Cosmic rays

Cosmic rays are energetic charged subatomic particles, originating from outer space, that impinge on Earth's atmosphere. They may produce secondary particles that may penetrate to the Earth's surface, and deeper. Cosmic rays are the same particles that are stable (nonradioactive) components of the types of atoms that normally occur on Earth, i.e. protons, atomic nuclei, or electrons. Cosmic rays thus resemble the particles that circulate inside particle accelerators, although cosmic ray energies may be far higher (see below). The term ray comes from the early days of radiation research, when a directed stream of any ionizing radiation was termed a "ray" (example, alpha rays). At the time they were named, the precise nature of cosmic rays was not understood, and it was thought they might be electromagnetic radiation, like gamma rays. Cosmic ray particles are now known to arrive individually, not in the form of a beam — although a single particle can produce a directed "shower" of many secondary particles. Today, when the particle nature of cosmic rays must be emphasized, the term "cosmic ray particle" is often used.

About 89% of incoming cosmic ray nuclei are simple protons (hydrogen nuclei), 10% are helium nuclei (alpha particles), and 1% of cosmic ray nuclei are those of the heavier elements. These nuclei together make up 99% of cosmic rays, and solitary electrons (much like beta particles, although their ultimate source is unknown) constitute the remaining 1% of the particles that make up galactic cosmic rays. It is unknown why electrons are accelerated less efficiently than atomic nuclei, during galactic cosmic ray production.

The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun (and presumably other stars as well), to as yet unknown events in the farthest reaches of the visible universe. Cosmic rays can have energies of over 10²⁰ eV, far higher than the 10¹² to 10¹³ eV that man-made particle accelerators can produce. (See Ultra-high-energy cosmic rays for a description of the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s [about 150 km/h].) There has been interest in investigating cosmic rays of even greater energies.

The energy spectrum for cosmic rays.

Cosmic rays have a primary role in the formation of the lithium, beryllium, and boron in the universe, through the process of "cosmic ray nucleosynthesis". They also produce some radioisotopes on Earth, such as carbon-14. In the history of science, cosmic rays were the source of the high energy reactions that resulted in the discovery of the positron, muon, and pi meson. The radiation from cosmic rays composes a large part of natural background radiation on the surface of Earth. It is far more intense outside the Earth's atmosphere and magnetic field, and because it is very difficult to shield against, is expected to have a major impact on the design of spacecraft that can safely transport humans in interplanetary space.