discovery of low-energy cosmic rays emanating from the Arches Cluster, produced via a previously unknown mechanism, may allow researchers to discover new sources of these rays and further investigate what role they might play in star formation.

Principal terms

COSMIC RAYS: ionizing radiation from space

ELECTROSCOPE: a device that detects small amounts of electric charge, usually by the repulsion of two thin, metallic foils

GAMMA RAYS: electromagnetic radiation with an energy higher than x-rays

IONIZING RADIATION: particles or light with sufficient energy to cause atoms to separate into positive ions and electrons

RADIOACTIVITY: the process by which one element decays, or transforms, into a different element by emission of a charged particle

X-RAYS: electromagnetic radiation with an energy between that of ultraviolet light and gamma rays

Bibliography

Friedlander, Michael W. Cosmic Rays. Cambridge: Harvard UP, 1989. Print. Well-illustrated account of the history of cosmic-ray astronomy. Emphasizes the methods of detection, the scientific results, and the implications for the sources.

Friedlander, Michael W. "Cosmic Rays and the Birth of Particle Physics." AIP Conference Proceedings 1516 (2013): 23–24. Print.

Ginzburg, V. L., and S. I. Syrovatskii. The Origin of Cosmic Rays. New York: Macmillan, 1964. Print. A technical account of the astrophysics of cosmic rays, suitable for college physics students. Includes mathematical descriptions of the acceleration mechanisms.

Linsley, John. "The Highest Energy Cosmic Rays." Scientific American July 1978: 60–70. Print. A description, suitable for general audiences, of the apparatus used to detect the showers of secondary particles produced when the most energetic of the cosmic rays interact high up in the earth's atmosphere.

"New Type of Cosmic Ray Discovered after 100 Years." National Center for Scientific Research. CNRS, 10 Oct. 2012. Web. 6 Dec. 2013.

Pérez-Peraza, Jorge A., ed. Homage to the Discovery of Cosmic Rays, the Meson-Muon and Solar Cosmic Rays. Hauppauge: Nova, 2013. Print.

Pomerantz, Martin A. Cosmic Rays. New York: Van Nostrand, 1971. Print. This well-illustrated book describes how cosmic rays interact with matter and with the magnetic field of the earth, and how these interactions can be used to detect and determine the properties of the cosmic rays. Geared for students with an introductory physics course.

Porter, Troy. "Cosmic Rays in Star-Forming Galaxies." AIP Conference Proceedings 1516 (2013): 136–40. Print.

Rossi, Bruno. Cosmic Rays. New York: McGraw, 1964. Print. This classic work, by one of the pioneers of cosmic-ray physics, provides a historical account of the discovery of cosmic rays. Describes how they are detected and discusses the various theories about their origins. Excellent nonmathematical discussion with many illustrations.

Rossi, Bruno. "Early Days in Cosmic Rays." Physics Today 34.10 (1981): 34–41. Print. A historical account, suitable for general audiences, of the early experiments in cosmic-ray research. Covering the years up to 1932, this article describes Rossi's contributions.

Stone, E. C., et al. "Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions." Science 341.6142 (2013): 150–53. Print.

Wefel, John P. "Matter from Outside Our Solar System: New Insights, Part I: The Astrophysical Framework." Physics Teacher 20.4 (1982): 222–29. Print.

Wefel, John P. "Matter from Outside Our Solar System: New Insights, Part II: Experimental Measurements and Interpretation." Physics Teacher 20.5 (1982): 289–97. Print. Discusses the history of cosmic-ray physics, describes the cosmic-ray detectors, and examines the implications of the composition on the sources. Well illustrated and written at a level appropriate for high school science students.

Cosmic Rays: Composition and Detection