Radioactivity is the spontaneous emission of energy in the form of particles or waves (electromagnetic radiation), or both, from the atomic nucleus of certain elements. HISTORY The discovery of radioactivity occurred in 1896 when Antoine Henri \TBecquerel\t observed that uranium emitted penetrating rays continuously and without initiation. The term radioactivity was coined by Pierre and Marie \TCurie\t to designate this phenomenon. They proved that the radioactivity of uranium was an atomic property and not a chemical one. Marie Curie later discovered the radioactive elements polonium and radium in uranium ore. These elements possess shorter HALF-LIVES (the time it takes for the radioactive decay of one-half of a radioactive sample) and are more highly radioactive than uranium. Ernest \TRutherford\t showed that one of the components of this radiation was deflected upon passage through a thin sheet of metal. He concluded that this phenomenon was due to positive electric charge repulsion between the metal ions of the lattice and a positively charged particle emitted by the radioactive sample and later shown to be a helium ion (an alpha particle). This led to the postulation of the nucleus and was one of the foundations for the formulation of the structure of the \Tatom\t. ATOMIC STRUCTURE AND NUCLEAR TRANSFORMATIONS An atom is composed of two major components, a positively charged nucleus surrounded by a cloud of negatively charged particles (electrons). The nucleus is composed of protons and neutrons, which are collectively called nucleons. Protons are positively charged nucleons, whereas neutrons are uncharged nucleons. The sum of protons and neutrons is the atom's mass number. An atom is neutral with respect to charge, with the number of protons equaling the number of electrons. The number of protons is equivalent to the atomic number, whereas the number of neutrons may vary for a given atomic number; each variation is called an \Tisotope\t. Therefore, isotopes of a given atomic number will vary in their mass numbers. Usually, the lighter elements have fewer isotopes than the heavier elements. An isotope of a given element may be designated with the atomic symbol for the element either preceded or followed by a superscript number representing the mass number. Another accepted designation is the element name followed by a hyphen and the mass number of the isotope, as in nitrogen-15. Stable and Unstable Nuclei In general, the proton-neutron ratio as well as their total number determines the stability of the nucleus. Unstable nuclei tend to adjust their proton-neutron ratio to a more stable form by means of the spontaneous disintegration via expulsion of one or more of the nucleons, that is, radioactivity. Stable isotopes vary in number from element to element, with the naturally occurring element uranium having no stable isotopes. It is not clear why certain combinations of protons and neutrons are stable while others are not. Unstable nuclei, or radioisotopes, undergo radioactive decay resulting in particle or electromagnetic radiation. Radioisotopes may occur naturally or be produced artificially (see \Tisotope\t; \Tradiochemistry\t). The vast majority of radioisotopes are artificially produced, since the only naturally occurring radionuclides (radioisotopes) surviving from the time of the creation of matter are those that decay very slowly, and members of a decay series (naturally produced continuously) are very scarce. Some radioisotopes are produced continually by \Tcosmic ray\t bombardment of atmospheric atoms. Carbon-14 and hydrogen-3 are formed from nitrogen-14 in this manner. Artificially produced radioisotopes were first synthesized more than 40 years ago when Curie and Joliot transformed aluminum-27 to phosphorus-30. Bombardment of stable nuclei with either charged or uncharged particles is the usual method for radioisotope production. This result may be achieved by nuclear reactors, which are the primary source of radioisotopes for biological purposes, by particle accelerators such as the cyclotron and linear accelerator, or by other neutron sources such as a neutron generator. Types of Nuclear Transformations In undergoing spontaneous nuclear changes, radioisotopes decay to more stable forms while giving off one or more of three types of emissions: alpha particles, beta particles, and gamma rays. The emission of alpha or beta particles converts one element into another via a nuclear charge change. Gamma radiation is a form of energy dissipation in which a nucleus in a high-energy state drops to a lower, more stable energy state. Alpha particle emission occurs only in elements of high atomic weight. An alpha particle is a helium nucleus consisting of two protons and two neutrons. Emission of an alpha particle from a radioisotope results in the formation of another element four mass units lighter and two atomic numbers lower. Beta particles are of two types, negatrons (electrons) and positrons. Radioisotopes with excess neutrons may decay into a more stable form by the conversion of a neutron into a proton, with the concurrent emission of a negative beta particle. Negatron emission results in an increase in atomic number of one unit. Any excess energy in the nucleus is dissipated as gamma rays. Radioisotopes with excess protons may become more energetically stable by positive beta particle emission. A nuclear proton is converted to a neutron with the concurrent emission of a positron. The atomic number decreases by one unit and any excess energy is emitted as gamma radiation. (See \Tbeta decay\t.) Gamma radiation is emitted as \Lphoton\ls, or discrete quanta of energy. Gamma radiation most often results in concurrence with beta emission; however, some nuclides decay by gamma ray emission alone. This means of decay does not change the mass number or the atomic number but provides a delayed means of disposing of excess energy from an energetic nucleus. Several other such forms of radioactive decay involving emission of groups of nucleons have also been observed. In fact, fragments of almost any size relative to a given nucleus may be ejected. Such events occur rarely and at random, when nuclei rearrange themselves spontaneously. Further types of radioactive decay include the extremely rare form known as double beta decay (see \Tbeta decay\t). UNITS AND STANDARDS The rates of emission of radiation for different radioisotopes vary considerably. Each individual radioisotope, however, has its own intrinsic decay rate. The decay constant is defined as the given fraction of atoms disintegrating in a specific unit of time. A more useful way of expressing the decay constant is the half-life. The half-life of a radioisotope is the time it takes for its radioactivity to decrease by one-half. Half-lives vary from fractions of seconds to billions of years. The standard unit of radioactivity is the curie, which is defined as the number of disintegrations occurring in one gram of radium per second. Radium was chosen because it was available in pure form and has a long half-life, 1,600 years. The curie is equivalent to 3.7 X 10 to the power of 10 disintegrations per second (dps). The curie is a rather large unit, so several fractions of this unit have found wide use. The millicurie (mC) is equal to one-thousandth of a curie, or 3.7 X 10 to the power of 7 dps, while the microcurie is equal to one-millionth of a curie, or 3.7 X 10 to the power of 4 dps, or 2.22 X 10 to the power of 6 disintegrations per minute (dpm). The usual state of a radioisotope is as a mixture with a large amount of the stable isotopes of the same element. Specific activity, defined as the amount of radioactivity per given weight or weight equivalent of a sample, expresses the relative abundance of a radioisotope in a sample. Specific activity is often expressed as dps or dpm, counting rates (counts per minute, cpm), or curies, mC, or micro C (using the Greek lower-case letter mu to represent micro) per unit weight. MEASUREMENT OF RADIOACTIVITY Radioactivity is quantitated in several ways. Absolute counting measures every disintegration occurring in the sample (dps or dpm), whereas relative counting measures a given detected fraction of the true disintegrations occurring (cps or cpm). Relative counting is much easier and is the more frequently used approach. Several different methods of relative counting are used successfully. These include gas ionization, scintillation, and autoradiography. Gas ionization techniques use the principle of ion pairs formed in gases upon exposure to radiation. An electric potential is applied between two electrodes in a gas-filled ion chamber. The negatively charged ions move to the anode while the positively charged ions move to the cathode. This creates a pulse, which is amplified and recorded. Gas ionization without gas amplification may be achieved using ionization chambers equipped with Lauritsen electroscopes or with a vibrating reed electrometer. Gas-ionization counting with gas amplification may be achieved using proportional counters or the well-known \TGeiger counter\t Scintillation in a solid fluor is a counting technique in which a fraction of the ionizing radiation is transferred to solid compounds that fluoresce. The absorbed energy in the fluor gives rise to visible or near ultraviolet energy emissions (scintillations) that are detected and amplified by a photomultiplier tube and recorded. Scintillation in a liquid fluor is a similar technique, except that the radioisotope and the fluor are dissolved in a liquid medium. Energy transfer goes through the solvent to the fluor and is finally detected as scintillations. Detection, amplification, and recording are similar. Autoradiography is a photochemical detection method in which a radioactive sample is placed on a photographic emulsion on film. Radiation from the sample interacts with the silver halide in the emulsion. The resulting development of the film allows an estimate of the radioactivity in the sample to be taken. USES AND APPLICATIONS A general classification of the uses and applications of radioisotopes in industrial processes or in scientific research based on the radioisotopic properties results in five major divisions: 1. Uses based on the effect of ionizing radiation on matter; 2. Uses based on the effect of matter on ionizing radiation; 3. Age-dating based on the decay rates of specific naturally occurring radioisotopes; 4. Direct energy transformation; 5. Physical and biological radiotracer applications. The interaction of radiation with matter has many practical applications. Gamma radiation has been used in food sterilization, polymer manufacture, and cancer therapy. Cobalt-60 and radium-226 are commonly used for these purposes. Other uses include thickness gauging and tool-wear estimation. A number of products are manufactured in a roll or continuous sheet that needs to be of uniform and known thickness. The amount of radiation passing through a sheet of material depends upon its thickness. Thus, with proper calibration, radioactivity can be used to monitor the thickness of these types of products. In age-dating, the half-life of the radioisotope is critical. Isotopes such as carbon-14, potassium-40, uranium-238, tritium (hydrogen-3), and others are used to determine the age of specific items or events. For instance, the carbon-14 content of living matter is in equilibrium with the carbon-14 content of the atmospheric carbon dioxide. Upon death, the carbon-14 in the dead material would no longer be exchangeable with carbon dioxide in the air, so the carbon-14 would diminish as dictated by its half-life, without replenishment from atmospheric carbon-14 dioxide. Comparison of the carbon-14 content of the dead material with the carbon-14 content of the air, which is assumed to have remained approximately the same since creation, allows the calculation of the age of the material (see \Tradiometric age-dating\t). Conversion of the energy (heat) from selected radioisotopes--such as plutonium-238 or strontium-90--to electrical energy can be achieved by an array of thermocouples and provides an ideal energy source for satellites or remote automated weather stations. The use of radioisotopes in biological research is widespread because of the extreme sensitivity of radioactive assays and the fact that the metabolic rate of a compound may be traced in living systems. A typical radiotracer experiment may result in a detection level of 10 to the power of - 8 with respect to the detection level of the unlabeled compound. Carbon-14, tritium, phosphorus-32, sulfur-35, and others are used for these purposes. HAZARDS AND SAFETY The effect of radiation on living matter can be quite devastating (see \Tradiation injury\t). Therefore, radiation safety and monitoring is particularly important. Radiation safety problems fall into three categories: personnel protection, contamination control, and waste disposal (see \Tnuclear energy\t). Energy dissipation must be quantitated to determine exposure levels. Biological effects of radiation are determined by the amount of energy absorbed. Therefore the time of exposure and rate of exposure must be defined. The roentgen (R) was defined as the quantity of gamma or X radiation required to produce one electrostatic unit of electricity of either sign per cubic centimeter of dry air. However, the roentgen is valid only for photon interaction with air and does not relate to tissue absorption or particulate radiation. Normally, exposure is expressed in roentgens/hour or milliroentgens/hour. A unit based on the energy dissipation of radiation in biological tissue was devised in 1953 and called the \Trad\t. It was defined as 100 ergs of energy imparted by any ionizing radiation that is dissipated in one gram of irradiated material. The rad is the unit of choice when tissue irradiation is concerned. Monitoring of radiation is essential to determining exposure. Area monitoring is usually accomplished with portable monitors such as Geiger-Muller survey meters or portable ionization chambers. Personnel monitoring is often achieved by film badges, which develop upon exposure to certain levels or total quantities of radiation. Disposal of radioactive wastes may be accomplished by maximum dilution or maximum concentration. Disposal by concentration and storage is necessary for high levels of radioactivity. RONALD D. \TJohnson\t Bibliography: Greiner, Walter, and Sandulescu, Aurel, "New Radioactivities," Scientific American, March 1990; Jenkins, E.N., and Lewis, I., Radioactivity (1979); Kathren, R.L., Radioactivity in the Environment (1984); Mann, W.B., and Ayres, R.L., eds., Radioactivity and Its Measurement, 2d ed. (1980); Miller, D.G., Radioactivity and Radiation Detection (1972); Pizzarello, D.J., ed., Radiation Biology (1982); Stewart, D.C., Handling Radioactivity (1981; repr. 1988).