Discovering Particles

Cosmic rays

Artist's impression of cosmic-ray shower over London

Cosmic rays are charged subatomic particles of extraterrestrial origin, with sources including the sun and other stars. The cosmic rays that reach the Earth consist of about 89% protons (hydrogen nuclei), 9% alpha particles (helium nuclei), 1% nuclei of heavier elements and 1% electrons. These particles collide with the atoms and ions that make up the Earth’s atmosphere, and the collision energy may be converted into new particles. Each new particle can collide with another atom or ion, so that showers of particles are produced. As a shower develops, the energy of the initial cosmic ray is shared between more and more particles, so that the average energy per particle decreases.

Most of the particles that reach the Earth’s surface from cosmic-ray showers are either muons or neutrinos. These tend to lose less energy in interactions than other types of particle, and so are more likely to pass through the atmosphere. At sea level, the number of muons that pass through a horizontal surface with an area of 1 square metre is about 150 per second. An energetic muon can penetrate the Earth’s surface to a depth of several kilometres. Neutrinos, which only experience the weak force, can pass through the entire Earth without interacting.

Although the background of ionizing radiation due to cosmic rays had been detected earlier, its nature wasn’t understood until ionisation levels at altitudes up to 5.3 kilometres were measured by Victor Hess, in a series of balloon flights, during 1911 and 1912. His results led him to the conclusion that: “a radiation of very great penetrating power enters our atmosphere from above.”

Studies using cosmic rays in the 1930s and 1940s saw the discovery of particles such as the anti-electron or positron (first antiparticle); the muon (first second-generation matter particle); the pion (first meson); and both the kaon and the lambda (first particles containing the strange quark). Experiments were often performed at high altitudes, in aeroplanes or on top of mountains, so as to catch the cosmic-ray showers earlier in their development.

At the Large Hadron Collider, cosmic-ray muons were used in the first tests of detector performance.

Further information:

Credits and licensing information