Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation spontaneously. Radioactivity is a physical, not a biological, phenomenon.
Radioactivity can occur both naturally and through human intervention. An example of artificially induced radioactivity is neutron activation. A neutron fired into a nucleus can cause nuclear fission (the splitting of atoms); this is the basic concept behind the atomic bomb. Neutron activation is also the underlying principle of boron-neutron capture therapy for certain brain cancers. A solution containing boron is injected into a patient and is absorbed more by cancer than by other cells. Neutrons fired at the area of the brain cancer are readily absorbed (captured) by the boron nuclei. These nuclei then become unstable and emit radiation that attacks the cancer cells.
The radioactivity of a sample can be measured by counting how many atoms are spontaneously decaying each second. This can be done with instruments designed to detect the particular type of radiation emitted with each “decay” or disintegration. The actual number of disintegrations per second may be quite large. Scientists have agreed upon standard units to use as a form of shorthand. Thus, a curie (abbreviated “Ci” and named after Pierre and Marie Curie, the discoverers of radium) is simply a shorthand way of writing “37,000,000,000 disintegrations per second,” the rate of disintegration occurring in 1 gram of radium.
The more modern International System of Measurements (SI) unit for the same type of measurement is the becquerel (abbreviated “Bq” and named after Henri Becquerel, the discoverer of radioactivity), which is simply a shorthand for “1 disintegration per second.”
The emitted radiation is done by an atomic nucleus that, for some reason, is unstable; it “wants” to give up some energy in order to shift to a more stable configuration.
- Too many neutrons in a nucleus lead it to emit a negative beta particle, which changes one of the neutrons into a proton.
- Too many protons in a nucleus lead it to emit a positron (positively charged electron), changing a proton into a neutron.
- Too much energy leads a nucleus to emit a gamma ray, which discards high energy without changing any of the particles in the nucleus.
- Too much mass leads a nucleus to emit an alpha particle, discarding four heavy particles (two protons and two neutrons).
Radioactive decay is determined by quantum mechanics – which is inherently probabilistic. So it is impossible to work out when any particular atom will decay, but we can make predictions based on the statistical behavior of large numbers of atoms.
Being unstable does not lead an atomic nucleus to emit radiation immediately. Instead, the probability of an atom disintegrating is constant, as if unstable nuclei continuously participate in a sort of lottery, with random drawings to decide which atom will next emit radiation and disintegrate to a more stable state.
The half-life of a radioactive isotope is the time after which, on average, half of the original material will have decayed. After two half-lives, half of that will have decayed again and a quarter of the original material will remain, and so on.
The time it takes for half of the atoms in a given mass to emit radiation and change to a more stable state, is called the half-life.
Half-lives vary significantly among types of atoms, from less than a second to billions of years. For example, it will take about 4.5 billion years for half of the atoms in a mass of uranium 238 to spontaneously disintegrate, but only 24,000 years for half of the atoms in a mass of plutonium 239 to spontaneously disintegrate. Iodine 131, commonly used in medicine, has a half-life of only eight days.