cosmic rays were discovered in 1912 by Victor Hess, when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above, and in 1936 was awarded the Nobel prize for his discovery.
For some time it was believed that the radiation was electromagnetic in nature (hence the name cosmic “rays”), and some textbooks still incorrectly include cosmic rays as part of the electromagnetic spectrum.
source: the Big Bang - the cosmic background radiation is the leftover from the matter-antimatter annihilations
Cosmic rays from outer space were the first high energy particles ever studied. From the 1930s to the 1950s, before man-made particle accelerators reached very high energies, cosmic rays served as a source of particles for high energy physics investigations, and led to the discovery of subatomic particles that included the positron and muon.
They gave a tantalizing glimpse of the subatomic world before accelerators were invented. A few cosmic rays pass through your body every second of every day, no matter where you are.
It is difficult to work out the exact origin of cosmic rays because they are arriving from all directions. Many were probably thrown into space by supernovae, the huge explosions of dying stars.
The energy of cosmic rays is usually measured in units of MeV, for mega-electron volts, or GeV, for giga-electron volts. (One electron volt is the energy gained when an electron is accelerated through a potential difference of 1 volt). Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light). The number of cosmic rays with energies beyond 1 GeV decreases by about a factor of 50 for every factor of 10 increase in energy.
It is believed that most galactic cosmic rays derive their energy from supernova explosions, which occur approximately once every 50 years in our Galaxy.
Because cosmic rays are electrically charged they are deflected by magnetic fields, and their directions have been randomized, making it impossible to tell where they originated.
When high energy cosmic rays undergo collisions with atoms of the upper atmosphere, they produce a cascade of “secondary” particles that shower down through the atmosphere to the Earth's surface. Secondary cosmic rays include pions (which quickly decay to produce muons, neutrinos and gamma rays), as well as electrons and positrons produced by muon decay and gamma ray interactions with atmospheric atoms. The number of particles reaching the Earth's surface is related to the energy of the cosmic ray that struck the upper atmosphere.
Most secondary cosmic rays reaching the Earth's surface are muons, with an average intensity of about 100 per m2 per second. Although thousands of cosmic rays pass through our bodies every minute, the resulting radiation levels are relatively low, corresponding, at sea level, to only a few percent of the natural background radiation. However, the greater intensity of cosmic rays in outer space is a potential radiation hazard for astronauts, especially when the Sun is active, and interplanetary space may suddenly be filled with solar energetic particles. Cosmic rays are also a hazard to electronic instrumentation in space; impacts of heavily-ionizing cosmic ray nuclei can cause computer memory bits to “flip” or small microcircuits to fail.
Cosmic rays hitting the outer atmosphere are mainly fast-moving, high-energy protons. As they hurtle towards the Earth, they collide with atoms in the air. Some of the collision energy reappears as the mass of new pairs of particles and antiparticles, following Einstein's famous equation E=mc2.
Cosmic rays are thus a natural source of antiparticles - and in 1932 Carl Anderson's studies of cosmic rays revealed the first antiparticle ever seen, the antielectron, or “positron”.