The nucleus of an atom is composed of protons and neutrons, which are in turn composed of fundamental particles known as quarks. Each element has a characteristic number of protons but may take a variety of forms, or isotopes, each with a different number of neutrons. Elements can decay into other ones if the process results in a lower energy state. Gamma radiation is a decay emission of pure energy.
The laws of quantum physics predict that an unstable atom will lose energy through decay but cannot forecast precisely when a particular atom will undergo this process. The most that quantum physics can predict is the average amount of time a collection of particles will take to decay. The first three types of nuclear decay discovered were dubbed radioactive decay and consist of the alpha, beta and gamma decay. Alpha and beta decay transmute one element into another and are often accompanied by gamma decay, which releases excess energy from the decay products.
Gamma decay is a typical byproduct of nuclear particle emission. In alpha decay, an unstable atom emits a helium nucleus consisting of two protons and two neutrons. For example, one isotope of uranium has 92 protons and 146 neutrons. It can undergo alpha decay, becoming the element thorium and consisting of 90 protons and 144 neutrons. Beta decay occurs when a neutron becomes a proton, emitting an electron and antineutrino in the process. For example, beta decay turns a carbon isotope with six protons and eight neutrons into nitrogen containing seven protons and seven neutrons.
Particle emission often leaves the resulting atom in an excited state. Nature, however, prefers that particles assume the state of least energy, or ground state. To this end, an excited nucleus can emit a gamma ray that carries away the excess energy as electromagnetic radiation. Gamma rays have much higher frequencies than those of light, which means they have a higher energy content. Like all forms of electromagnetic radiation, gamma rays move at the speed of light. An example of gamma ray emission occurs when cobalt undergoes beta decay to become nickel. The excited nickel gives off two gamma rays in order to drop down to its ground state of energy.
It usually takes very little time for an excited nucleus to emit a gamma ray. However, certain excited nuclei are “metastable,” meaning they may delay gamma ray emission. The delay may last only a portion of a second but could stretch out over minutes, hours, years or even longer. The delay occurs when the spin of the nucleus prohibits gamma decay. Another special effect occurs when an orbiting electron absorbs an emitted gamma ray and is ejected from orbit. This is known as the photoelectric effect.