Cosmic rays

Cosmic rays were first considered after hot air balloon flights by the Austrian Victor Hess in 1911 and 1912 showed that air becomes more electrically conductive with altitude and reasoned that the effect is probably not from terrestrial causes. The Italian, Domenico Pacini simultaneously showed that the effect decreased with depth of water with the same conclusion (that the effect is not terrestrial in origin), though only Victor Hess received the Nobel Prize in 1934. Robert Millikan thought this effect came from electromagnetic waves like gamma rays and named the cause Cosmic Rays, though later, since they were deflected by magnetic fields, cosmic rays were found to be charged atomic particles rather than gamma rays or x-rays. Cosmic background radiation which some think was left over from the big bang are photons (at microwave wavelengths), and thus not cosmic rays (which are high speed atomic particles).

Before particle accelerators were made, cosmic rays were the main source of high energy particles for research, and figured in the discovery of muons and positrons. They are now being investigated as clues to stellar nucleosynthesis, indicators for the content of interstellar gas, indicators of the size of the sun's magnetic field, possible factors for global warming and cooling, and even as a way to calculate the water content of soils.

Makeup of Cosmic Rays
Most cosmic rays are protons and alpha particles (sometimes referred to as Hydrogen and Helium because they are the nuclei of these atoms). Some particles, which are called galactic cosmic rays, are believed to be released by supernovas and accelerated to high speeds while others, called solar cosmic rays, are simply released from solar flares. Particles from supernovas are thought to create other particles when they strike atoms in interstellar space, these are called secondary cosmic rays. About one percent of galactic cosmic rays have nuclei of heavier atoms, and research into their proportions and energies has become a branch of astronomy.

When cosmic rays hit the earth's upper atmosphere they strike atoms in the air with enough force to create new particles which make up most of the charged particles that reach the ground. These air shower particles are also sometimes called secondary cosmic rays which can cause confusion for readers because some galactic particles are called secondary cosmic rays. The number of these air shower particles is highest at about 15 kilometers above the ground, but is reduced to about one twentieth of that highest number at sea level because of the protective absorption of air molecules. People who fly regularly in commercial aircraft have a slightly higher exposure to radiation because they are above the thicker part of the atmosphere and so enjoy less protection.

In 1990, an analysis of satellite data gave the following abundances of cosmic rays averaged for the energies of .6 Gev to 35 Gev. The information was normalized in terms of 100 silicon nuclei. They assumed that the interstellar medium was 90% Hydrogen and 10% Helium, and that the cosmic rays of these energies all originated from other stars than our sun.

Most cosmic rays follow curved paths because of the galactic magnetic field so no one is sure where they come from. However, some very high energy cosmic rays that are thought to come from other galaxies are believed to follow paths that allow astronomers to trace them back to their source. It was assumed that almost all of the very high cosmic rays were protons (hydrogen nuclei), but a recent find shows that a surprising quantity of these rays fit the models best if they are actually iron nuclei.

Cosmic Ray Energies
Energies of cosmic rays, like energies of nuclear particles, are measured in electronvolts (abbreviated eV). Joules measure energy or work done, if you push one coulomb of electric charge into a capacitor sized so that its voltage comes up to one volt, you have a joule of energy stored in the capacitor. The amount of energy that each electron of that coulomb stores is one electronvolt of energy. This measure of energy is used for atomic particles because it is easier than using minute fractions of a joule and is equal to 1.60218 x 10-19 joules. Like a joule, it can represent energy, work, or quantity of heat, so, electronvolts can also be a measure of the temperature of molecules. Electronvolts are even used for representing a particle's mass (but you must assume that their mass is divided by the speed of light squared which is defined as 1 for this case). It takes about 35 electron volts of energy to ionize one atom of hydrogen by removing its electron.

Cosmic Ray Detectors
There were originally two main types of detectors, cloud chambers and spark chambers, both relying on the tendency of a cosmic ray and its secondary particles to ionize air molecules.