Radioactive decay is a natural process. An atom of a radioactive isotope will spontaneously decay into another element through one of three common processes:
In the process, four different kinds of radioactive rays are produced:
Americium-241, a radioactive element best known for its use in smoke detectors, is a good example of an element that undergoes alpha decay. An americium-241 atom will spontaneously throw off an alpha particle. An alpha particle is made up of two protons and two neutrons bound together, which is the equivalent of a helium-4 nucleus. In the process of emitting the alpha particle, the americium-241 atom becomes a neptunium-237 atom. The alpha particle leaves the scene at a high velocity -- perhaps 10,000 miles per second (16,000 km/sec).
If you were looking at an individual americium-241 atom, it would be impossible to predict when it would throw off an alpha particle. However, if you have a large collection of americium atoms, then the rate of decay becomes quite predictable. For americium-241, it is known that half of the atoms decay in 458 years. Therefore, 458 years is the half-life of americium-241. Every radioactive element has a different half-life, ranging from fractions of a second to millions of years, depending on the specific isotope. For example, americium-243 has a half-life of 7,370 years.
Tritium (hydrogen-3) is a good example of an element that undergoes beta decay. In beta decay, a neutron in the nucleus spontaneously turns into a proton, an electron, and a third particle called an antineutrino. The nucleus ejects the electron and antineutrino, while the proton remains in the nucleus. The ejected electron is referred to as a beta particle. The nucleus loses one neutron and gains one proton. Therefore, a hydrogen-3 atom undergoing beta decay becomes a helium-3 atom. If you click on the "Go" button in the figure below, you can see the neutron change.
In spontaneous fission, an atom actually splits instead of throwing off an alpha or beta particle. The word "fission" means "splitting." A heavy atom like fermium-256 undergoes spontaneous fission about 97 percent of the time when it decays, and in the process, it becomes two atoms. For example, one fermium-256 atom may become a xenon-140 and a palladium-112 atom, and in the process it will eject four neutrons (known as "prompt neutrons" because they are ejected at the moment of fission). These neutrons can be absorbed by other atoms and cause nuclear reactions, such as decay or fission, or they can collide with other atoms, like billiard balls, and cause gamma rays to be emitted.
Neutron radiation can be used to make nonradioactive atoms become radioactive; this has practical applications in nuclear medicine. Neutron radiation is also made from nuclear reactors in power plants and nuclear-powered ships and in particle accelerators, devices used to study subatomic physics.
In many cases, a nucleus that has undergone alpha decay, beta decay or spontaneous fission will be highly energetic and therefore unstable. It will eliminate its extra energy as an electromagnetic pulse known as a gamma ray. Gamma rays are like X-rays in that they penetrate matter, but they are more energetic than X-rays. Gamma rays are made of energy, not moving particles like alpha and beta particles.
While on the subject of various rays, there are also cosmic rays bombarding the Earth at all times. Cosmic rays originate from the sun and also from things like exploding stars. The majority of cosmic rays (perhaps 85 percent) are protons traveling near the speed of light, while perhaps 12 percent are alpha particles traveling very quickly. It is the speed of the particles, by the way, that gives them their ability to penetrate matter. When they hit the atmosphere, they collide with atoms in the atmosphere in various ways to form secondary cosmic rays that have less energy. These secondary cosmic rays then collide with other things on Earth, including humans. We get hit with secondary cosmic rays all of the time, but we are not injured because these secondary rays have lower energy than primary cosmic rays. Primary cosmic rays are a danger to astronauts in outer space.