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 420 J (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 33,600 J (8,000 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 only a few thousandths of a centimetre into aluminium, 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 towards the positive electric pole, and that of the alpha particles to a lesser extent towards the negative pole, while 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 half that of the hydrogen ion. Physicists supposed that the particles could be doubly charged ions of helium (helium atoms with two electrons removed). The helium ion has approximately four times the mass of the hydrogen ion, which meant that the charge-to-mass ration would indeed be half that of the hydrogen ion. This supposition was proved by Rutherford when he allowed an alpha-emitting substance to decay near an evacuated vessel made of thin glass. 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. Then alpha and beta particles were recognized to be discrete units of matter and radioactivity to be a process in which atoms are transformed by the emission of one or the other of these particles into new kinds of atoms possessing new chemical properties. This 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 an elaborate theory of atomic structure, in terms of which the entire phenomenon of radioactivity can be explained. Briefly, the atom has been found to consist of a dense central nucleus surrounded by a cloud of electrons. The nucleus, in turn, is composed of protons, which are equal in number to the electrons (in an electrically neutral atom), and of neutrons. Neutrons are electrically neutral and of approximately the same mass as protons. 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 each about one unit). An atom of the uranium isotope of mass number 238, upon emitting an alpha particle, becomes an atom of another element of mass number 234. (The mass number of a nucleus is the total number of neutrons and protons; it is nearly, but not exactly, equal to its actual mass expressed in atomic mass units.) Each of the two protons that form part of the alpha particle 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 Physics.

Thorium-234 emits beta particles, which are electrons. 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, so 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 usually occurs 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 differing in energy. 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 signalling 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 (1.5 in) through air before being completely stopped, and those of polonium-212 travel 8.5 cm (3.3 in) under the same conditions. Measurement of the distance travelled 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 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 by the characteristic maximum and average speeds of their beta particles. The distribution in the beta-particle energies (speeds) necessitated the hypothesis of the existence of an uncharged, massless particle called the neutrino; neutrino emission accompanies all beta decays. Gamma rays have ranges several times greater than those of beta particles and can in some cases pass through several centimetres 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 so readily. Beta rays produce t to z of the ionization generated by alpha rays per centimetre of their path in air. Gamma rays produce about t 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 per gram of radium. See Radiation Effects, Biological.

There are modes of radioactive decay other than the three mentioned above. Some isotopes are capable of emitting positrons, which are identical with electrons but opposite in charge. The positron-emission process is usually classified as 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. A number of isotopes, notably uranium-235 and several isotopes of the artificial transuranic elements, are capable of decaying by a spontaneous-fission process, in which the nucleus is split into two fragments (see Nuclear Energy). 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.