Cosmic rays: composition and detection

Type of physical science: Astronomy; Astrophysics

Field of study: Observational techniques

The high-energy cosmic rays are samples of material from outside the solar system. The elemental and isotopic compositions of the cosmic rays constrain the models for element production in a variety of astrophysical sources.

Overview

The cosmic rays are charged particles, electrons, and positively charged ions ranging from protons to the heaviest elements, which arrive at Earth from space. About 98 percent of the cosmic rays are positively charged nuclei, with most of the remainder being negatively charged electrons. Although some of the lowest-energy cosmic rays are particles emitted by Earth's sun, most of the cosmic rays are too energetic to be confined to the solar system and are samples of material from other parts of the galaxy. Because the cosmic rays are charged particles, their paths from the sources to Earth are bent by the magnetic fields in the galaxy. As a result, traditional astronomy in which electromagnetic radiation intercepted by a detector, such as a telescope, is traced back in a straight line to its source is not possible with the cosmic rays. Nevertheless, the cosmic rays provide important clues to the processes that occur in stars, supernova, and other astrophysical sources.

Measurement of the composition of the cosmic rays permits comparison to the composition of the earth, the lunar samples returned by the Apollo missions, the meteorites, and the sun. This allows the processes by which elements are produced within stars to be examined and compared to theoretical models for nucleosynthesis.

The nucleus of each element has a unique charge, so the methods of determining the composition of the cosmic rays require a measurement of the charge of each individual cosmic-ray particle. Generally, these techniques require two independent measurements. The first measurement might determine the rate at which the cosmic ray loses energy in traversing the detector. This rate of energy loss is proportional to the square of the ratio of the charge to velocity of the particle. A second measurement might then determine the velocity, or some other property that depends on velocity in a different manner than the rate of energy loss. From these two measurements, the charge can be determined.

A number of innovative charge measurement techniques have been developed. These detectors can be divided into three general categories: recording detectors, such as photographic emulsions; visual detectors, such as cloud chambers; and electronic detectors, such as Geiger-Muller counters.

In the late 1940's, groups of cosmic-ray investigators at the University of Minnesota and the University of Rochester employed photographic emulsions carried to high altitudes, frequently above 27,000 meters, by balloons to determine the charge and energy of the cosmic rays. These high altitudes were required because collisions between incoming cosmic rays and air molecules can cause the cosmic rays to fragment into several lighter nuclei, thus altering their composition. At high altitudes, the probability of such a collision is low; therefore, the balloon detectors measure the primary composition (that is, the true composition) of the particles in space. These early experiments demonstrated that of the nuclei in the cosmic rays, about 87 percent are hydrogen, or protons; 12 percent are helium; and the remaining 1 percent are nuclei heavier than helium. It is the composition of these heavier nuclei that contain the clues to the nucleosynthetic processes.

Following the initial discovery of heavy nuclei among the cosmic rays, the emphasis in cosmic-ray research shifted to the determination of the charge spectrum, or relative abundances, of each element. The early experiments made use of the magnetic field of the earth as a velocity selector. The paths of charged particles are bent when they encounter a magnetic field, so only particles exceeding a given cutoff energy can penetrate through a region of given magnetic field intensity. The magnetic field of the earth is so strong near the equator that only particles with velocities very close to the speed of light can penetrate. Thus, for cosmic rays detected near the equator, the magnetic cutoff identifies the velocity to be approximately the speed of light. A single measurement of the rate of energy loss for these particles provides a measurement of their charge.

These early experiments indicated the difficulty of detection of the heavy nuclei among the cosmic rays. A 1-square-meter detector placed in space, above the earth's atmosphere and outside the earth's magnetic field, would register several hundred nonsolar protons per second and about one-seventh that number of helium nuclei. Yet, only one or two nuclei heavier than carbon would be measured every second, and the detector would register a single iron nucleus every fifteen seconds. To observe a single lead nucleus would require several months of detector operation. Cosmic-ray astrophysicists recognized that large detectors with long exposure times would be required to determine accurately the composition of the heavy cosmic rays.

In 1956, Frank McDonald, a physicist at Iowa State University, developed a combination of two electronic detectors--a scintillation counter and a Cherenkov counter--to determine the charge and velocity of the cosmic rays. This combination of detectors provided good measurements of the elemental abundances for elements up to iron. The elements heavier than iron were so rare that their identification required a new technique. In the mid-1960's, Robert Fleischer, Buford Price, and Robert Walker, researchers at the General Electric Research and Development Center, found that the trails of ionizing particles were recorded in certain types of plastics and that these trails could be revealed later by etching the plastic in an appropriate chemical agent. They demonstrated that if the rate at which the trail was etching as well as the total etchable length were both measured, the charge and energy of the particle could be determined. Balloon flights with these plastic detectors provided information on the composition of the heavier elements in the cosmic rays.

In the 1970's, cosmic-ray researchers employed orbiting Earth satellites to increase the duration of their measurements. Large plastic detectors were flown for several months on the U.S. Skylab space station. The IMP-7 and IMP-8 satellites, launched in 1972 and 1973, respectively, provided good measurements on the isotopic composition of the lighter cosmic rays. In 1978, the third High Energy Astronomical Observatory (HEAO-3) carrying a 6-square-meter electronic detector, provided high-quality measurements of the abundances of nuclei up to the element bismuth.

Applications

Astrophysicists generally believe that the only elements present in the early universe were hydrogen, helium, and perhaps small amounts of lithium, beryllium, and boron. Most of the elements now present were produced by nuclear reactions in stars, in stellar explosions, or by other astrophysical mechanisms in a process called nucleosynthesis. Theoretical calculations show that the elemental and isotopic abundances produced depend on the particular conditions of the nucleosynthetic event. Thus, the abundances of the elements and isotopes of material from outside Earth's solar system might be different from those of solar system material, and those differences would provide clues to the differing nucleosynthetic conditions at those sites. This comparison requires a knowledge of the cosmic-ray composition at the source.

The composition of the cosmic rays can be altered during their journey through space to Earth. Radioactive decay will remove those radioactive elements with short half-lives, compared to the time it took for the cosmic rays to reach Earth. Collisions between the cosmic rays and interstellar gas atoms will cause fragmentation of some of the cosmic rays.

Measurements of the cosmic-ray composition provide clues to the "age" of the cosmic rays, that is, the duration of their journey through space. The light nuclei, lithium, beryllium, and boron, are much more abundant in the cosmic rays than in solar system materials. These excess light nuclei are believed to have been produced by spallation, or collisional fragmentation with interstellar gas atoms. Since the abundance of interstellar gas atoms is known from other astronomical measurements, the amount of excess light elements provides a measure of the duration of the cosmic-ray journey. Astrophysicists indicate that the cosmic rays presently arriving at Earth began their journey about 10 million years ago. Since the solar system formed about 4.5 billion years ago, the cosmic rays may be sampling a much younger type of material than the solar system.

Once the age of the cosmic rays is known, the abundances of the heavier elements detected at Earth can be corrected back to the source by removing the spallation contribution.

Generally, the elemental composition of the cosmic rays is similar to that of the solar system, but the differences provide clues to differences in the nucleosynthetic processes.

The largest difference in the heavy element composition is for the isotope neon 22, which is four times more abundant relative to the other neon isotopes in the cosmic rays than in solar system matter. Isotopic measurements also show excesses of magnesium 25, magnesium 26, silicon 29, and silicon 30 in the cosmic rays when compared to solar system matter. These latter discrepancies could be explained if the cosmic ray sources were stars with initial abundances of carbon, nitrogen, and oxygen about twice that seen in Earth's sun. Nevertheless, even this alteration of the stellar composition cannot explain the unusually high neon-22 abundance.

The abundances of the heavier elements may provide clues to the site of the production of cosmic rays. Astrophysicists have identified several different nucleosynthetic processes. The two major ones both proceed by the addition of neutrons to light target nuclei. In the "s-process," the time between successive neutron capture events is long enough that the new nucleus can be transformed (β decay) to a stable nucleus before the next capture. This process occurs in the interior of stars. In the "r-process," neutron capture proceeds so rapidly that β decay is not possible between individual capture events, leading to the production of more neutron-rich elements. This process is believed to occur in explosive processes such as supernova. Only the r-process can produce elements heavier than bismuth.

Since astrophysicists have suggested that supernova are a likely source of the cosmic rays, they would be expected to contain the r-process elemental and isotopic abundance signatures. The presence of elements heavier than bismuth in the cosmic rays would suggest an r-process origin. Thus far, the experimental results are ambiguous. Rare events, possibly attributable to elements heavier than bismuth, were reported from balloon flights carrying photographic emulsions and plastic detectors; however, the HEAO-3 detected no such events.

Because of the scarcity of these heavy elements, longer duration, large area cosmic-ray detectors will be required to resolve the question.

Elemental and isotopic measurements on the cosmic rays indicate that their sources differ in significant ways from the source of solar system material. Because of these differences, more precise measurements of the elemental and isotopic compositions of the heavier elements are required for detailed comparisons to the nucleosynthetic models.

Context

The formulation of a detailed model of the nucleosynthesis of the heavy elements by Geoffrey Burbidge, Margaret Burbidge, William A. Fowler, and Fred Hoyle in 1957 provided predictions of the elemental and isotopic abundances expected from the r-process and s-process. This information, coupled with the discovery of heavy elements in the cosmic rays in the late 1940's, suggested comparison of the cosmic-ray composition with the predictions of the nucleosynthetic models. Rapid advances in electronic detectors in the 1950's made such comparisons possible, but the limited flight duration of high-altitude balloons restricted the number of elements that could be measured because of the low abundance of heavy elements.

The use of Earth satellites in the 1970's significantly increased the duration of cosmic-ray composition experiments. Nevertheless, even these long-duration satellite experiments were inadequate to answer the question of the abundance of heavy elements in the cosmic rays.

The development of high-resolution electronic detectors, permitting high-quality determinations of the isotopic composition, showed significant differences between the neon, magnesium, and silicon isotopic abundances in the cosmic rays and solar system matter.

Advances in the modeling of the nuclear processes in stellar interiors allowed astrophysicists to calculate that most of these discrepancies were consistent with nucleosynthesis in a star with carbon, nitrogen, and oxygen abundances approximately double that of Earth. Long duration, large area cosmic-ray detectors, possibly on a space station, will be required to determine the abundances of elements heavier than bismuth, allowing direct comparison of the cosmic-ray composition with that expected for r-process nucleosynthesis in supernova, which are suggested as the cosmic-ray source.

Principal terms

BETA DECAY: radioactive decay in which a nucleus is transformed into another nucleus having a charge one unit higher by emission of an electron

COSMIC RAYS: ionizing radiation from space

ISOTOPES: two or more nuclei of the same element having different masses, or numbers of neutrons

NUCLEOSYNTHESIS: the process by which heavier elements are produced from hydrogen and helium in stars

SPALLATION: fragmentation of a nucleus into two or more lighter nuclei by collision

Bibliography

Friedlander, Michael W. COSMIC RAYS. Cambridge, Mass.: Harvard University Press, 1989. Well-illustrated account of the history of cosmic-ray astronomy. Deals with methods of detection, elemental and isotopic composition, and implications for the cosmic-ray sources.

Ginzburg, V. L., and S. I. Syrovatskii. THE ORIGIN OF COSMIC RAYS. New York: Macmillan, 1964. A technical account of cosmic-ray astrophysics. Includes a good discussion of how the light element abundances provide an age for the cosmic rays. Suitable for college physics students.

Pomerantz, Martin A. COSMIC RAYS. New York: Van Nostrand Reinhold, 1971. Suitable for readers with only an introductory physics background. Describes cosmic-ray interaction with matter and how these interactions are used to detect and determine the properties of the cosmic rays. Well illustrated.

Rossi, Bruno. COSMIC RAYS. New York: McGraw-Hill, 1964. A firsthand account by one of the pioneers of cosmic-ray physics. Describes how cosmic rays are detected and discusses ideas about their origins.

Wefel, John P. "Matter from Outside Our Solar System--New Insights. Part 1: The Astrophysical Framework." THE PHYSICS TEACHER 20 (April, 1982): 222-229.

Wefel, John P. "Matter from Outside Our Solar System--New Insights. Part 2: Experimental Measurements and Interpretation." THE PHYSICS TEACHER 20 (May, 1982): 289-297. Discusses the history of cosmic-ray physics, the mechanisms of nucleosynthesis, the construction of cosmic-ray detectors, and the implications of the composition on the sources. Well illustrated. Suitable for high school science students.

Nuclear Synthesis in Stars