cosmic rays

In space, away from the earth, there is a very small flux of electrons and high speed nuclei of light atoms moving in all directions. Their origin and how they are accelerated is not always certain but they are probably formed in our galaxy in supernova explosions, or in collisions or outbursts of stars. Those nuclei which enter the region of the earth's atmosphere are found to have energies which are anything up to millions of GeV. Their spectrum is such that the higher the energy the fewer there are (see graph). Those with energies less than a certain value are deflected away again into space by the earth's magnetic field. These primary cosmic rays, which impinge on the earth's atmosphere, are chiefly protons (approx 90%), helium 4 nuclei (approx. 9%) and the nuclei of atoms up to zirconium (Z=40). The predominant flux is from the solar wind of particles boiled off the sun. There are no high energy electrons in primary cosmic rays probably because in their long journey through space they have lost most of their energy in collisions with interstellar matter (whose density could be of the order of 1 particle/cm3). Some low energy electrons and protons get trapped by the earth's magnetic field into Van Allan radiation belts around the world and eventually leak to earth, giving rise to the spectacular aurora . Some primary cosmic ray particles in coming down through the atmosphere dissipate their energy entirely by ionisation, while others terminate in collision with atomic nuclei and induce a large variety of nuclear reactions. In such cases a primary proton or alpha particle has sufficient energy to knock many nucleons out of a nucleus (a process termed spallation) and also materialise its kinetic energy into entirely new short lived particles such as mesons . Particles in this jet induce further nuclear reactions or decay individually into lighter particles. A large number of pi-zero mesons are produced that decay into 2 gamma photons and result in the large background gamma ray flux experienced by ourselves. In this way a shower of secondary cosmic rays of many thousands is spread over square kilometres penetrating metres into the earth.