Cosmic Ray

Cosmic Ray

Discovery of cosmic rays

The nature and origin of cosmic rays

Resources

The term cosmic ray refers to tiny particles of matter that travel through space. Cosmic rays generally possess an electromagnetic charge and are highly energetic. Physicists divide cosmic rays into two categories: primary and secondary. Primary cosmic rays originate far outside Earth’s atmosphere. Secondary cosmic rays are particles produced within Earth’s atmosphere as a result of collisions between primary cosmic rays and molecules in the atmosphere. Cosmic rays that originate from sources outside the solar system are called galactic cosmic rays (GCRs).

Discovery of cosmic rays

The existence of cosmic radiation was first discovered in 1912, in experiments performed by Austrian-born American physicist Victor Franz Hess (1883–1964). His experiments were sparked by a desire to better understand phenomena of electric charge. A common instrument of the day for demonstrating such phenomena was the electroscope. An electroscope contains thin metal leaves or wires that separate from one another when they become charged, due to the fact that like charges repel. Eventually the leaves (or wires) lose their charge and collapse back together. It was known that this loss of charge had to be due to the attraction by the leaves of charged particles (called ions) in the surrounding air. The leaves would attract those ions having a charge opposite to that of the leaves, due to the fact that opposite charges attract; eventually the accumulation of ions in this way would neutralize the charge that had been acquired by the leaves, and they would cease to repel each other. Scientists wanted to know where these ions came from. It was thought that they must be the result of radiation emanating from Earth’s crust, since it was known that radiation could produce ions in the air. This belief led scientists to predict that there would be fewer ions present the further one traveled away from Earth’s surface. Hess’s experiments, in which he took electroscopes high above Earth’s surface in a balloon, showed that this was not the case. At high altitudes, the electroscopes lost their charge even faster than they had on the ground, showing that there were more ions in the air and, thus, that the radiation responsible for the presence of the ions was stronger at higher altitudes. Hess concluded that there was a radiation coming into the atmosphere from outer space.

As physicists became interested in cosmic radiation, they developed new ways of studying it. The Geiger-Muller counter consists of a wire attached to an electric circuit and suspended in a gaseous chamber. The passage of a cosmic ray through the chamber produces ions in the gas, causing the counter to discharge an electric pulse. Another instrument, the cloud chamber, contains a gas that condenses into vapor droplets around ions when produced by the passage of a cosmic ray. In the decades following Hess’ discovery, physicists used instruments such as these to learn more about the nature of cosmic radiation.

The nature and origin of cosmic rays

An atom of a particular element consists of a nucleus surrounded by a cloud of electrons, which are negatively charged particles. The nucleus consists of protons, which have a positive charge, and neutrons, which have no charge. These particles can be further broken down into smaller constituents; all of these particles are known as subatomic particles. Cosmic rays consist of nuclei and of various subatomic particles. Almost all of the primary cosmic rays are nuclei of various atoms. The great majority of these are single protons, which are nuclei of hydrogen atoms. The next most common primary cosmic ray is the nucleus of the helium atom, made up of a proton and a neutron. Hydrogen and helium nuclei make up about 99% of the primary cosmic radiation. The rest consists of nuclei of other elements and of electrons.

When primary cosmic rays enter the Earth’s atmosphere, they collide with molecules of gases present there. These collisions result in the production of more high-energy subatomic particles of different types; these are the secondary cosmic rays. These include photons, neutrinos, electrons, positrons, and other particles. In turn, these particles may collide with other particles, producing still more secondary radiation. If the energy of the primary particle that initiates this process is very high, this cascade of collisions and particle production can become quite extensive. This is known as a shower, air shower, or cascade shower.

The energy of cosmic rays is measured in units called electron volts (abbreviated eV). Primary cosmic rays typically have energies on the order of billions of electron volts. Some are vastly more energetic than this; a few particles have been measured at energies in excess of 1019 eV. This is in the neighborhood of the amount of energy required to lift a weight of 2.2 lb (1 kg) to a height of 3.3 ft (1 m). Energy is lost in collisions with other particles, so secondary cosmic rays are typically less energetic than primary ones. The showers of particles described above diminish as the energies of the particles produced decrease. The energy of cosmic rays was first determined by measuring their ability to penetrate substances such as gold or lead.

Since cosmic rays are mostly charged particles (some secondary rays such as photons have no charge), they are affected by magnetic fields. The paths of incoming primary cosmic rays are deflected by the Earth’s magnetic field, somewhat in the way that iron filings will arrange themselves along the lines of force emitted by a magnet. More energetic particles are deflected less than those having less energy. In the 1930s it was discovered that more particles come to the Earth from one particular direction. Because of the nature of Earth’s magnetic field, this led scientists to the conclusion that most of the incoming cosmic radiation consists of positively charged particles. This was an important step towards the discovery that the primary cosmic rays are mostly bare atomic nuclei, since atomic nuclei carry a positive charge.

The ultimate origin of cosmic radiation is still not completely understood. Some of the radiation is believed to have been produced in the Big Bang at the origin of the universe. Other cosmic rays are produced by the sun, particularly during solar disturbances such as solar flares. Exploding stars, called supernovas, are also a source of cosmic rays.

The fact that cosmic ray collisions produce smaller subatomic particles has provided a great deal of insight into the fundamental structure of matter. The construction of experimental equipment such as particle accelerators has been inspired by a desire to reproduce the conditions under which high-energy radiation is produced, in order to gain better experimental control of collisions and the production of particles.

See also Particle detectors.

Resources

BOOKS

Dorman, L.I. Cosmic Rays in the Atmosphere and Underground. Dordrecht, Netherlands, and Boston, MA: Kluwer Academic Publishers, 2004.

Schlickeiser, Reinhard. Cosmic Ray Astrophysics. Berlin, Germany, and London, UK: Springer, 2002.

KEY TERMS

Electron— A negatively charged particle, ordinarily occurring as part of an atom. The atom’s electrons form a sort of cloud about the nucleus.

Electron volt (eV)— The unit used to measure the energy of cosmic rays.

Electroscope— A device for demonstrating the presence of an electric charge, which may be positive or negative.

Ion— An atom or molecule that has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).

Neutron— Particle found in the nucleus of an atom, possessing no charge.

Nucleus— The central mass of an atom. The nucleus is composed of neutrons and protons.

Primary cosmic ray— Cosmic ray originating outside the Earth’s atmosphere.

Proton— Positively charged particle composing part of the nucleus of an atom. Primary cosmic rays are mostly made up of single protons.

Secondary cosmic ray— Cosmic ray originating within the Earth’s atmosphere as a result of a collision between another cosmic ray and some other particle or molecule.

Shower (also air shower or cascade shower)— A chain reaction of collisions between cosmic rays and other particles, producing more cosmic rays.

PERIODICALS

“Cosmic Rays: Are Air Crews At Risk?” Occupational and Environmental Medicine 59, no. 7 (2002): 428-432.

“Radiation Risk During Long-Term Spaceflight.” Advances in Space Research 30, no. 4 (2002): 989-994.

John Bishop