Radioactive decay is the process by which an
atomic nucleus of an unstable atom loses energy by emitting ionizing particles (
ionizing radiation). The emission is spontaneous, in that the atom decays without any interaction with another particle from outside the atom (i.e., without a
nuclear reaction). Usually, radioactive decay happens due to a process confined to the nucleus of the unstable atom, but, on occasion (as with the different processes of
electron capture and
internal conversion), an inner electron of the radioactive atom is also necessary to the process.
Radioactive decay is a
stochastic (i.e., random) process at the level of single atoms, in that, according to
quantum theory, it is impossible to predict when a given atom will decay.
[1] However, given a large number of identical atoms (nuclides), the decay rate for the collection is predictable, via the
Law of Large Numbers.
The decay, or loss of energy, results when an atom with one type of nucleus, called the
parent radionuclide, transforms to an atom with a nucleus in a different state, or a different nucleus, either of which is named the
daughter nuclide. Often the parent and daughter are different
chemical elements, and in such cases the decay process results in
nuclear transmutation. In an example of this, a
carbon-14 atom (the "parent") emits radiation (a
beta particle,
antineutrino, and a
gamma ray) and transforms to a
nitrogen-14 atom (the "daughter"). By contrast, there exist two types of radioactive decay processes (
gamma decay and
internal conversion decay) that do not result in transmutation, but only decrease the energy of an excited nucleus. This results in an atom of the same element as before but with a nucleus in a lower energy state. An example is the
nuclear isomer technetium-99m decaying, by the emission of a gamma ray, to an atom of
technetium-99.
Nuclides produced as daughters are called
radiogenic nuclides, whether they themselves are
stable or not. A number of naturally occurring
radionuclides are short-lived radiogenic nuclides that are the daughters of radioactive
primordial nuclides (types of radioactive atoms that have been present since the beginning of the Earth and solar system). Other naturally occurring radioactive nuclides are
cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust. For a summary table showing the number of stable nuclides and of radioactive nuclides in each category, see
Radionuclide.
The
SI unit of activity is the
becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 10
9 decays per second) or TBq (terabecquerel, 1 x 10
12 decays per second) are commonly used. Another unit of radioactivity is the
curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with one gram of pure
radium,
isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 10
10 Bq. The use of Ci is presently discouraged by the SI.
Explanation
The interplay of these forces produces a number of different phenomena in which energy may be released by rearrangement of particles in the nucleus or the change of one particle into others. The rearrangement is hindered energetically, so that it does not occur immediately. Random quantum
vacuum fluctuations are theorized to promote relaxation to a lower energy state (the "decay") in a phenomenon known as
quantum tunneling.
One might draw an analogy with a snowfield on a mountain: While
friction between the ice crystals may be supporting the snow's weight, the system is inherently unstable with regard to a state of lower potential energy. A disturbance would thus facilitate the path to a state of greater
entropy: The system will move towards the ground state, producing heat, and the total energy will be distributable over a larger number of
quantum states. Thus, an
avalanche results. The
total energy does not change in this process, but, because of the
law of entropy, avalanches happen only in one direction and that is toward the "
ground state" — the state with the largest number of ways in which the available energy could be distributed.
Such a collapse (a
decay event) requires a specific
activation energy. For a snow avalanche, this energy comes as a disturbance from outside the system, although such disturbances can be arbitrarily small. In the case of an excited
atomic nucleus, the arbitrarily small disturbance comes from quantum
vacuum fluctuations. A radioactive nucleus (or any excited system in quantum mechanics) is unstable, and can, thus,
spontaneously stabilize to a less-excited system. The resulting transformation alters the structure of the nucleus and results in the emission of either a photon or a high-velocity particle that has mass (such as an electron,
alpha particle, or other type).
Danger of radioactive substances
The dangers of radioactivity and radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when electrical engineer and physicist
Nikola Tesla intentionally subjected his fingers to X-rays in 1896.
[3] He published his observations concerning the burns that developed, though he attributed them to ozone rather than to X-rays. His injuries healed later.
The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927,
Hermann Joseph Muller published research showing genetic effects, and in 1946 was awarded the
Nobel prize for his findings.
Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as
patent medicine and
radioactive quackery. Examples were radium
enema treatments, and radium-containing waters to be drunk as tonics.
Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood (Curie later died from
aplastic anemia, which was likely caused by exposure to ionizing radiation). By the 1930s, after a number of cases of bone necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.
Types of decay
As for types of radioactive radiation, it was found that an
electric or
magnetic field could split such emissions into three types of beams. For lack of better terms, the rays were given the
alphabetic names
alpha,
beta, and
gamma, still in use today. While alpha decay was seen only in heavier elements (atomic number 52,
tellurium, and greater), the other two types of decay were seen in all of the elements.
In analyzing the nature of the decay products, it was obvious from the direction of
electromagnetic forces produced upon the radiations by external magnetic and electric fields that
alpha rays carried a positive charge,
beta rays carried a negative charge, and
gamma rays were neutral. From the magnitude of deflection, it was clear that
alpha particles were much more massive than
beta particles. Passing alpha particles through a very thin glass window and trapping them in a
discharge tube allowed researchers to study the
emission spectrum of the resulting gas, and ultimately prove that alpha particles are
helium nuclei. Other experiments showed the similarity between classical beta radiation and
cathode rays: They are both streams of
electrons. Likewise gamma radiation and X-rays were found to be similar high-energy
electromagnetic radiation.
The relationship between types of decays also began to be examined: For example, gamma decay was almost always found associated with other types of decay, occurring at about the same time, or afterward. Gamma decay as a separate phenomenon (with its own half-life, now termed
isomeric transition), was found in natural radioactivity to be a result of the gamma decay of excited metastable
nuclear isomers, in turn created from other types of decay.
Although alpha, beta, and gamma were found most commonly, other types of decay were eventually discovered. Shortly after the discovery of the
positron in cosmic ray products, it was realized that the same process that operates in classical
beta decay can also produce positrons (
positron emission). In an analogous process, instead of emitting positrons and neutrinos, some proton-rich nuclides were found to capture their own atomic electrons (
electron capture), and emit only a neutrino (and usually also a gamma ray). Each of these types of decay involves the capture or emission of nuclear electrons or positrons, and acts to move a nucleus toward the ratio of neutrons to protons that has the least energy for a given total number of
nucleons (neutrons plus protons).
Shortly after discovery of the
neutron in 1932, it was discovered by
Enrico Fermi that certain rare decay reactions yield neutrons as a decay particle (
neutron emission). Isolated
proton emission was eventually observed in some elements. It was also found that some heavy elements may undergo
spontaneous fission into products that vary in composition. In a phenomenon called
cluster decay,
specific combinations of neutrons and protons (atomic nuclei) other than alpha particles (helium nuclei) were found to be spontaneously emitted from atoms, on occasion.
Other types of radioactive decay that emit previously seen particles were found, but by different mechanisms. An example is
internal conversion, which results in electron and sometimes high-energy photon emission, even though it involves neither beta nor gamma decay. This type of decay (like
isomeric transition gamma decay) did not transmute one element to another.
Rare events that involve a combination of two beta-decay type events happening simultaneously (see below) are known. Any decay process that does not violate conservation of energy or momentum laws (and perhaps other particle conservation laws) is permitted to happen, although not all have been detected. An interesting example (discussed in a final section) is
bound state beta decay of
rhenium-187. In this process, an inverse of
electron capture, beta electron-decay of the parent nuclide is not accompanied by beta electron emission, because the beta particle has been captured into the K-shell of the emitting atom. An antineutrino, however, is emitted.
