Isotope, Radioactivity and Half-life
Isotope is one of two or more species of atom having the same atomic number, hence constituting the same element, but differing in mass number. As atomic number is equivalent to the number of protons in the nucleus, and mass number is the sum total of the protons plus the neutrons in the nucleus, isotopes of the same element differ from one another only in the number of neutrons in their nuclei.
Experiments carried out early in the 20th century indicated that radioactive substances that were chemically inseparable might differ from each other only in the structure of their nuclei. The British physicist Sir Joseph Thomson demonstrated in 1912 the existence of stable isotopes by passing neon through a discharge tube and deflecting the neon ions by means of magnetic and electric fields; this showed that the stable element neon exists in more than one form. Thomson found two isotopes of neon, one of mass number 20 and another of mass 22. Later experiments showed that naturally occurring neon contains 90% of neon-20 (the isotope with mass 20), 9.73% neon-22, and 0.27% neon-21. Research on isotopes was continued by many scientists, notably the British physicist Francis William Aston; their work in detecting and studying isotopes was accelerated by the development of the mass spectrograph.
It is now known that most elements in the natural state consist of a mixture of two or more isotopes. Among the exceptions are beryllium, aluminum, phosphorus, and sodium. The chemical atomic weight (atomic wt.) of an element is the weighted average of the individual atomic weights, or mass numbers, of the isotopes. For example, chlorine, atomic wt. 35.457, is composed of chlorine-35 and chlorine-37, the former occurring with an abundance of 76% and the latter of 24%. All the isotopes of elements with atomic numbers higher than 83 (above bismuth in the periodic table) are radioactive, and a few of the lighter isotopes, such as potassium-40, are radioactive. About 280 naturally occurring stable isotopes (not radioactive) are known.
Artificial radioactive isotopes, known also as radioisotopes, were produced for the first time in 1933 by the French physicists Irène and Frédéric Joliot-Curie. Radioisotopes are prepared by the bombardment of naturally occurring atoms with nuclear particles, such as neutrons, electrons, protons, and alpha particles using particle accelerators.
The separation of isotopes of the same element from each other is difficult. Full separation in one step by chemical methods is impossible, because isotopes of the same elements have the same chemical properties; physical methods are generally based on the extremely small differences in physical properties caused by the differences in mass of the isotopes. Electrolytic separation and various exchange procedures for isotope separation, however, depend on chemical rate or equilibrium differences that are based primarily on the difference in energy of chemical bonds, which are a function of isotope mass. The isotopes of hydrogen, deuterium (hydrogen-2) and ordinary hydrogen (hydrogen-1) were the first to be separated in appreciable quantities. This accomplishment is credited to the American chemist Harold Urey, who discovered deuterium in 1932.
Before 1940 many methods were used for the separation of small amounts of isotopes for research purposes. Some of the most successful were the centrifuge method, fractional distillation, thermal diffusion, electrolysis, gaseous diffusion, and electromagnetic separation. Each of these methods depends on the small difference in weight of the isotopes to be separated, and is most effective with the hydrogen isotopes, where the difference in mass between the two substances amounts to 100%; by contrast, the difference in mass between the carbon isotopes carbon-12 and carbon-13 or between the neon isotopes neon-20 and neon-22 amounts to only about 10%, and between the uranium isotopes uranium-235 and uranium-238 to only a little over 1%. This factor of 10 to 1 or 100 to 1 makes the separation far more than 10 or 100 times as difficult. In all processes except the electromagnetic, which is the sole one-stage procedure, isotope separation involves a series of production stages. The net result of any single stage is the separation of the original material into two fractions, one of which contains a slightly higher percentage of the heavy isotope than the original mixture and the other contains slightly more of the light isotope.
To obtain an appreciable concentration, or enrichment, in the desired isotope, it is necessary to separate further the enriched fraction. This process is usually carried out by means of a cascade, comprising a large number of stages. The enriched fraction from any stage becomes the raw material for the next stage, and the depleted fraction, which still contains a considerable percentage of the desired isotope, is mixed with the raw material for the preceding stage. Even the depleted material from the original stage is stripped in additional stages when the raw material (for example, uranium) is scarce. Suitable apparatus is designed to make the flow from stage to stage automatic and continuous.
Such a cascade is extremely flexible, and units can be shifted from one stage of the separation to another as desired. For example, in the separation of uranium, a large amount of material must be handled at the beginning, where the desired uranium-235 is mixed with about 140 times as much uranium-238; at the end of the process the uranium-235 is almost pure, and the volume of material is much smaller. Furthermore, by merely changing the piping, it is possible to shift stages to compensate for addition at an intermediate stage of material that results from preliminary enrichment by a different process.
Centrifuge and Distillation
In the centrifuge method the apparatus is so arranged that vapor flows downward in the outer part of the rotating cylinder and upward in the central region of the cylinder. The centrifugal force acts more strongly on the heavy molecules than on the light ones, increasing the concentration of the heavy isotopes in the outer region. In separation by fractional distillation a mixture containing various isotopes is distilled. The molecules of the fraction having the lower boiling point (the lighter isotopes) tend to concentrate in the vapor stream and are collected.
This method utilizes the tendency of lighter molecules of a liquid or gas to concentrate in a hot region and for heavier molecules to concentrate in a cold region. A simple form of thermal-diffusion apparatus consists of a tall vertical tube with a wire electrically heated to about 500° C (932° F) running down its center, producing a temperature gradient between the center and wall of the tube. The heavier isotopes tend to concentrate in the outer portions of the tube, and the lighter isotopes, to concentrate toward the center. At the same time, because of thermal convection, the gas or liquid near the wire tends to rise, and the cooler outer gas or liquid tends to fall. The overall effect is that the heavier isotopes collect at the bottom of the tube and the lighter at the top.
The electrolytic method of separation is of historical as well as current interest, because it was the first method used to separate practically pure deuterium. This method depends on the fact that when water undergoes electrolysis, the lighter hydrogen isotope tends to come off first, leaving behind a residue of water that is enriched in the heavier isotope.
This and the electromagnetic method of separating isotopes of uranium afforded the first large-scale separation ever achieved. The problem of separating uranium-235 from uranium-238 arose in 1940 after the demonstration of the susceptibility of the 235 isotope to fission by neutrons. Uranium-235 exists in naturally occurring uranium to the extent of 7 parts to 1000 of uranium-238. Under the auspices of the atomic bomb project, the various methods for separating isotopes were considered, and the gaseous-diffusion and electromagnetic methods were put into large-scale operation for the production of about 1 kg (2.2 lb) per day of uranium-235 to be used in atomic bombs. See NUCLEAR WEAPONS.
The gaseous-diffusion method exploits the different rate of diffusion of gases of different molecular weight. The rate of diffusion of a gas is inversely proportional to the square root of the mass; light atoms diffuse through a porous barrier faster than heavier atoms. In the separation of uranium isotopes, the only gaseous compound of uranium, the fluoride of uranium, UF6, is used. The uranium hexafluoride is pumped continuously through porous barriers. The difference in weight between uranium-235 and uranium-238 is slightly greater than 1%, but the difference in weight between the fluorides is slightly less than 1%. The enrichment factor, which depends on the square root of the above difference, is theoretically 0.43% for an instantaneous process or 0.30% for a continuous process, but in practice an enrichment factor of only about 0.14% per stage has been achieved. To produce 99% uranium-235 from natural uranium, which contains about 0.7% uranium-235, 4000 such stages are required. The process requires the use of thousands of miles of pipe, thousands of pumps and motors, and intricate control mechanisms.
Although the gaseous-diffusion method yields large amounts of uranium-235, the first comparatively large amounts of the isotope were produced by electromagnetic means at Oak Ridge, Tennessee. A series of separator units was built in which an ionic beam obtained from a uranium compound was passed through a magnetic field. Because the radius of the curvature of the path of the ions deflected by the beam depends on the mass of the ion, ions of different mass complete their path at different positions, and the uranium isotopes are appreciably separated. Only a small amount of material, however, can be treated in one operation. Because of this limitation on production, the use of the electromagnetic process for large-scale isotope separation was abandoned after the war in favor of the gaseous-diffusion process.
The concept of laser separation and enrichment of isotopes arose soon after the invention of the laser in 1960. It gained further incentive six years later with the development of the tunable dye laser, which provides photon beams in a selectably narrow range of infrared to ultraviolet wavelengths. According to this concept, if an element is first vaporized its atoms can then be selectively excited and ionized by an accurately tuned laser beam to separate out the desired isotope. Isotopes can also be separated in molecular form by selective laser-beam dissociation of those molecules of the compound that contain the desired isotope. Since 1972 such processes have been under development, particularly for uranium and plutonium enrichmentfor nuclear power and nuclear weapons, respectively. Much of the work in the U.S. is classified, but a pilot plant may be operational by the later 1980s. The method is costly and technically difficult, but only a few stages are required for production of highly enriched material.
Radioactivity, spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles (see ALPHA PARTICLE; BETA PARTICLE), or of electromagnetic rays called X rays (see X RAY) and gamma rays. The phenomenon was discovered in 1896 by the French physicist Antoine Henri Becquerel when he observed that the element uranium can blacken a photographic plate, although separated from it by glass or black paper. He also observed that the rays that produce the darkening are capable of discharging an electroscope, indicating that the rays possess an electric charge. In 1898 the French chemists Marie Curie and Pierre Curie deduced that radioactivity is a phenomenon associated with atoms, independent of their physical or chemical state. They also deduced that because the uranium-containing ore pitchblende is more intensely radioactive than the uranium salts that were used by Becquerel, other radioactive elements must be in the ore. They carried through a series of chemical treatments of the pitchblende that resulted in the discovery of two new radioactive elements, polonium and radium. Marie Curie also discovered that the element thorium is radioactive, and in 1899 the radioactive element actinium was discovered by the French chemist André Louis Debierne. In that same year the discovery of the radioactive gas radon was made by the British physicists Ernest Rutherford and Frederick Soddy, who observed it in association with thorium, actinium, and radium.
Radioactivity was soon recognized as a more concentrated source of energy than had been known before. The Curies measured the heat associated with the decay of radium and established that 1 g (0.035 oz) of radium gives off about 100 cal of energy every hour. This heating effect continues hour after hour and year after year, whereas the complete combustion of a gram of coal results in the production of a total of only about 8000 cal of energy. Radioactivity attracted the attention of scientists throughout the world following these early discoveries. In the ensuing decades many aspects of the phenomenon were thoroughly investigated.
Types of Radiations
Rutherford discovered that at least two components are present in the radioactive radiations: alpha particles, which penetrate into aluminum only a few thousandths of a centimeter, and beta particles, which are nearly 100 times more penetrating. Subsequent experiments in which radioactive radiations were subjected to magnetic and electric fields revealed the presence of a third component, gamma rays, which were found to be much more penetrating than beta particles. In an electric field the path of the beta particles is greatly deflected toward the positive electric pole, that of the alpha particles to a lesser extent toward the negative pole, and gamma rays are not deflected at all. Therefore, the beta particles are negatively charged, the alpha particles are positively charged and are heavier than beta particles, and the gamma rays are uncharged.
The discovery that radium decayed to produce radon proved conclusively that radioactive decay is accompanied by a change in the chemical nature of the decaying element. Experiments on the deflection of alpha particles in an electric field showed that the ratio of electric charge to mass of these particles is about twice that of the hydrogen ion (see ION). Physicists supposed that the particles could be doubly charged ions of helium (helium atoms with two electrons removed). This supposition was proved by Rutherford when he allowed an alpha-emitting substance to decay near an evacuated thin-glass vessel. The alpha particles were able to penetrate the glass and were then trapped in the vessel, and within a few days the presence of elemental helium was demonstrated by use of a spectroscope. Beta particles were subsequently shown to be electrons (see ELECTRON), and gamma rays to consist of electromagnetic radiation of the same nature as X rays but of considerably greater energy.
The Nuclear Hypothesis
At the time of the discovery of radioactivity physicists believed that the atom was the ultimate, indivisible building block of matter. The recognition of alpha and beta particles as discrete units of matter and of radioactivity as a process by means of which atoms are transformed into new kinds of atoms possessing new chemical properties because of the emission of one or the other of these particles brought with it the realization that atoms themselves must have structure and that they are not the ultimate, fundamental particles of nature. In 1911 Rutherford proved the existence of a nucleus within the atom by experiments in which alpha particles were scattered by thin metal foils (see ATOM AND ATOMIC THEORY). The nuclear hypothesis has since grown into a refined and fully accepted theory of atomic structure, in terms of which the entire phenomenon of radioactivity can be explained. Briefly, the atom is thought to consist of a dense central nucleus surrounded by a cloud of electrons. The nucleus, in turn, is composed of protons (see PROTON) equal in number to the electrons (in an electrically neutral atom), and neutrons (see NEUTRON). An alpha particle, or doubly charged helium ion, consists of two neutrons and two protons, and hence can be emitted only from the nucleus of an atom. Loss of an alpha particle by a nucleus results in the formation of a new nucleus, lighter than the original by four mass units (the masses of the neutron and of the proton are about one unit each). An atom of the uranium isotope of mass 238, upon emitting an alpha particle, becomes an atom of another element of mass 234. Each of the two protons that form part of the alpha particle emitted from an atom of uranium-238 possesses a unit of positive electric charge. The number of positive charges in the nucleus, balanced by the same number of negative electrons in the orbits outside the nucleus, determines the chemical nature of the atom. Because the charge on the uranium-238 nucleus decreases by two units as a result of alpha emission, the atomic number of the resultant atom is 2 less than that of the original, which was 92. The new atom has an atomic number of 90 and hence is an isotope of the element thorium.
Thorium-234 emits beta particles, which are electrons. According to current theory, beta emission is accomplished by the transformation of a neutron into a proton, thus resulting in an increase in nuclear charge (or atomic number) of one unit. The mass of the electron is negligible, thus the isotope that results from thorium-234 decay has mass number 234 but atomic number 91 and is, therefore, a protactinium isotope.
Gamma emission is usually found in association with alpha and beta emission. Gamma rays possess no charge or mass; thus emission of gamma rays by a nucleus does not result in a change in chemical properties of the nucleus but merely in the loss of a certain amount of radiant energy. The emission of gamma rays is a compensation by the atomic nucleus for the unstable state that follows alpha and beta processes in the nucleus. The primary alpha or beta particle and its consequent gamma ray are emitted almost simultaneously. A few cases are known of pure alpha and beta emission, however, that is, alpha and beta processes unaccompanied by gamma rays; a number of pure gamma-emitting isotopes are also known. Pure gamma emission occurs when an isotope exists in two different forms, called nuclear isomers, having identical atomic numbers and mass numbers, but different in nuclear-energy content. The emission of gamma rays accompanies the transition of the higher-energy isomer to the lower-energy form. An example of isomerism is the isotope protactinium-234, which exists in two distinct energy states with the emission of gamma rays signaling the transition from one to the other.
Alpha, beta, and gamma radiations are all ejected from their parent nuclei at tremendous speeds. Alpha particles are slowed down and stopped as they pass through matter, primarily through interaction with the electrons present in that matter. Furthermore, most of the alpha particles emitted from the same substance are ejected at very nearly the same velocity. Thus nearly all the alpha particles from polonium-210 travel 3.8 cm through air before being completely stopped, and those of polonium-212 travel 8.5 cm under the same conditions. Measurement of distance traveled by alpha particles is used to identify isotopes. Beta particles are ejected at much greater speeds than alpha particles, and thus will penetrate considerably more matter, although the mechanism by means of which they are stopped is essentially similar. Unlike alpha particles, however, beta particles are emitted at many different speeds, and beta emitters must be distinguished from one another through the existence of the characteristic maximum and average speeds of their beta particles. The distribution in the beta-particle energies (speeds) necessitates the hypothesis of the existence of an uncharged, massless particle called the neutrino, and neutrino emission is now thought to accompany all beta decays.
Gamma rays have ranges several times greater than those of beta particles and can in some cases pass through several inches of lead. Alpha and beta particles, when passing through matter, cause the formation of many ions; this ionization is particularly easy to observe when the matter is gaseous. Gamma rays are not charged, and hence cannot cause such ionization directly, but when they interact with matter they cause the ejection of electrons from atoms; the atoms minus some of their electrons are thereby ionized. Beta rays produce @ to A of the ionization generated by alpha rays per centimeter of their path in air. Gamma rays produce about @ of the ionization of beta rays. The Geiger-Müller counter and other ionization chambers , which are based on these principles, are used to detect the amounts of individual alpha, beta, and gamma rays, and hence the absolute rates of decay of radioactive substances. One unit of radioactivity, the curie, is based on the decay rate of radium-226, which is 37 billion disintegrations per sec per g of radium.
Modes of radioactive decay, other than the three above mentioned, exist. Some isotopes are capable of emitting positrons, which are identical with electrons but opposite in charge. The positron-emission process is usually classified as a beta decay and is termed beta-plus emission to distinguish it from the more common negative-electron emission. Positron emission is thought to be accomplished through the conversion, in the nucleus, of a proton into a neutron, resulting in a decrease of the atomic number by one unit. Another mode of decay, known as K-electron capture, consists of the capture of an electron by the nucleus, followed by the transformation of a proton to a neutron. The net result is thus also a decrease of the atomic number by one unit. The process is observable only because the removal of the electron from its orbit results in the emission of an X ray. In recent years it has been shown that a number of isotopes, notably uranium-235 and several isotopes of the artificial transuranium elements, are capable of decaying by a spontaneous-fission process, in which the nucleus is split into two fragments. In the mid-1980s a unique decay mode was observed, in which isotopes of radium of masses 222, 223, and 224 emit carbon-14 nuclei rather than decaying in the usual way by emitting alpha radiation.
Half-life of Uranium
Half-life is the length of time required for half of a given number of initial number of atoms of that isotope to decay.
Example of three different isotopes of uranium and its half-life period is given below.
uranium-234: half life = 244 thousand years, 0.0055% of all uranium.
uranium-235: half life = 704 million years, 0.72% of all uranium.
uranium-238: half life = 4.5 billion years, 99.28% of all uranium.
The decay of some substances, such as uranium-238 and thorium-232, appears to continue indefinitely without detectable diminution of the decay rate per unit mass of the isotope (specific-decay rate). Other radioactive substances show a marked decrease in specific-decay rate with time. Among these is the isotope thorium-234 (originally called uranium X), which, after isolation from uranium, decays to half its original radioactive intensity within 25 days. Each individual radioactive substance has a characteristic decay period or half-life; because their half-lives are so long that decay is not appreciable within the observation period, the diminution of the specific-decay rate of some isotopes is not observable under present methods. Thorium-232, for example, has a half-life of 14 billion years.
Radioactive Decay Series
When uranium-238 decays by alpha emission, thorium-234 is formed; thorium-234 is a beta emitter and decays to form protactinium-234. Protactinium-234 in turn is a beta emitter, forming a new isotope of uranium, uranium-234. Uranium-234 decays by alpha emission to form thorium-230, which decays in turn by alpha emission to yield the predominant isotope, radium-226. This radioactive decay series, called the uranium-radium series, continues similarly through five more alpha emissions and four more beta emissions until the end product, a nonradioactive (stable) isotope of lead (element 82) of mass 206 is reached. Every element in the periodic table between uranium and lead is represented in this series, and each isotope is distinguishable by its characteristic half-life. The members of the series all share a common characteristic: Their mass numbers can be made exactly divisible by four if the number 2 is subtracted from them, that is, their mass numbers can be expressed by the simple formula 4n + 2, in which n is a whole number. Other natural radioactive series are the thorium series, called the 4n series, because the mass numbers of all its members are exactly divisible by four, and the actinium series, or 4n + 3 series. The parent of the thorium series is the isotope thorium-232, and its final product is the stable isotope lead-208. The actinium series begins with uranium-235 (named actinouranium by early investigators) and ends with lead-207. A fourth series, the 4n + 1 series, all the members of which are artificially radioactive, has in recent years been discovered and thoroughly characterized. Its initial member is an isotope of the synthetic element curium, curium-241. It contains the longest-lived isotope of the element neptunium, and its final product is bismuth-209.
An interesting application of knowledge of radioactive elements is made in determining the age of the earth. One method of determining geologic time is based on the fact that in many uranium and thorium ores, all of which have been decaying since their formation, the alpha particles have been trapped (as helium atoms) in the interior of the rock. By accurately determining the relative amounts of helium, uranium, and thorium in the rock, the length of time during which the decay processes have been going on (the age of the rock) can be calculated. Another method is based on the determination of the ratio of uranium-238 to lead-206 or of thorium-232 to lead-208 in the rocks (that is, the ratios of concentration of the initial and final members of the decay series). These and other methods give values for the age of the earth of between 3 billion and 5 billion years. Similar values are obtained for meteorites that have fallen to the surface of the earth, as well as samples of the moon brought back by Apollo 11 in July 1969, indicating the possibility that the entire solar system could be about the same age as the earth.
All the naturally occurring isotopes above bismuth in the periodic table are radioactive and in addition naturally radioactive isotopes of bismuth, thalium, vanadium, indium, neodymium, gadolinium, hafnium, platinum, lead, rhenium, lutetium, rubidium, potassium, hydrogen, carbon, lanthanum, and samarium exist. In 1919 Rutherford carried out the first nuclear reaction when he bombarded ordinary nitrogen gas (nitrogen-14) with alpha particles and found that the nitrogen nuclei captured alpha particles and emitted protons very rapidly, forming a stable isotope of oxygen, oxygen-17. This reaction can be written symbolically as wN + nHe ± xO + eH
in which the atomic numbers of the participating nuclei are conventionally written below and to the left of the chemical symbols and their mass numbers above and to the left. In the above reaction the alpha particle is shown as a helium nucleus and the proton as a hydrogen nucleus.
Not until 1933 was it demonstrated that such nuclear reactions could sometimes result in the formation of new radioactive nuclei. The French chemists Irène and Frédéric Joliot-Curie prepared the first artificially radioactive substance in that year when they bombarded aluminum with alpha particles. The aluminum nuclei captured alpha particles and then emitted neutrons with the consequent formation of an isotope of phosphorus, which decayed by positron emission with a short half-life. They also produced an isotope of nitrogen from boron and one of aluminum from magnesium. Since that time a great many nuclear reactions have been discovered, and the nuclei of elements throughout the periodic table have been bombarded with different particles, including alpha particles, protons, neutrons, and deuterons (ions of the hydrogen isotope of mass 2). As a result of this intensive investigation, more than 400 artificial radioactivities are now known. This research has been aided immeasurably by the development of particle accelerators that accelerate the bombarding particles to enormous speeds, thus in many cases increasing the probability of their capture by the target nuclei.
The vigorous investigation of nuclear reactions and the search for new artificial radioactivities, especially in connection with the search for such activities among the heavier elements, was responsible for the discovery of nuclear fission and the subsequent development of the atomic bomb. The investigations have also resulted in the discovery of several new elements that do not exist in nature. The development of nuclear reactors has made possible the production on a large scale of radioactive isotopes of nearly all the elements of the periodic table, and the availability of these isotopes is an incalculable aid to chemical research and to biological and medical research. Of great importance among the artificially produced radioactive isotopes is an isotope of carbon, carbon-14, which has a half-life of about 5730 ± 40 years. The availability of this substance has made possible the investigation of numerous aspects of life processes, such as the process of photosynthesis, in a more fundamental manner than hitherto considered possible.
Scientists have recently shown that a very minute but unchanging amount of carbon-14 is present in the atmosphere of the earth and that all livIng organisms assimilate traces of this isotope during their lifetime. After death this assimilation ceases and the radioactive carbon, constantly decaying, is no longer maintained at a steady concentration. Estimation of the ages of a number of objects, such as bones and mummies, of historical and archaeological interest has been made possible by carbon-14 measurements.
In neutron-activation analysis, a sample of a substance is made radioactive in a nuclear reactor. A number of impurities that cannot be detected by other means can then be found by detecting the particular types of radioactivity that are associated with radioisotopes of these impurities. Other applications of radioactive isotopes are in medical therapy, industrial radiography, and specific devices such as phosphorescent light sources, static eliminators, thickness gauges, and nuclear batteries.
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