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Ionising radiations many times stronger than gamma rays are continuously entering the earth’s atmosphere from cosmos.They are called cosmic rays.These radiations come from all directions and the intensity is practically the same through out day and night.
Primary Cosmic Rays:
These cosmic rays are broadly classified into primary cosmic rays and secondary cosmic rays.The primary cosmic rays are those coming from outer space and hitting the outer layer of the atmosphere of the earth.The primary cosmic rays mostly consist of protons and rest is made up of alpha particles and heavy nuclei.The energy of primary cosmic rays is of the order of billion electron volt.(109 eV).
Secondary Cosmic Rays:
The secondary cosmic rays are formed as a result of collisions of primary rays with the nuclei of the oxygen and nitrogen atoms of the air in the upper layers of the atmosphere.They are made up of nuclear particles like alpha particles,protons,electrons,mesons and photons in different proportions.
Cosmic Ray Showers:
When an ionisation chamber is used to study cosmic ray intensities, it is found that occationally the intensity rises momentarily to several times its normal value. This indicates sudden burst of ionisation.This is known as cosmic ray showers.