When the nucleus of an atom decays, it ejects one or more types of the following radiation:
- Alpha particles (α) are composed of two protons and two neutrons, or a “helium nucleus”. Alpha radiation can be easily stopped by your skin or a piece of paper; however it has the highest ionizing power and if it gets inside you, is least likely to escape.
- Beta particles (β) can be split into normal β- and β+ particles. When a neutron decays into a proton, a β- particle, or an electron is emitted (because a neutron is made out of a positive and negative charge). In the same decay process, along with the β- comes an “electron anti-neutrino” — a particle with negligible mass; theoretically conceived to balance decay reactions. (It was experimentally found some years later.) On the other hand, β+ radiation happens when a proton decays into a neutron, by emitting a positron (the anti-particle of an electron, which has a positive charge) and an electron neutrino. β particles are easily stopped by a sheet of aluminium.
- Gamma radiation (γ) is a form of energy, and thus has neither mass nor charge. It also has the shortest wavelength in the EM (electro-magnetic) wave spectrum. It has the highest penetrating power, and is really only slowed down significantly by many centimeters of lead. γ radiation cannot originate from a nucleus that doesn’t spontaneously emit some other type of radiation.
Most atoms have a stable nucleus that doesn’t change; however, a radioactive substance has a nucleus that is unstable. To stabilize itself, the nucleus emits radiation.
Radioactive decay is a spontaneous process that cannot be controlled and is not affected by temperature; however, each radioactive element has its own particular decay rate, also known as its half-life. The half-life of a radioactive element is the time that it takes half the atoms in the sample to decay or for the count rate to decay to half.