ESA title
Victor Hess
Science & Exploration

Particle radiation: radioactivity and cosmic rays

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Particle radiation is around us all the time but it is harder to notice than wave radiation. The most common type of particle radiation comes from radioactive atoms. Almost everything is radioactive to some extent: rocks, soil, plants, animals and YOU.

There are two main types of radioactive particles:
alpha α which is really the nucleus of a helium atom moving at about 5% of the speed of light, and
beta β, an electron moving at around 90% of the speed of light.

Gamma γ is also given out by radioactive atoms but it is a wave and part of the electromagnetic (EM) spectrum. However, because of the wave/particle duality it is often better to treat it like a particle. The radioactivity we detect comes mainly from the Earth.

There are other types of particle radiation; they come from space and are called ‘cosmic rays’.

What are cosmic rays?

Cosmic rays were discovered in 1912 by the Austrian physicist Victor Hess. During a series of balloon flights he found that the atmospheric ionisation rate increased with height. He concluded that “a radiation of very great penetrating power enters our atmosphere from above”. Some years later Robert Millikan coined the name "cosmic rays" for this “extra terrestrial radiation”.

Cosmic rays are particles that hit the Earth’s atmosphere from space. They include protons, helium nuclei (like radiation), electrons (like β radiation) and the nuclei of heavier atoms such as iron.

Crab supernova remnant (Hubble telescope)
Crab supernova remnant (Hubble telescope)

All of them are moving extremely fast - the fastest travel at the speed of light - and can carry billions more energy per particle than those from radioactivity on Earth. The most energetic cosmic rays are millions of times more energetic than the highest energy particles we can produce in our most advanced particle accelerators, such as the Large Hadron Collider (LHC) at CERN in Geneva.

Some of these particles come from the Sun but others are more of a mystery. It is most likely that these other particles are spat out by supernovae, huge stars that explode violently at the end of their lives.

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