Introduction to Radioactivity
Radioactivity, the process by which the nuclei (cores) of unstable atoms of an element emit radiation (particles of matter and rays of energy), and in so doing become atoms of other elements. It is a property of certain types of matter. Substances in which radioactivity takes place are called radioactive. The particles and energy given off by these substances are formed of nuclear radiation. In making the emissions, the nucleus of a radioactive atom is said to decay. The radiation of a radioactive substance is harmful to life. Properly used, however, this radiation is extremely useful in science, medicine, agriculture, and industry.
Some chemical elements, including uranium, radium, thorium, and polonium, are naturally radioactive. Any element that is not naturally radioactive can be made radioactive in a nuclear reactor or particle accelerator.
The principal particles emitted by radioactive substances are alpha particles and beta particles. Most of the naturally radioactive elements emit only one of these two types of particles. The emission of either alpha or beta particles is often accompanied by the emission of gamma rays. Radium and certain other elements produce all three types of radiation.
The discovery of radioactivity late in the 19th century helped lay the foundations of modern physical science. It put an end to the theory that atoms are indivisible and indestructible, and led to great advances in the knowledge of the structure of matter, and of the relationship between matter and energy. From the research inspired by the discovery of radioactivity have come nuclear weapons on the one hand, and, on the other, nuclear power and the use of radioactive substances for the benefit of mankind.
Causes of Radioactivity
To understand how radioactivity occurs, it is necessary to know the structure of an atom and how it can change. An atom is built up of electronstiny negatively charged particlesthat revolve around a heavy, positively charged nucleus. The nucleus of an atom is made up of two basic kinds of particlesprotons, which carry a positive electrical charge; and neutrons, which carry no charge. Protons and neutrons are made up of even smaller particles called quarks. A nucleus is said to be stable when the forces of attraction and repulsion acting upon its particles are in balance. When these forces are out of balanceas occurs when there are too many or too few of one kind of particlethe nucleus is unstable. An unstable nucleus becomes stable by undergoing radioactive decay. Particles that have opposite electric charges attract each other, and those that have the same electric charge repel (push away) each other. That is why the negatively charged electrons are attracted to the positively charged nucleus and stay within it, and the positively charged protons repel one another. The protons and neutrons stay in the nucleus only because they are bound together by an extremely powerful force, called the strong nuclear force or the strong interaction.
Nuclei are most stable when they contain even numbers of both protons and neutrons. Of some 1,000 different kinds of nuclei with odd numbers of both protons and neutrons, only six are not radioactive. Protons and neutrons make up the nucleus of every element except that of the most common form of hydrogen. Only one proton makes up the nucleus of a hydrogen atom.
Protons and neutrons are arranged in layers, or shells, within the nucleus. Each shell can hold only a certain number of particles. A nucleus is stable when all its shells are filled with the proper number of particles. If there are too few or too many particles in one or more shells, the nucleus tends to be unstable and therefore radioactive.
The number of protons and neutrons changes in an atoms nucleus when an atom absorbs radiation (elementary particles or energy) or emits it. When the number of protons changes, an atom of a different element is produced. A radioactive atom gives off radiation automatically to become more stable. This process continues until it becomes stable and nonradioactive. During this process, a radioactive atom changes into different elements or different forms of the same element.
There is a relationship also between radioactivity and the atomic number of an element. (Atomic number is the number of protons in the nucleus.) Every element with an atomic number greater than that of lead (82) is radioactive. The nuclei of some of these elements can decay by splitting in two. This process is called spontaneous fission.
Radioactive Decay
The process of giving off atomic particles by radioactive atoms is called radioactive decay. In radioactive decay one element is changed into another. Its rate depends on the form of an element and on the element. A group of radioactive elements formed by the decay of one element into another is called a radioactive series. The rate at which elements decay is expressed in terms of half-lives.
TransmutationsRadioactivity produces transmutations in elements, that is, it changes one element into another. In the Periodic Table, elements are classified by their atomic numbers. When an atom emits an alpha or beta particle, the atom becomes a different element, thereby changing its place in the Periodic Table.
An atom that emits an alpha particle loses two protons and two neutrons. The loss of the two protons reduces its atomic number by two. This change is shown by the diagram Radioactive Decay. For example, the first element in the diagram is uranium, which has an atomic number of 92. When it emits an alpha particle, it becomes thorium, which has an atomic number of 90.
There are three main types of beta decay. In the most common type, a neutron in the nucleus of an atom changes into a proton. A negative beta particle (an electron) and a particle called an antineutrino are formed in the process and are emitted from the nucleus. This type of beta decay results in the atomic number of the nucleus increasing by one. Each case of beta decay shown in the diagram is of this type. For example, thorium (atomic number 90) becomes protactinium (atomic number 91). In the other two types of beta decay, the atomic number decreases by one. In both, a proton changes into a neutron and a particle called a neutrino is formed and emitted from the nucleus. In one, a positive beta particle (a positron) is also formed and emitted from the nucleus; in the other, an inner electron of the atom is captured by the nucleus and destroyed.
The weight of both neutrons and protons is roughly equal to one unit of atomic weight. In comparison, the weight of electrons, positrons, neutrinos, and antineutrinos is negligible. Thus when an atom emits an alpha particle, its atomic weight decreases by four units, but when an atom emits a beta particle, its atomic weight remains the same. For example, when thorium 230 decays, it emits an alpha particle and becomes radium 226. On the other hand, when lead 214 decays, its atomic weight remains at 214, although it becomes an atom of a different element (bismuth).
Isotopes are atoms with the same atomic number but differing atomic weights because they have unequal numbers of neutrons. Isotopes of the same element have identical chemical properties but slightly different physical ones. Many of the isotopes of naturally occurring elements are radioactive. The decay properties of radioactive isotopes of the same element usually differ greatly.
The Radioactive SeriesAs certain radioactive isotopes decay they produce other isotopes of other elements that are also radioactive. These daughter isotopes produce still other radioactive daughters, and so on until a stable isotope of some element is formed. Radioactive isotopes of this kind form radioactive series.
Three radioactive series occur in nature: the thorium series, the uranium series, and the actinium series. (The diagram Radioactive Decay shows the uranium series.) The thorium series begins with thorium 232, the uranium series with uranium 238, and the actinium series with uranium 235. Each series ends in a stable lead isotope. The neptunium series is produced artificially in nuclear reactors. It begins with neptunium 237 and ends with bismuth 209.
Half-livesEvery radioactive isotope decays at its own fixed, unchangeable rate. As time passes, more and more of the isotope changes into an isotope of another element. Eventually half the parent isotope changes. In an equal length of time, half of what now remains of the parent isotope will change, and so on. The time it takes for half of a radioactive isotope to change into another isotope is called the half-life of the parent isotope.
Uranium 238 has a half-life of 4.5 billion years. This means that in 4.5 billion years, half of a given amount of uranium 238 will have changed to thorium 234. In 24.1 days, half of a sample of thorium 234 will have become protactinium 234. The half-lives of other radioactive substances range from a very small fraction of a second to more than 100,000,000,000 years.
Radioisotopes
A radioactive isotope is called a radioisotope. Scientists produce more than 900 different kinds of artificial radioisotopes for use in science, medicine, agriculture, and industry. Some are valuable because of the kind of radiation they emit. Cobalt 60, for example, emits high-energy gamma rays that can be used for the same purposes as x-rays. Other kinds of radioisotopes are valuable because their radioactivity makes it possible to trace them with radiation detectors when, for example, they pass through a living body. Radioisotopes used in this way are called tracers, or tagged elements.
An artificial radioisotope is produced by making a stable isotope radioactive. Some artificial radioisotopes are produced by smashing atoms in particle accelerators, such as the cyclotron. Most are produced by bombarding atoms with particles and rays emitted by radioactive elements in a nuclear reactor.
A number of radioisotopes are obtained by fission, the splitting of an atom into two approximately equal parts. Others are produced by spallation, which occurs when a bombarded atom loses a number of particles but the nucleus does not split. Still other radioisotopes are formed when the nuclei of atoms capture particles with which they are bombarded. The captured particles unbalance the nuclei and make them radioactive.
Studying Particles and Rays
The particles and rays given off in radioactivity are much too small to be seen. They can be studied only through their properties. One of these properties is that of producing fluorescence (glowing) in certain substances. Another is the property of producing ionizationthe stripping of electrons from molecules or atoms to give them an electric charge.
A widely used instrument for detecting and measuring radioactivity is the Geiger-Mller counter, which detects the particles emitted by a radioactive substance by recording the ionization they produce in passing through a gas-filled chamber. Similar instruments are designed to record the ionization the particles produce in passing through a solid material. The material is connected to an electric circuit and acts as an electrical insulator until some of its atoms are ionized. It then briefly generates an electrical signal that is recorded electronically. In semiconductor radiation counters, the material used is a semiconductor, usually silicon or germanium. In crystal counters, the material is a crystal with high electrical resistance.
Scintillation counters measure radioactivity through the flashes produced by a fluorescent substance. In a common type of scintillation counter, the energy of the light flashes is converted into electrical energy, which is then amplified to operate the counter.
The bubble chamber is a clean, smooth-walled container in which a liquid can be heated above its boiling point without boiling. When an alpha or beta particle passes through a superheated liquid in a bubble chamber, the particle ionizes molecules of the liquid. The ionization energy triggers boiling along the path of the particle, producing a trail of tiny bubbles.
Other detectors include photographic film or plates and cloud chambers. One photographic technique is called autoradiography, or radioautography. It consists of placing photographic film against an object containing a radioactive substance, usually a tracer. The particles emitted by the substance expose a pattern on the film. The pattern reveals the distribution of the radioactive substance in the object. A cloud chamber, like a bubble chamber, makes visible the paths followed by alpha and beta particles.
The unit of radioactivity is the curie. One curie is equal to 37,000,000,000 emissions a second.
Radioactivity Discoveries
Radioactivity was discovered in 1896 by Antoine Henri Becquerel, a French physicist. His discovery led to the important research of Pierre and Marie Curie and others. Radium and other radioactive elements were discovered, and much was learned about the nature and effects of radioactivity. About 1900 Ernest Rutherford discovered alpha and beta particles in radioactive emissions.
In 1934, Frdric and Irne Joliot-Curie discovered that when aluminum foil was bombarded with alpha particles, the foil continued to emit radiation after the bombardment was stopped. The foil was the first known artificial radioisotope. In the late 1930s, as the result of the work of a number of scientists, it was discovered that uranium fissioned (split into two approximately equal parts) when bombarded with neutrons. This discovery led to the atomic bomb and, later, to nuclear power.