Cosmic rays
High-energy particles, mostly protons, that stream in from space and smash into the atmosphere, showering secondary particles to the ground.
Cosmic rays are not really rays but particles: mostly atomic nuclei, with a few electrons, travelling at nearly the speed of light. About 90 per cent of the nuclei are single protons. Some come from the Sun; the most energetic come from far beyond the Solar System.
They were discovered by going up. In the early 20th century scientists knew that air slowly becomes electrically charged, and assumed radioactivity in the ground was to blame. In 1912 the Austrian physicist Victor Hess carried sensitive instruments in a balloon to 5,300 metres and found the rate of ionisation there was twice that at ground level: the radiation was coming from above. He shared the 1936 Nobel Prize in Physics.
Few of the original particles reach the ground. Instead they collide with atoms high in the atmosphere, producing a cascade, or air shower, of secondary particles. Muons from these showers arrive at sea level all the time; roughly one passes through a volume the size of your head every second. A cloud chamber makes their tracks visible.
Cosmic rays supply about 0.39 millisieverts of the roughly 3 millisieverts of natural background radiation a person receives each year, more at high altitude; airline crews on long high routes receive extra. They also make the carbon-14 in the atmosphere on which radiocarbon dating depends.
Coming up in future stories
- A Brazilian on a Bolivian Mountain Caught a New Particle 1947 · Mount Chacaltaya, Bolivia, and Bristol, England
- A Cosmic Ray May Have Voted in Belgium and Moved Mario 2003 · Schaerbeek, Brussels / 2013 · a Super Mario 64 speedrun
- The Man Who Ballooned Into Space Rays 1912 · Austria-Hungary