Radioactivity

III

Half-Life

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 with present methods. Thorium-232, for example, has a half-life of 14 billion years.

IV

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 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 it reaches the end product, a nonradioactive (stable) isotope of lead (element 82) of mass 206. 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 artificial 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 the ages of rocks 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 the concentrations of the initial and final members of the decay series). These and other methods give values for the age of the Earth of around 4.6 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 that the entire solar system is probably about the same age as the Earth.

V

Artificial Radioactivity

All the naturally occurring isotopes above bismuth in the periodic table are radioactive, and there are in addition naturally radioactive isotopes of bismuth, thallium, vanadium, indium, neodymium, gadolinium, hafnium, platinum, lead, rhenium, lutetium, rubidium, potassium, hydrogen, carbon, lanthanum, and samarium. In 1919 Rutherford brought about the first artificially induced 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

¨N + ¸He → ©O + §H

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 aluminium with alpha particles. The aluminium 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. The Joliot-Curies also produced an isotope of nitrogen from boron and one of aluminium 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 (nuclei of deuterium, 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 among the heavier elements, was responsible for the discovery of nuclear fission and the subsequent development of the atomic bomb (see Nuclear Energy; Nuclear Weapons). 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 (see Isotopic Tracers). Of great importance among the artificially produced radioactive isotopes is carbon-14, which has a half-life of 5730 ± 40 years. The availability of this substance has made possible the investigation of numerous aspects of life processes, such as photosynthesis, in a more fundamental manner than hitherto considered possible.

A minute amount of carbon-14 is present in the atmosphere of the Earth and 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. Estimates of the ages of objects of historical and archaeological interest, such as bones and mummies, has been made possible by carbon-14 measurements. See Dating Methods.

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 (see Radiology), industrial radiography, and specific devices such as phosphorescent light sources, static eliminators, thickness gauges, and nuclear batteries.