Cosmic Rays: Detection and Composition

Although some of the lowest energy cosmic rays are particles emitted by Earth's sun, most of the rays are too energetic to be confined to the solar system, and are samples of material from other parts of the galaxy. Because these 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 cosmic rays. Nevertheless, the rays provide important clues to the processes that occur in stars, supernova, and other astrophysical sources.

Measurement of the composition of cosmic rays permits comparison to the composition of the Earth, the lunar samples returns 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 cosmic rays require a measurement of the charge of each individual cosmic ray particle. Generally, these techniques require two independent measurement techniques.

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 1940s, group 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 cosmic rays.

These high altitudes were required because collisions between incoming rays and air molecules can cause 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 of the particles in space.

These early experiments demonstrated that of the nuclei in 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 nucleosynthesis processes.

Following the initial discovery of heavy nuclei among the cosmic rays, the emphasis in cosmic ray related research shifted to the determination of the charge spectrum, or relative abundances, of each of the elements. 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 lights. 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 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 above 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 nuclei 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 cosmic rays.

This combination of detectors provided good measurements of the elemental abundance for elements up to iron. Elements heavier that iron were so rare that their identification required a new technique.

In mid 1960s 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 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 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 the earth.

Long duration, large area cosmic-ray detectors, possibly on a space station, will be required to determine the abundance of elements heavier than bismuth, allowing direct comparison of cosmic-ray composition with that expected for r-process nucleosynthesis in supernova, which are suggested as the source of cosmic rays.

1) Friedlander, Michael W. Cosmic Rays Harvard University Press, 1989.
2) Ginzburg V. L. and Syrovatskii S. I. The Origin of Cosmic Rays Macmillan, 1964
3) Rossi, Bruno Cosmic Rays McGraw-Hill, 1964.