Radioactivity: The Unstable Nucleus and its Uses
WHEN THE FRENCH PHYSICIST Henri Becquerel (1852-1908) discovered "his" uranium rays in 1896 and when Marie Curie began to study them, one of the givens of physical science was that the atom was indivisible and unchangeable. The work of Becquerel and Curie soon led other scientists to suspect that this theory of the atom was untenable.
Scientists soon learned that some of the mysterious "rays" emanating from radioactive substances were not rays at all, but tiny particles. Radioactive atoms emit three different kinds of radiation. One kind of radiation is a particle of matter, called the alpha particle. It has a positive electric charge and about four times the mass of a hydrogen atom. (We now know that it consists of two protons and two neutrons, the same as the nucleus of the helium atom.) Alpha particles exit radioactive atoms with high energies, but they lose this energy as they move though matter. An alpha particle can pass through a thin sheet of aluminum foil, but it is stopped by anything thicker. Beta "rays," a second form of radiation, turned out to be electrons, very light particles with a negative electric charge. The beta particles travel at nearly the speed of light and can make their way through half a centimeter of aluminum. Gamma rays, a third type of radiation, are true rays, electromagnetic waves--the same kind of thing as radio waves and light, with no mass and no electrical charge. They are similar to, but more energetic than, the X-rays, an energetic form of electromagnetic radiation discovered by the German physicist Wilhelm Conrad Roentgen (1845-1923) in 1895. Gamma rays emitted by radioactive atoms can penetrate deeper into matter than alpha or beta particles. A small fraction of gamma rays can pass through even a meter of concrete.
The point was that radioactivity was no more nor less than the emission of tiny particles and energetic waves from the atom. Building on the research of Marie Curie and others, scientists soon realized that if atoms emitted such things they could not be indivisible and unchangeable. Atoms are made up of smaller particles, and these can be rearranged.
It began with a vexing puzzle--in any laboratory where people worked with radium or other radioactive minerals, radioactivity tended to spread around, turning up in unexpected corners. In fact the labs were being contaminated by a radioactive gas. In 1900 Ernest Rutherford (1871-1937) found that the radioactivity of the "emanation" (as he called it) from thorium diminished with time. This decay of radioactivity was a vital clue.
Rutherford, working in Canada with the chemist Frederick Soddy (1877-1956), developed a revolutionary hypothesis to explain the process. They realized that radioactive elements can spontaneously change into other elements. As they do so, they emit radiation of one type or another. The spontaneous decay process continues in a chain of emissions until a stable atom is formed. It was, as Rutherford and Soddy boasted, the transmutation of elements that had eluded alchemists for thousands of years. They recognized at once that the ceaseless emissions pointed to a vast store of energy within atoms--energy that might someday be released for useful power or terrible weapons, however people chose.
TO UNDERSTAND WHAT HAPPENS when radioactive atoms emit radiation, scientists had to understand how the atom is built. As Rutherford first explained in 1911, each atom is made of a small, massive nucleus, surrounded by a swarm of light electrons. It is from the nucleus that the radioactivity, the alpha or beta or gamma rays, shoot out. By around 1932 Rutherford's colleagues had found that the nucleus is built of smaller particles, the positively charged protons and the electrically neutral neutrons. A proton or a neutron each has about the mass of one hydrogen atom. All atoms of a given element have a given number of protons in their nuclei, called the atomic number. To balance this charge they have an equal number of electrons swarming around the nucleus. It is these shells of electrons that give the element its chemical properties.
However, it turned out that atoms of a given element can have different numbers of neutrons, and thus different atomic mass. Soddy named the forms of an element with different atomic masses the isotopes of the element. For example, the lightest element, hydrogen, has the atomic number 1. Its nucleus normally is made of one proton and no neutrons, and thus its atomic mass is also 1. But hydrogen has isotopes with different atomic masses. "Heavy" hydrogen, called deuterium, has one proton and one neutron in its nucleus, and thus its atomic mass is 2. Hydrogen also has a radioactive isotope, tritium. Tritium has one proton and two neutrons, and thus its atomic mass is 3. The three forms of hydrogen each have one electron, and thus the same chemical properties.
When a radioactive nucleus gives off alpha or beta particles, it is in the process of changing into a different nucleus--a different element, or a different isotope of the same element. For example, radioactive thorium is formed when uranium-238--an isotope of uranium with 92 protons and 146 neutrons--emits an alpha particle. Since the alpha particle consists of two protons and two neutrons, when these are subtracted what is left is a nucleus with 90 protons and 144 neutrons. Thorium is the element of atomic number 90, and this isotope of thorium has an atomic mass of 234. The results of decay may themselves be unstable, as is the case with thorium-234. The chain of decays continues until a stable nucleus forms, in this case the element lead.
Rutherford and Soddy discovered that every radioactive isotope has a specific half-life. Half the nuclei in a given quantity of a radioactive isotope will decay in a specific period of time. The half-life of uranium-238 is 4.5 billion years, which means that over that immense period of time half the nuclei in a sample of uranium-238 will decay (in the next 4.5 billion years, half of what is left will decay, leaving one quarter of the original, and so forth). The isotopes produced by the decay of uranium themselves promptly decay in a long chain of radiations. Radium and polonium are links in this chain.
Radium caught Marie Curie's attention because its half-life is 1600 years. That's long enough so that there was a fair amount of radium mixed with uranium in her pitchblende. And it was short enough so that its radioactivity was quite intense. A long-lived isotope like uranium-238 emits radiation so slowly that its radioactivity is scarcely noticeable. By contrast, the half-life of the longest-lived polonium isotope, polonium-210, is only 138 days. This short half-life helps explain why Marie Curie was unable to isolate polonium. Even as she performed her meticulous fractional crystallizations, the polonium in her raw material was disappearing as a result of its rapid radioactive decay.
You can read Marie Curie's description of radioactivity as understood in 1904.
Uses of Radioactivity
THE EARLY WORK OF MARIE AND PIERRE CURIE led almost immediately to the use of radioactive materials in medicine. In many circumstances isotopes are more effective and safer than surgery or chemicals for attacking cancers and certain other diseases. Over the years, many other uses have been found for radioactivity. Until electrical particle accelerators were invented in the 1930s, scientists used radiation from isotopes to bombard atoms, uncovering many of the secrets of atomic structure. To this day radioactive isotopes, used as "tracers" to track chemical changes and the processes of life, are an almost indispensable tool for biologists and physiologists. Isotopes are crucial even for geology and archeology. As soon as he understood radioactive decay, Pierre Curie realized that it could be used to date materials. Soon the age of the earth was established by uranium decay at several billion years, far more than scientists had supposed. Since the 1950s radioactive carbon has been used to pin down the age of plant and animal remains, for example in ancient burials back to 50,000 years ago.
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