radioactivity

1. Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
2. The radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.

A phenomenon resulting from an instability of the atomic nucleus in certain atoms whereby the nucleus experiences a spontaneous but measurably delayed nuclear transition or transformation with the resulting emission of radiation. The discovery of radioactivity by H. Becquerel in 1896 marked the birth of nuclear physics.

All chemical elements may be rendered radioactive by adding or by subtracting (except for hydrogen and helium) neutrons from the nucleus of the stable ones. Studies of the radioactive decays of new isotopes far from the stable ones in nature continue as a major frontier in nuclear research. The availability of this wide variety of radioactive isotopes has stimulated their use in a wide variety of fields, including chemistry, biology, medicine, industry, artifact dating, agriculture, and space exploration. See also Alpha particles; Beta particles; Gamma rays; Isotope; Radioactivity and radiation applications.

A particular radioactive transition may be delayed by less than a microsecond or by more than a billion years, but the existence of a measurable delay or lifetime distinguishes a radioactive nuclear transition from a so-called prompt nuclear transition, such as is involved in the emission of most gamma rays. The delay is expressed quantitatively by the radioactive decay constant, or by the mean life, or by the half-period for each type of radioactive atom, discussed below.

The most commonly found types of radioactivity are alpha, beta negatron, beta positron, electron capture, and isomeric transition. Each is characterized by the particular type of nuclear radiation which is emitted by the transforming parent nucleus. In addition, there are several other decay modes that are observed more rarely in specific regions of the periodic table.

Transition rates and decay laws

The rate of radioactive transformation, or the activity, of a source equals the number A of identical radioactive atoms present in the source, multiplied by their characteristic radioactive decay constant &lgr:.
1.
Thus Eq. (1) holds, where the decay constant &lgr: has dimensions of s⁻¹. The numerical value of &lgr: expresses the statistical probability of decay of each radioactive atom in a group of identical atoms, per unit time. For example, if &lgr: = 0.01 s⁻¹ for a particular radioactive species, then each atom has a chance of 0.01 (1%) of decaying in 1 s, and a chance of 0.99 (99%) of not decaying in any given 1‐s interval. The constant &lgr: is one of the most important characteristics of each radioactive nuclide: &lgr: is essentially independent of all physical and chemical conditions such as temperature, pressure, concentration, chemical combination, or age of the radioactive atoms.

Many radioactive nuclides have two or more independent and alternative modes of decay. For example, ²³⁸U can decay either by alpha-particle emission or by spontaneous fission. When two or more independent modes of decay are possible, the nuclide is said to exhibit dual decay. The competing modes of decay of any nuclide have independent partial decay constants given by the probabilities &lgr:₁, &lgr:₂, &lgr:₃, …, per second, and the total probability of decay is represented by the total decay constant &lgr:, defined by Eq. (2).
2.

The actual life of any particular atom can have any value between zero and infinity. The average or mean life of a large number of identical radioactive atoms is, however, a definite and important quantity. The total L of the life-spans of all the A₀ atoms initially present is given by Eq. (3). Then the average lifetime L/A₀, which is called the mean life τ, is given by Eq. (4).
3.

4.

The time interval over which the chance of survival of a particular radioactive atom is exactly one-half is called half-period T (also called the half-life, written T_1/2). The half-period T is related to the total radioactive decay constant &lgr:, and to the mean life τ, by Eq. (5). For mnemonic
5.
reasons, the half-period T is much more frequently employed than the total decay constant &lgr: or the mean life τ.

Radioactive series decay

In a number of cases a radioactive nuclide A decays into a nuclide B which is also radioactive; the nuclide B decays into C which is also radioactive, and so on. For example, ²³²₉₀Th decays into a series of 10 successive radioactive nuclides. Substantially all the primary products of nuclear fission are negatron beta-particle emitters which decay through a chain or series of two to six successive beta-particle emitters before a stable nuclide is reached as an end product.

Alpha-particle decay

Alpha-particle decay is that type of radioactivity in which the parent nucleus expels an alpha particle (a helium nucleus), which contains two protons and two neutrons. Thus, the atomic number, or nuclear charge, of the decay product is 2 units less than that of the parent, and the nuclear mass of the product is 4 atomic mass units less than that of the parent, because the emitted alpha particle carries away this amount of nuclear charge and mass. This decrease of 2 units of atomic number or nuclear charge between parent and product means that the decay product will be a different chemical element, displaced by 2 units to the left in a periodic table of the elements.

In the simplest case of alpha decay, every alpha particle would be emitted with exactly the same velocity and hence the same kinetic energy. However, in most cases there are two or more discrete energy groups called lines. For example, in the alpha decay of a large group of ²³⁸U atoms, 77% of the alpha decays will be by emission of alpha particles whose kinetic energy is 4.20 MeV, while 23% will be by emission of 4.15-MeV alpha particles. When the 4.20-MeV alpha particle is emitted, the decay product nucleus is formed in its ground (lowest energy) level. When a 4.15-MeV alpha particle is emitted, the decay product is produced in an excited level, 0.05 MeV above the ground level. This nucleus promptly transforms to its ground level by the emission of a 0.05-MeV gamma ray or alternatively by the emission of the same amount of energy in the form of a conversion electron and the associated spectrum of characteristic x-rays. Thus in all alpha-particle spectra, the alpha particles are emitted in one or more discrete and homogeneous energy groups, and alpha-particle spectra are accompanied by gamma-ray and conversion electron spectra whenever there are two or more alpha-particle groups in the spectrum.

Among all the known alpha-particle emitters, most alpha-particle energy spectra lie in the domain of 4–6 MeV, although a few extend as low as 2 MeV and as high as 10 MeV. There is a systematic relationship between the kinetic energy of the emitted alpha particles and the half-period of the alpha emitter. The highest-energy alpha particles are emitted by short-lived nuclides, and the lowest-energy alpha particles are emitted by the very-long-lived alpha-particle emitters. H. Geiger and J. M. Nuttall showed that there is a linear relationship between log &lgr: and the energy of the alpha particle.

The Geiger-Nuttall rule is inexplicable by classical physics, but emerges clearly from quantum, or wave, mechanics. In 1928 the hypothesis of transmission through nuclear potential barriers was shown to give a satisfactory account of the alpha-decay data, and it has been altered subsequently only in details.

Beta-particle decay

Beta-particle decay is a type of radioactivity in which the parent nucleus emits a beta particle. There are two types of beta decay established: in negatron beta decay (β⁻) the emitted beta particle is a negatively charged electron (negatron); in positron beta decay (β⁺) the emitted beta particle is a positively charged electron (positron). In beta decay the atomic number shifts by one unit of charge, while the mass number remains unchanged. In contrast to alpha decay, when beta decay takes place between two nuclei which have a definite energy difference, the beta particles from a large number of atoms will have a continuous distribution of energy.

For each beta-particle emitter, there is a definite maximum or upper limit to the energy spectrum of beta particles. This maximum energy, E_max, corresponds to the change in nuclear energy in the beta decay. As in the case of alpha decay, most beta-particle spectra include additional continuous spectra which have less maximum energy and which leave the product nucleus in an excited level from which gamma rays are then emitted.

For nuclei very far from stability, the energies of these excited states populated in beta decay are so large that the excited states may decay by proton, two-proton, neutron, two-neutron, three-neutron, or alpha emission, or spontaneous fission.

The continuous spectrum of beta-particle energies implies the simultaneous emission of a second particle besides the beta particle, in order to conserve energy and angular momentum for each decaying nucleus. This particle is the neutrino. The neutrino has zero charge and extremely small rest mass, travels at nearly the same speed as light (3 × 10¹⁰ cm/s), and is emitted as a companion particle with each beta ray. By postulating the simultaneous emission of a beta particle and a neutrino, E. Fermi developed in 1934 a quantum-mechanical theory which satisfactorily gives the shape of beta-particle spectra, and the relative half-periods of beta-particle emitters for allowed beta decays. See also Neutrino.

When the ground state of a nucleus differing by two units of charge from nucleus A has lower energy than A, then it is theoretically possible for A to emit two beta particles, either β⁺β⁺ or β⁻β⁻ as the case may be, and two neutrinos or antineutrinos, and go from Z to Z ± 2. Here two protons decay into two neutrons, or vice versa. This is a second-order process and so should go much slower than beta decay. There are a number of cases where such decays should occur, but their half-lives are of the order of 10²⁰ years or greater. Such decay processes are obviously very difficult to detect. The first direct evidence for two-neutrino double-beta-minus decay of selenium-82, was found only in 1987.

Whenever it is energetically allowed by the mass difference between neighboring isobars, a nucleus Z may capture one of its own atomic electrons and transform to the isobar of atomic number Z − 1. Usually the electron-capture (EC) transition involves an electron from the K shell of atomic electrons, because these innermost electrons have the greatest probability density of being in or near the nucleus. See also Electron capture.

Gamma-ray decay

Gamma-ray decay involves a transition between two excited levels of a nucleus, or between an excited level and the ground level. A nucleus in its ground level cannot emit any gamma radiation. Therefore gamma-ray decay occurs only as a sequel of another radioactive decay process or of some other process whereby the product nucleus is left in an excited state. Such additional processes include the fusion of two nuclei, Coulomb excitation, and induced nuclear fission. See also Coulomb excitation; Nuclear fission; Nuclear fusion.

A gamma ray is high-frequency electromagnetic radiation (a photon) in the same family with radio waves, visible light, and x-rays. The energy of a gamma ray is given by hν, where h is Planck's constant and ν is the frequency of oscillation of the wave in hertz. The gamma-ray or photon energy hv lies between 0.05 and 3 MeV for the majority of known nuclear transitions. Higher‐ energy gamma rays are seen in neutron capture and some reactions. See also Electromagnetic radiation.

Gamma rays carry away energy, linear momentum, and angular momentum, and account for changes of angular momentum, parity, and energy between excited levels in a given nucleus. This leads to a set of gamma-ray selection rules for nuclear decay and a classification of gamma-ray transitions as “electric” or as “magnetic” multipole radiation of multipole order 2^l, where l = 1 is called dipole radiation, l = 2 is quadrupole radiation, and l = 3 is octupole, l being the vector change in nuclear angular momentum. The most common type of gamma-ray transition in nuclei is the electric quadrupole (E2). There are cases where several hundred gamma rays with different energies are emitted in the decays of atoms of only one isotope. See also Multipole radiation.

An alternative type of deexcitation which always competes with gamma-ray emission is known as internal conversion. Instead of the emission of a gamma ray, the nuclear excitation energy can be transferred directly to a bound electron of the same atom. Then the nuclear energy difference is converted to energy of an atomic electron, which is ejected from the atom.

When the energy between two states in the same nucleus exceeds 1.022 MeV, twice the rest mass energy of an electron, it is also possible for the nucleus to give up its excess energy to an electron-positron pair—a pair creation process. See also Electron-positron pair production.

Spontaneous fission

This involves the spontaneous breakup of a nucleus into two heavy fragments and neutrons. Spontaneous fission can occur when the sum of the masses of the two heavy fragments and the neutrons is less than the mass of the parent undergoing decay. After the discovery of fission in 1939, it was subsequently discovered that isotopes like ²³⁸U had very weak decay branches for spontaneous fission, with branching ratios on the order of 10⁻⁶. Some isotopes with relatively long half-lives such as ²⁵²Cf have large (3.1%) spontaneous fission branching.

Heavy cluster decays

Alpha-particle decay and spontaneous fission are two natural phenomena in which an atomic nucleus spontaneously breaks into two fragments, but the fragments are of very different size in one case and almost equal size in the other. On the basis of fragmentation theory and the two-center shell model, new kinds of radioactivities that are intermediate between alpha particle decay and fission were predicted in 1980. Subsequently, it was shown theoretically that the new processes should occur throughout a very broad region of the nuclear chart, including elements with atomic numbers higher than 40. However, experimentally observable emission rates could be expected only for nuclei heavier than lead, in a breakup leading to a very stable heavy fragment with proton and neutron numbers equal or very close to Z = 82, N = 126 (²⁰⁸₈₂Pb or its neighborhood). The main competitor is always alpha-particle decay. In 1984, a series of experimental confirmations began with the discovery of ¹⁴ ₆C radioactivity of ²²³₈₈Ra. A very promising technique uses solid-state track-recording detectors with special plastic films and glasses that are sensitive to heavier clusters but not to alpha particles.

Proton radioactivity

Proton radioactivity is a mode of radioactive decay that is generally expected to arise in proton-rich nuclei far from the stable isotopes, in which the parent nucleus changes its chemical identity by emission of a proton in a single-step process. Its physical interpretation parallels almost exactly the quantum-mechanical treatment of alpha-particle decay. For many years only a few examples of this decay mode were observed, because of the narrow range of half-lives and decay energies where this mode can compete with other modes. However, in the late 1990s, experimental techniques using new recoil mass spectrometers, which can separate rare reaction products, and new double-sided silicon strip detectors became available and opened up the discovery of many new proton radioactivities. Two-proton radioactivity (the simultaneous emission of two protons) was first observed in 2001 in an excited state of neon-18 and in 2002 in the ground state of iron-45.

Delayed particle emissions

Twelve types of beta-delayed particle emissions have been observed. Beta-delayed deuteron (²H) emission, which is not shown there, also can be expected. Over 100 beta-delayed particle radioactivities are now known. Theoretically, the number of isotopes which can undergo beta-delayed particle emission could exceed 1000. Thus, this mode, which was observed in only a few cases prior to 1965, is among the important ones in nuclei very far from the stable ones in nature. Studies of these decays can provide insights into the nucleus which can be gained in no other way.

When Henri Becquerel established the existence of ‘uranic rays’ in March 1896, there was no way of appreciating the far-reaching implications of this discovery. Only 75 elements had been discovered by this time, two of which, uranium and thorium, were radioactive, although this was not known. The periodic table has since been expanded to 81 stable and 31 radioactive elements.

Radioactivity is the process of emission of radiation as a radioactive material changes form, often to a different element. To understand this process, we need to be familiar with a number of concepts and terms. Atoms of each element contain a different and defining number of protons and an equal number of electrons. The nucleus of the atom contains neutrons as well as protons and different numbers of neutrons are present in different isotopes of the same element. Isotopes of an element may be either stable, or unstable and radioactive — radioisotopes. Isotopes of all elements are referred to collectively as nuclides; those that are radioactive, as radionuclides. Radionuclides are specified by the elemental name and the mass number — the combined number of protons and neutrons — for example, carbon-14 (¹⁴C), iodine-131 (¹³¹I), plutonium-239 (²³⁹Pu). Where an element is referred to as radioactive, as in the paragraph above, this is intended to mean that all isotopes of the element are radioactive. Radionuclides differ in their rate of decay as well as the radiation emitted. The rate of decay in a given mass of the radionuclide is measured in units called becquerels (Bq), where 1 Bq equals one transformation per second. Alpha-emitting radionuclides emit alpha particles, each consisting of 2 protons and 2 neutrons. Beta-emissions involve the loss of an electron from the nucleus as a beta particle during the conversion of a neutron into a proton. Gamma rays are high energy photons, often emitted together with beta or alpha radiations when the transformation has left the atom with excess energy. An important characteristic of a radionuclide, as well as the radiation emitted, is its half-life — the time taken for half the atoms present to decay to the daughter nuclide. Thus ¹³¹I is a beta-emitter with a half-life of 8 days, while ²³⁹Pu is an alpha-emitter with a half-life of 24 000 years.

We are exposed to radionuclides throughout our lives, mainly from natural sources. The greatest exposures are due to inhalation of radon-222 (²²²Rn) gas, present in the atmosphere due to the decay of uranium-238 contained in rocks and soil. Artificial sources include the medical use of radiopharmaceuticals and small amounts released by the nuclear industry. There is, of course, the potential for greater exposures from nuclear installations if accidents occur, the most noteable example being the accident at Chernobyl in the former Soviet Union in 1986.

The health risk associated with exposure to a particular radionuclide will depend on the radiation emitted and its chemical behaviour. Beta and gamma radiations can penetrate through the skin and may pose an external radiation hazard, but the main concern generally is the entry of radionuclides into the body by inhalation and ingestion. Intake will lead to dose delivery to the respiratory and alimentary tracts, and absorption into the blood followed by entry into other organs and tissues. Depending on their chemical behaviour, some radionuclides concentrate in specific organs and tissues. For example, iodine-131 concentrates in the thyroid gland because iodine is an essential constituent of the hormone, thyroxine. Consequently, the dose to the thyroid is much greater than doses to other tissues, presenting a potential risk of thyroid cancer. Plutonium-239 is deposited mainly in the skeleton and liver and presents a potential risk of liver and bone cancer and leukaemia. Doses are calculated for intakes of radionuclides, taking account of their distribution and retention in the body and the pattern of deposition of radiation energy in different tissues. These calculations are done primarily by the International Commission on Radiological Protection, and the calculated values of dose per unit intake (Sv per Bq) are used as a basis for restrictions on radionuclide exposure in legislation in Europe, the UK, and elsewhere.

— John D. Harrison

See also imaging techniques; radiation, ionizing; radiology; radiotherapy.

Spontaneous nuclear disintegration with emission of corpuscular or electromagnetic radiations. The principal types of radioactivity are alpha disintegration, beta decay (negatron emission, positron emission, and electron capture), and isometric transition. Double beta decay is another type of radioactivity that has been postulated, and spontaneous fission and the spontaneous transformations of mesons are sometimes considered to be types of radioactivity. To be considered radioactive, a process must have a measurable lifetime between approximately 1 and 10 seconds and 1017 years, according to present experimental techniques. Radiations emitted within a time too short for measurement are called prompt; however, prompt radiations, including gamma rays, characteristic x-rays, conversion and auger electrons, delayed neutrons, and annihilation radiation, are often associated with radioactive disintegrations because their emission may follow the primary radioactive process.

n. the spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nuclei of an unstable isotope.

See the Introduction, Abbreviations and Pronunciation for further details.

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The emission of elementary particles by some atoms when their unstable nuclei disintegrate (see half-life). Materials composed of such atoms are radioactive. (See alpha radiation, beta radiation, and gamma radiation.)

The quality of emitting or the emission of particulate or electromagnetic radiation as a consequence of the decay of the nuclei of unstable elements, a property of all chemical elements of atomic number above 83, and possible of induction in all other known elements.
The chemical elements are made up of atoms, each of which consists of a nucleus around which orbits a cloud of negatively charged electrons. The nucleus itself is made up of two kinds of particles: neutrons, which have no electrical charge; and protons, each of which has a single positive charge. A neutral atom has an equal number of protons and electrons and no electric charge. The atomic number of an element is the number of protons in the nucleus of each of its atoms. The mass number of an element is the sum of the number of protons and neutrons in the nucleus.
All of the atoms of a particular element have the same atomic number, but can have different numbers of neutrons. An isotope of a chemical element consists of atoms having the same number of protons, but a different number of neutrons. When an atomic nucleus is unstable it decomposes or decays spontaneously, emitting high-energy particles. The emissions from radioactive decay can consist of electrons (beta particles), or electromagnetic energy in the form of photons, or helium ions (alpha particles). (See also radiation.) The process of decay can produce a product that is itself unstable, in which case it too will decay. The process continues until a stable nuclide is finally formed.
The radioactivity of a substance can be measured by determining the rate at which atoms decay in a given period of time. The basic unit of measurement of radioactivity is the becquerel (Bq), which is equal to 1 disintegration per second. The now outdated but possibly still used unit is the curie (Ci), which equals 37 thousand-million disintegrations per second. One-thousandth of a curie is a millicurie; one-millionth is a microcurie. These units of measure are used to calculate the dosage of radioactivity administered for various therapeutic procedures in much the same way that units of measure such as the gram and milligram are used to measure dosages of medications.
The half-life of an element is the time necessary for one-half of a given amount of the isotope to decay. Half-lives can range from thousands of millions of years to fractions of a second. The rate at which atomic decay occurs in a particular isotope cannot be altered by any outside force such as temperature, pressure or chemical reaction. The knowledge of the half-life of a particular isotope is essential to the proper handling of the substance for the protection of the medical staff and the animal receiving some form of radiation therapy.

The spontaneous breaking apart, or decay, of unstable nuclei in isotopes. The unstable radioactive isotope is called the parent, and the products of the decay of the parent are called the daughter isotopes.

(DOD) The spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nuclei of an unstable isotope.