Cosmic Rays
Introduction
Cosmic rays are high-energy particles originating from outer space that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are atomic nuclei: most are hydrogen nuclei, some are helium nuclei, and the rest heavier nuclei. A small fraction are solitary electrons (similar to beta particles). Cosmic rays can have energies of over 10^20 eV, far higher than the 10^12 to 10^13 eV that terrestrial particle accelerators can produce. There has been interest in investigating cosmic rays of even greater energies. High-energy cosmic rays can damage electronics and life in space.
Discovery and Initial Studies
The term "cosmic rays" was coined by Robert A. Millikan in 1925. However, the discovery of cosmic rays predates this. In 1912, physicist Victor Francis Hess gave proof of radiation from space with a series of daring balloon flights, for which he won the Nobel Prize in Physics in 1936. Hess ascended in a balloon to an altitude of 5,300 meters, and observed that the rate of ionization increased approximately fourfold over the rate at ground level. Hess concluded that "a radiation of very high penetrating power enters our atmosphere from above." In 1913–1914, Werner Kolhörster confirmed Victor Hess' results by conducting similar balloon flights.
Composition and Sources
Cosmic rays are composed primarily of protons and alpha particles, but can also contain nuclei of many heavier elements. Cosmic rays originate from the sun, from outside of the solar system in our own galaxy, and from distant galaxies. Solar cosmic rays come from the sun, and are low in energy. Galactic cosmic rays, which are from outside our solar system, can be extremely high in energy. The exact sources of these particles are still the subject of much research. Supernovae are a source of cosmic rays, but other sources may contribute as well.
Propagation and Interaction with Matter
Cosmic rays can propagate through the galaxy and interact with matter in various ways. They can ionize atoms, penetrate and pass through matter, and create showers of secondary particles that rain down through the atmosphere. When cosmic rays enter the Earth's atmosphere, they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of lighter particles, a so-called air shower. This process is known as cosmic ray spallation.
Effects on Electronics and Space Travel
Cosmic rays can have a number of effects on electronics and space travel. They can cause soft errors in electronic devices and upset the operation of microelectronics. This is a significant problem for spacecraft, as the effects of cosmic rays can damage electronic systems. Cosmic rays also pose a health threat to astronauts. Prolonged exposure to cosmic rays can lead to an increased risk of cancer and other health effects. This is a major concern for long-duration space missions, such as a manned mission to Mars.
Detection and Study
Cosmic rays can be detected and studied in several ways. Ground-based detectors can measure the showers of particles produced by cosmic rays striking the atmosphere. Balloons and satellites can detect cosmic rays above the atmosphere. The largest cosmic ray detector in the world is the Pierre Auger Observatory in Argentina, which has been operational since 2004. The study of cosmic rays is a branch of space physics and is interlinked with particle physics.