Radioactivity

 

 

I    INTRODUCTION

 

Radioactivity, spontaneous disintegration of atomic nuclei by the emission of subatomic particles called alpha particles and beta particles, or of electromagnetic rays called X rays 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.

 

 

II   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. 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, and gamma rays to consist of electromagnetic radiation of the same nature as X rays but of considerably greater energy.

 

 

A   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). 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 equal in number to the electrons (in an electrically neutral atom), and neutrons. 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. See Elements, Chemical; Nuclear Chemistry; Periodic Law.

 

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.

 

 

B   Gamma Radiation

 

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 (see Radiation Effects, Biological). Beta rays produce  to  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 (see Particle Detectors), 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 second. The newer and preferred unit for measuring radioactivity in the International System of Units is called the becquerel. It is equal to one disintegration per second.

 

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.

 

 

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 under 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 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.

 

 

V     ARTIFICIAL RADIOACTIVITY

 

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

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 Irčne 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 (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 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 have 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, industrial radiography, and specific devices such as phosphorescent light sources, static eliminators, thickness gauges, and nuclear batteries.

 


Contributed By: Seymour Z. Lewin

 

Microsoft ® Encarta ® Reference Library 2003. © 1993-2002 Microsoft Corporation. All rights reserved.

 

 

 

 

 

Discovery of radioactivity

 

Like Thomson's discovery of the electron, the discovery of radioactivity in uranium by the French physicist Henri Becquerel in 1896 forced scientists to radically change their ideas about atomic structure. Radioactivity demonstrated that the atom was neither indivisible nor immutable. Instead of serving merely as an inert matrix for electrons, the atom could change form and emit an enormous amount of energy. Furthermore, radioactivity itself became an important tool for revealing the interior of the atom.

 

The German physicist Wilhelm Conrad Röntgen had discovered X rays in 1895, and Becquerel thought they might be related to fluorescence and phosphorescence, processes in which substances absorb and emit energy as light. In the course of his investigations, Becquerel stored some photographic plates and uranium salts in a desk drawer. Expecting to find the plates only lightly fogged, he developed them and was surprised to find sharp images of the salts. He then began experiments that showed that uranium salts emit a penetrating radiation independent of external influences. Becquerel also demonstrated that the radiation could discharge electrified bodies. In this case, discharge means the removal of electric charge, and it is now understood that the radiation ionizing molecules of air allows the air to conduct an electric current. Early studies of radioactivity relied on measuring ionization power or on observing the effects of radiation on photographic plates.

 

In 1898 the French physicists Pierre and Marie Curie discovered the strongly radioactive elements polonium and radium, which occur naturally in uranium minerals. Marie coined the term radioactivity for the spontaneous emission of ionizing, penetrating rays by certain atoms (see below).

 

Experiments conducted by the British physicist Ernest Rutherford in 1899 showed that radioactive substances emit more than one kind of ray. It was determined that part of the radiation is 100 times more penetrating than the rest and can pass through aluminum foil 1/50 of a millimetre thick. Rutherford named the less penetrating emanations alpha rays and the more powerful ones beta rays, after the first two letters of the Greek alphabet. Investigators who, in 1899, found that beta rays were deflected by a magnetic field concluded that they are negatively charged particles similar to cathode rays. In 1903 Rutherford found that alpha rays were deflected slightly in the opposite direction, showing that they are massive, positively charged particles. Much later, Rutherford proved that alpha rays are nuclei of helium atoms by collecting the rays in an evacuated tube and detecting the buildup of helium gas over several days. A third kind of radiation was identified by the French chemist Paul Villard in 1900. Designated as the gamma ray, it is not deflected by magnets and is much more penetrating than alpha particles. Gamma rays were later shown to be a form of electromagnetic radiation, like light or X rays, but with much shorter wavelengths. Because of these shorter wavelengths, gamma rays have higher frequencies and are even more penetrating than X rays. In 1902, while studying the radioactivity of thorium, Rutherford and the English chemist Frederick Soddy discovered that radioactivity was associated with changes inside the atom that transformed thorium into a different element. They found that thorium continually generates a chemically different substance that is intensely radioactive. The radioactivity eventually makes the new element disappear. Watching the process, Rutherford and Soddy formulated the exponential decay law, which states that a fixed fraction of the element will decay in each unit of time. For example, half of the thorium product decays in four days, half the remaining sample in the next four days, and so on.

 

Until the 20th century, physicists had studied such subjects as mechanics, heat, and electromagnetism that they could understand by applying common sense or by extrapolating from everyday experiences. The discovery of the electron and radioactivity, however, showed that classical Newtonian mechanics could not explain phenomena at atomic and subatomic levels. As the primacy of classical mechanics crumbled during the early 20th century, quantum mechanics was developed to replace it. Since then, experiments and theories have led physicists into a world that is often extremely abstract and seemingly contradictory.

 

property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei.

 

 

 

radioactivity

 

An unstable nucleus will decompose spontaneously, or decay, into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy. Radioactive decay is a property of several naturally occurring elements as well as of artificially produced isotopes of the elements. The rate at which a radioactive element decays is expressed in terms of its half-life; i.e., the time required for one-half of any given quantity of the isotope to decay. Half-lives range from more than 1,000,000,000 years for some nuclei to less than 10-9 second (see below Rates of radioactive transitions ). The product of a radioactive decay process--called the daughter of the parent isotope--may itself be unstable, in which case it, too, will decay. The process continues until a stable nuclide has been formed.

 

 

The nature of radioactive emissions

 

The emissions of the most common forms of spontaneous radioactive decay are the alpha () particle, the beta () particle, the gamma () ray, and the neutrino. The alpha particle is actually the nucleus of a helium-4 atom, with two positive charges 4/2He. Such charged atoms are called ions. The neutral helium atom has two electrons outside its nucleus balancing these two charges. Beta particles may be negatively charged (beta minus, symbol e-), also called a negatron, or positively charged (beta plus, symbol e+), also called a positron. (The beta minus [-] particle is actually an electron created in the nucleus during beta decay without any relationship to the orbital electron cloud of the atom.) The positron is regarded as the antiparticle of the negatron because the two particles, when brought together, will mutually annihilate each other. Gamma rays are electromagnetic radiations like radio waves, light, and X rays. Beta radioactivity also produces the neutrino and antineutrino, particles that have no charge and no rest mass, symbolized by and , respectively.

 

In the less common forms of radioactivity, fission fragments, neutrons, or protons may be emitted. Fission fragments are themselves complex nuclei with usually between one-third and two-thirds the charge Z and mass A of the parent nucleus. Neutrons and protons are, of course, the basic building blocks of complex nuclei, having approximately unit mass on the atomic scale and having zero charge or unit positive charge, respectively. The neutron cannot long exist in the free state. It is rapidly captured by nuclei in matter; otherwise, in free space it will undergo beta-minus decay to a proton, a negatron, and an antineutrino with a half-life of 12.8 minutes. The proton is the nucleus of ordinary hydrogen and is stable.