Radioactive decay

Radioactive decay is the release of energy by nuclei through the emission of electromagnetic energy or several types of charged particles. In geology, radioactive decay is important not only as the basis for most of the standard dating techniques and as a tracer for fluid flows and chemical reactions but also for its role in heating the interior of the earth and changing the character of minerals.

Behavior of Atomic Nuclei

Radioactive decay is the release of energy by the nucleus of an atom. Nuclei discharge energy either through the emission of electromagnetic radiation—a form of pure energy that does not alter the chemical nature of the atom—or through the emission of a particle that changes the atom into an atom of a different chemical element. To understand radioactive decay, it is necessary to understand the behavior of atomic nuclei.

Atomic nuclei occupy a very tiny central portion of the atom. If the atom were the size of a two-story house, the nucleus would be the size of the head of a pin. In spite of its small size, the atomic nucleus contains nearly all the mass of the atom. Nuclei are composed of two particles with nearly identical masses: the proton, which carries one unit of positive electric charge; and the neutron, which is uncharged. The nucleus is orbited by the electrons. Each electron carries one unit of negative electric charge equal in size to the positive charge of the proton—even though the mass of the electron is only five-hundredths of a percent of that of a proton or neutron. The atom is electrically neutral. Therefore, the number of electrons orbiting the nucleus under normal conditions equals the number of protons contained in the nucleus. The number of protons or electrons determines the chemical element to which the atom belongs.

The particles in the nucleus are held together by the nuclear force, which is strong enough to overpower the electrical repulsion of the protons at very short distances. Just as the atomic electrons orbit the nucleus in patterns with definite energies, the nuclear particles fill states of definite energy within the nucleus. The energies of the neutron states are a little lower than those of the protons, because the neutrons are not forced apart by electrical repulsion. For elements with few protons in the nucleus, the numbers of protons and neutrons in the nucleus are nearly equal. For elements with larger numbers of protons, the electrical repulsion becomes strong enough so that neutrons in heavier atoms considerably outnumber protons to help stabilize the nucleus. For most elements, there are several types of nuclei that contain different numbers of neutrons but have the same number of protons. Such atoms with equal numbers of protons but different numbers of neutrons are called isotopes of an element.

Half-Life of the Decay

If a nucleus has extra energy, it will seek to rid itself of that extra energy by emitting either electromagnetic radiation or a particle. This emission is called radioactive decay. The time at which an excited nucleus (one with extra energy) will decay is not predictable except as a probability, which depends on the time elapsed since the nucleus was formed. This probability is described in terms of the half-life of the decay, which is the time it takes for half the excited nuclei in a sample to decay. For example, if there are originally four hundred excited nuclei in a sample, two hundred of them will undergo radioactive decay during the first half-life, one hundred of them in the second, fifty in the third, and twenty-five in the fourth. After the first half-life, there will be two hundred excited nuclei left in the sample; one hundred will be left after the second, fifty left after the third, and twenty-five excited nuclei left after the fourth.

Nuclei with short half-lives decay rapidly and disappear quickly, but the large number of energized particles they emit may do much damage to their surroundings. Nuclei with longer half-lives exist much longer. Their radioactive decay will do less immediate damage to their surroundings but will continue to do damage over a considerable period of time.

Types of Decay

Three major types of radioactive decays are found in nature. They are called alpha, beta, and gamma decay (named for the first three letters of the Greek alphabet). The particles emitted in the decay are called alpha, beta, and gamma particles or rays. The mechanisms for the decays, their half-lives, and their effect on their surroundings differ widely.

Emission of high-energy electromagnetic radiation consisting of higher-frequency X-rays, called gamma decay, allows the protons and neutrons to settle into lower energy states without changing the number of protons in the nucleus that decayed. Gamma decay particles carry no charge and have no mass. Because they are electromagnetic radiation, they travel long distances through matter and do little damage to the atoms through which they pass compared to the damage done by the passage of charged particles. The half-lives of gamma decays are usually very short. Common half-lives are about a billionth of a second; it is rare to find a gamma half-life as long as a second. The energy of the gamma radiation is characteristic of the energy levels of the protons and neutrons in the nucleus that emitted it. The pattern of emitted gamma rays can be used to identify a particular nuclear species.

Alpha decay occurs mostly in heavy nuclei that emit an alpha particle—the nucleus of a helium atom, consisting of two protons and two neutrons. This massive particle is believed to form a tightly bound unit inside these heavy nuclei. It takes advantage of a unique phenomenon of quantum mechanics to escape far enough from the vicinity of the nucleus that the positive electrical repulsion of the nucleus acts on the alpha's positive charge to drive it out of the atom. Because they are very massive and carry two units of positive charge, alpha particles heavily damage their surroundings even though they travel very short distances in matter and are easily stopped by a thin sheet of paper. Like gamma decay, each alpha decay is characterized by a unique energy determined by the energy structure of the nucleus from which it has escaped. The remaining nucleus now forms an atom of a different element, with two fewer protons and therefore two fewer electrons in the neutral atom. The half-lives of alpha decays are usually very long. Uranium-238, for example, has a half-life of 4.51 billion years, which is believed to be approximately the age of the earth.

Beta Decay

There are two types of beta decay: negative (the emission of an electron and changing of a neutron to a proton in the nucleus) and positive (the emission of a positron—a positively charged electron—and changing of a proton to a neutron in the nucleus). Both types of beta particles lie between alpha and gamma decay in terms of the damage they do to their environment. They are typically stopped by thin blocks of aluminum and do less damage than alpha particles. Positrons—a form of antimatter—destroy themselves by uniting with an electron and annihilating themselves with the emission of two gamma rays that damage their surroundings. The half-lives of beta decays range from a few seconds to thousands of years.

One of the more important beta decays is the decay of the carbon-14 isotope of carbon with six protons and eight neutrons to the isotope of nitrogen with seven protons and seven neutrons by the emission of an electron. This decay has a half-life of 5,730 years. If a positron is emitted, the remaining nucleus has one proton fewer than it did before. Several varieties of beta decay involve phenomena such as the capture of one of the atomic electrons by a proton to turn itself into a neutron. In this case, there is no emitted positron, but an observer sees an X-ray as other electrons fall close to the nucleus to replace the electron that was captured.

When they were first discovered, beta decays puzzled researchers because they did not exhibit the definite energies that characterized alpha and gamma decays. It was finally realized that nuclei undergoing beta decay emit not only an electron or a positron but also a tiny uncharged particle called a neutrino. Neutrinos carry off part of the definite energy of the nuclear decay so that the beta particles from a particular nuclear transition exhibit a statistical energy distribution. Because they are uncharged and interact very little with other atomic particles, neutrinos can pass through the mass of several earths with less than a 50 percent chance of interacting. Consequently, they are very difficult to detect. (An example of a neutrino detector is a hole in a mine the size of a ten-story building, filled with water and surrounded by detectors.) Despite their small chance of interacting, neutrinos are important to scientists' understanding of the structure of the universe, as they are believed to be produced in the nuclear reactions at the core of the sun and other stars. Thus, they may constitute a large portion of the mass of the universe. A debate rages over whether the neutrino is massless or has a very tiny mass that has not yet been measured. By 2010 several studies pointed to a tiny mass for the neutrino, including analysis of cosmic background radiation and evidence for neutrino oscillations requiring mass.

Radiation Detectors

Radioactive decay was discovered by accident when Antoine-Henri Becquerel (1852-1908) accidentally left a piece of uranium-bearing rock on top of a photographic plate in a darkened drawer. The rock left its image on the film. The first studies of radioactive decay used film as a detector for radiation. The next generation of radiation detectors used fluorescent screens which would glow when struck by a decay particle. The screens could not be connected to electronic timers, and the flashes had to be counted through a microscope.

The Geiger-Müller counter is a gas-filled tube with a charged wire running up its center. When a gamma or a beta particle enters the tube, it knocks electrons off the gas atoms and makes the gas a conductor. The electrons are collected by the center wire and can be counted electronically or used to activate a speaker and make the characteristic click of a counter in the presence of radiation. Geiger-Müller counters often are not sensitive to alpha particles, as the metal used to keep the gas inside the tube also keeps alpha particles from entering the counter.

The proportional counter, also a gas-filled tube, works on the same principle as does a Geiger-Müller counter except that it carefully measures the number of electrons that reach the central electrode. Because the number of electrons knocked off gas atoms is directly related to the energy of the particle that knocked them off, the number of electrons reaching the central electrode is directly related to the energy of the particle that produced them.

Scintillation counters utilize transparent materials that emit a flash of light when a particle passes through them. The amount of light is proportional to the kind of particle that passes through and its energy. The light is collected by a special tube, called a photomultiplier tube, that converts the light into a current pulse whose size is proportional to the amount of light emitted. The pulse can be used to drive an electronic counting system.

Modern Detectors

Many modern studies of gamma decay use solid-state detectors which take advantage of the fact that silicon and germanium crystals can be grown with very small amounts of impurities. If the crystals are carefully prepared, they will become conductors when radiation knocks electrons off the atoms in the regions containing the impurities. Once again, the number of electrons produced depends on the energy of the gamma that originated them. The crystal is placed between electrodes with a large voltage across them, and the electrodes collect the electrons produced by the gamma.

Modern detectors produce a pulse of electrons—an electric current—the strength of which is proportional to the energy of the radioactive decay particle that produced it. This current pulse is amplified and its size determined to study the energy of the decay particle that produced it. Such studies typically involve several stages of amplification during which the researcher must be careful not to alter the shape of the current pulses he or she is studying. The pulses are then fed into a device called a multichannel analyzer, which sorts them according to their strength and which stores each pulse as a count in a series of electronic bins. The bins can be displayed to show the number of counts received at each energy level. Called a spectrum of the decay, the result is used to identify the nucleus that emitted the decay product. In modern systems, the multichannel analyzer is replaced by a dedicated computer that automatically identifies the energy of the decay and, in many cases, can tell the researcher which nucleus produced it. Such systems have memories stored with data on energies and half-lives of numerous radioactive decays that have been accumulated since World War II and carefully tabulated.

Radiation detectors and their associated counting systems have become smaller and more rugged. Studies of radioactive decays were once conducted only in laboratories; however, portable systems can now be taken into the field and are found, for example, at petroleum drilling sites. Detection systems are frequently carried into space and have been used in studies of the cosmic radiation and numerous other phenomena. The more sensitive systems used in radioactive dating are still confined to the laboratory, as they must be protected from the radiation in the environment produced by cosmic radiation and by radiation from common minerals.

Research and Exploration Tool

In the decades following World War II, radioactive decay ceased to be a laboratory curiosity and became a widely used tool. The understanding of radioactive decay has led to the development of the means to date geological and archaeological specimens. Radioactive-dating techniques take advantage of the fact that each species of excited nucleus will decay so that half of it disappears after a particular amount of time elapses. If no new excited nuclei have been added to the sample since it was formed, one can compare the number of excited nuclei remaining in the sample to the number of nuclei in the sample formed by the radioactive decay and thus determine how long it has been since the sample was formed. The understanding of geologic time is based largely on radioactive dating.

In addition to its importance as a dating tool, radioactive decay is believed to be a major source of the heating of the earth. This heat flow is important to the overall heat budget of the earth and may be partially responsible for present temperatures on the earth's surface. Radioactive decay may also contribute to driving convection currents in the earth's interior, and it is probably at least partially responsible for the heat extracted as geothermal energy. Radioactive decay is also responsible for changing the nature of certain minerals, as in metamictization.

The presence of a large number of nuclei undergoing radioactive decay makes nuclear wastes hazardous. Although short-lived nuclei decay within a year and disappear, wastes from nuclear reactors are characterized by the presence of nuclei with long half-lives. They must therefore be stored in such a manner that they will not come in contact with the environment for tens of thousands of years. Only very stable geologic formations where wastes cannot be reached by groundwater will permit such storage. The search for suitable areas has covered many states. Radioactive decay has become a concern of many home owners with the discovery that seepage of radioactive radon gas, produced by the decay of minerals in the earth, has raised radiation levels in some homes above levels deemed healthy.

Despite its hazards, radioactive decay has become a useful tool in many types of geological research. For example, small amounts of short-lived radioactivity have been injected into geothermal systems along with cooled water to see how long it takes the reinjected water to reach the production end of the system. Radiation detectors are inserted into boreholes during petroleum exploration to map the presence of radioactive minerals along the walls of the borehole, which assists in the identification of shale layers within a sandstone formation. Radioactive decay has proved to be a useful tool for scientific research and exploration.

Principal Terms

alpha particle: the nucleus of a helium atom, which consists of a tightly bound group of two protons and two neutrons

atom: the smallest piece of an element that has all the properties of the element

electron: a negatively charged particle that forms the outer portion of the atom and whose negative charge is equal in magnitude to the positive charge of the proton

gamma radiation: high-energy electromagnetic radiation emitted when a nucleus emits excess energy

half-life: the time during which half the atoms in a sample of radioactive material undergo decay

neutron: the uncharged particle that is one of the two particles of nearly equal mass forming the nucleus

nucleus: the central portion of the atom, which contains all the positive charge and most of the mass of the atom

positron: a positively charged electron, a form of antimatter

proton: the positively charged particle that is one of the two particles of nearly equal mass forming the nucleus

Bibliography

Adolff, Jean Pierre, and Robert Guillaumont. Fundamentals of Radiochemistry. Boca Raton, Fla.: CRC Press, 1993. A thorough approach to radiochemistry, this book describes all aspects and applications associated with the field. An excellent source of information for someone without much knowledge or background in radiochemistry, this book is well illustrated with diagrams and charts. Bibliography and index.

Choppin, Gregory R., Jan Rydberg, and Jan-Olov Liljenzin. Radiochemistry and Nuclear Chemistry. 3rd ed. Oxford: Butterworth-Heinemann, 2001. This widely used college text introduces the reader to the basics of nuclear chemistry and radiochemistry. It explores the theories surrounding those fields and their applications. Well illustrated with clear diagrams and figures, this is a good introduction for someone without a strong background in chemistry.

Dosseto, Anthony, Simon P. Turner, and James A. Van-Orman, eds. Timescales of Magmatic Processes: From Core to Atmosphere. Hoboken, N.J.: Wiley-Blackwell, 2010. Covers many aspects of the earth's history, from the formation and differentiation of the earth, to magma ascent, cooling, and degassing. Uranium series isotopes are referenced multiple times in evaluating the time scales of multiple concepts.

Durrance, Eric M. Radioactivity in Geology: Principles and Applications. New York: John Wiley & Sons, 1987. This monograph presents a very thorough review of the principles of radioactive decay and its many applications in geology. In addition to its thoroughness, this book is written for a nonspecialist in nuclear physics and is fairly readable as well as complete, discussing not only dating methods but also exploration methods, the use of radioactivity in petroleum studies, environmental radioactivity, and the role of radioactivity in heating the earth's interior.

Keller, C. Radiochemistry. New York: John Wiley & Sons, 1988. In addition to covering the basic theory of radioactive decay, this volume stresses the applications of radioactive decay in the study of problems of geological interest. Unlike the preceding reference, it emphasizes radioactivity produced by humans for their own purposes and is of less general interest than is Durrance's monograph.

Lieser, Karl Heinrich. Nuclear and Radiochemistry: Fundamentals and Applications. 2d ed. New York: Wiley-VCH, 2001. Lieser's book gives the reader a basic understanding of the practices and principles involved in the fields of radiochemistry and nuclear chemistry. Although there is quite a bit of chemistry involved in the author's explanations, someone without a chemistry background will still find the book useful. Illustrations, charts, and diagrams help clarify difficult concepts and theories. Bibliography and index.

Mares, S., and M. Tvrdy. Introduction to Applied Geophysics. New York: Springer, 2011. Provides a good overview of the methods used in studying geophysics. Chapter 3 covers radioactivity and radiometric methods.

Mozumder, A. Fundamentals of Radiation Chemistry. San Diego: Academic Press, 1999. Mozumder presents a concise introduction to radiochemistry, examining the procedures and theories associated with it. Some background in chemistry would be helpful but is not necessary. Bibliography, illustrations, and index.

Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon & Schuster, 1986. The early chapters of this excellent history of nuclear physics are devoted to a detailed description of the discovery of nuclear decay and the development of a theory of the nucleus. The historically important experiments and ideas are presented in detail. Because the emphasis is on people and the development of a theory, Rhodes has written a very readable as well as technically accurate description of the theory of radioactive decay.

Wagner, Gunther A., and S. Schiegl. Age Determination of Young Rocks and Artifacts: Physical and Chemical Clocks in Quaternary Geology and Archaeology. New York: Springer, 2010. The authors cover various materials and dating methods. Well organized, accessible to advanced undergraduates and graduate students.

Walker, Mike. Quaternary Dating Methods. New York: Wiley, 2005. This text provides a detailed description of current dating methods, followed by content on the instrumentation, limitations, and applications of geological dating. Written for readers with some science background, but clear enough for those with no prior knowledge of dating methods.

Walther, John Victor. Essentials of Geochemistry. 2d ed. Jones & Bartlett Publishers, 2008. Contains chapters on radioisotope and stable isotope dating, and radioactive decay. Geared more toward geology and geophysics than toward chemistry, this text provides content on thermodynamics, soil formation, and chemical kinetics.