What are cosmic rays?

At the start of the 20th century scientists became very interested in a puzzling phenomena. There seemed to be rather more radiation in the environment than they could account for by the known sources of natural background radioactivity.

After much debate, the puzzle was partly solved by a daring Austrian scientist, Victor Hess. In 1912 he took a radiation counter (he used a gold leaf electroscope) on a balloon flight. He risked his life, by travelling to 17,500 feet without oxygen, but managed to observe that the amount of radiation increased as his balloon climbed. This demonstrated that the radiation was from outer space and eventually it was dubbed "Cosmic Radiation".

Left: Victor Hess before his balloon flight, during which he observed cosmic ray intensity increasing with altitude. Right: Hess's balloon.

Since 1912 we have learnt a lot about cosmic rays. We now know that they are sub-atomic particles and possess a large range of energies (usually measured in electron-volts [eV]) from a few billion eV to more than 10²⁰ eV.

The rate at which cosmic rays bombard the Earth varies enormously with their energy. Low energy cosmic rays are plentiful (many thousand per square metre every second). The highest energy cosmic rays are very rare (less than one hits a square kilometre of the Earth's surface each century). This makes detecting them very difficult.

We know from measurements made on board satellites and high altitude balloons that the vast majority of cosmic rays are protons, although other heavier atomic nuclei are also present, extending all the way up to uranium nuclei. The vast majority of cosmic ray particles therefore have a positive electrical charged.

A small fraction (0.1%) of cosmic rays are photons (in the form of gamma-rays). These gamma-ray photons are important when trying to find the origin of cosmic rays since they have no electrical charge and so arrive at the Earth undeflected by the galactic magnetic field.