The Discovery of Radioactivity: Gateway to Twentieth-Century Physics

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The Discovery of Radioactivity: Gateway to Twentieth-Century Physics

Overview

Radioactivity was one of several discoveries made at the turn of the twentieth century that led to revolutionary changes in physics. Unlike some discoveries, it was completely unexpected. The discoverer was looking for something else when he found it, the scientific world initially ignored it, and most of its ramifications were not apparent until much later. As radioactivity gradually transmuted into nuclear physics, its impact reverberated far beyond the confines of physics, forever changing society in its wake. The discovery of radioactivity changed our ideas about matter and energy and of causality's place in the universe. It led to further discoveries and to advances in instrumentation, medicine, and energy production. It increased opportunities for women in science. Radioactivity introduced new health hazards, and its military applications permanently changed world politics. Applications of radioactivity created ethical problems which have yet to be resolved.

Background

None of this was foreshadowed at the start of 1896, when the scientific world was agog at reports from Germany of a new invisible radiation which penetrated opaque bodies. The first x-ray photo of the bones in a human hand mes merized professors and the public alike. These rays seemed to come from the phosphorescent screens used to detect cathode rays (later identified as electron beams), a popular and controversial topic in the late nineteenth century.

It was natural to wonder whether other phosphorescent substances gave off invisible penetrating rays. One of the scientists who was impressed by the x rays, Antoine Henri Becquerel (1852-1908), had inherited a collection of phosphorescent minerals assembled by his father, a leading expert on optical luminescence. Becquerel was the third generation of a family of famous physicists who were professors at the Natural History Museum in Paris, and he had established his reputation with researches on optical phenomena. He returned to the Museum and began testing the minerals.

First Becquerel would wrap a photographic plate in black paper in order to block visible light. After placing a mineral on the paper, he would expose it to sunlight in order to make it phosphoresce. Then he would wait to see whether an image would form on the plate. Most of the minerals had no effect, but a uranium compound made a strong image on the plate. One day he set out a sample containing uranium, but the sun appeared only intermittently. When the weather did not improve, he finally developed the plate, and to his great surprise saw a sharp image on it!

This did not make sense, because phosphorescent materials needed light in order to glow. Perhaps there had been enough light on the cloudy days after all. But when he kept the sample in a light proof box, it still marked the plate. Apparently these invisible rays did not require light. What they did seem to require was uranium, since everything Becquerel tested that contained uranium worked, while the other minerals did not. (Exceptions were later attributed to errors.) Uranium metal worked even better than uranium compounds—and metals did not phosphoresce. Still, for some time Becquerel believed the rays he had found were a kind of invisible light.

Becquerel published his findings in 1896 and 1897, but most scientists were not very interested, since their journals were being flooded with reports on various kinds of invisible rays. Satisfied that he had established his discovery, Becquerel investigated a different topic for the next year and a half. An engineer in London, Silvanius P. Thompson (1851-1916), had also found in 1896 that uranium gave off invisible rays, but after he learned that Becquerel had already published this result, Thompson likewise dropped this topic.

The uranium rays nevertheless caught the attention of a young Polish student in Paris. Maria Sklodowska Curie (1867-1934), who had recently married the French physicist Pierre Curie (1859-1906), was looking for a subject for her doctoral thesis. She decided to search for other elements that might give off invisible rays, naming this property radioactivity. Becquerel had shown that uranium rays had electrical effects, and Curie used this process (later called ionization) to test mineral samples. First she found that thorium gave off rays, but G. C. Schmidt in Germany had already published this finding. Then she noticed that pitchblende, a uranium ore, emitted more radiation than uranium itself. Several new elements had been discovered during the late nineteenth century, and Curie wondered if one might be hidden in the mineral pitchblende. That prospect was so tantalizing that her husband decided to join her in the search. After backbreaking labor, and with the help of the chemist Gustav Bémont, the Curies announced the discovery of two new elements, which they named polonium and radium (1898).

Impact

This finding startled the scientific world, and soon more researchers were investigating the new elements and the radiations they emitted. Having more powerful sources made it easier to do experiments, and the electrical method allowed more sensitive and precise measurements than the cruder photographic method. Industrialists developed factories to process uranium ores and the market for uranium ores burgeoned.

Some scientists doubted the Curies' findings, and even questioned the existence of radioactivity. Marie Curie worked for years to obtain sufficient radium and polonium to determine their atomic weights. This feat convinced the skeptics, and eventually the electrical methods pioneered by radioactivity researches were accepted by the rest of the scientific community. New methods led to advances in instrumentation, which in turn led to further discoveries about atomic structure, subatomic particles, and cosmic rays.

While some were not sure of radioactivity's existence, others wanted to elevate it to a universal property of matter. As reports poured in from across Europe and beyond of radioactivity detected in springs, soils, snow, air, in fact almost everywhere, the hypothesis of universal radioactivity seemed plausible. Eventually experimenters found that radioactivity in the environment came from traces of radioactive elements, rather than from universal radioactivity. These studies contributed to the later discovery of cosmic rays.

At first most scientists believed the new rays were x rays. In 1898 a young physicist working in Canada, Ernest Rutherford (1871-1937), found that two types of rays (he believed two types of x rays) were emitted by uranium. In 1899 researchers in Germany, Austria, and France showed independently that some of the rays were actually charged particles, later known as electrons. Later three types of rays were identified. Two (alpha and beta rays) were shown to be particulate; the gamma rays were similar to x rays.

The idea that atoms could spontaneously lose part of their material substance stoked speculations about atomic transmutation, which mingled with various forms of spiritualism which were circulating in the popular press at the turn of the century. More reputable scientific opinion predicted conversions between matter and energy; could all the material world be nothing more than energy forms? Nineteenth-century electrical theory, and later Albert Einstein's special theory of relativity, gave formulas for computing the conversion of mass to energy, but instruments were not sensitive enough to test these predictions with radioactive substances.

From the beginning scientists had been puzzled by radioactivity's persistence, and by their inability to affect it. Phosphorescence eventually disappears if the phosphor is not reexposed to light, yet uranium's activity seemed to continue unremittingly, year after year. With the discovery of radium, which gave off much more energy per gram than uranium, the question became critical. What was radioactivity's energy source? Researchers took samples deep inside a mine, enclosed them in lead, sealed them from light, heated them, cooled them, altered them physically and chemically, yet the rays always continued unabated, carrying huge amounts of energy, many times more than any known chemical reaction. Radioactivity seemed to violate the principle of conservation of energy.

By 1899 scientists realized that some radioactive bodies gradually lose their activity. Rutherford determined that this loss followed an exponential law. All bodies gradually lose their radioactivity, but for some elements the loss was not detected because the process can take up to billions of years. The transmutation theory of radioactivity was published by Rutherford and Frederick Soddy (1877-1956) in 1903. This theory states that radioactivity's energy comes from the radioactive atoms themselves as they change themselves into new elements. Thus the law of energy conservation was preserved. Much later atomic energy was used for nuclear reactors and atomic bombs.

The transmutation theory caused scientists to change their ideas about atoms and about the energy available in matter. Atoms were not unchangeable, they contained huge stores of energy, and they were built out of smaller particles. The exponential law of decay meant that probability theory could be used to describe radioactivity. The realization that atomic disintegration was a random process changed physicist's ideas about causality in nature, affecting in turn areas as distant as modern art and literature. Radioactivity altered ideas about the earth's age, and later provided methods for measuring it.

Radioactivity's biological effects were scarcely recognized when the nineteenth century came to a close. Experimenters had noticed that radium caused burns, but many decades elapsed and many lives were lost before radiation induced illnesses were identified and adequate safety precautions were adopted. Scientists learned that radiation caused mutations in plants and animals. During the twentieth century radioactivity was used to treat cancer, detect illnesses, study physiological processes, and sterilize foods.

Radioactivity opened new career paths for both students and established scientists. Because of the progressive attitudes of leaders in the field, and because of Marie Curie's example, an unusual number of women did research in radioactivity. During the 1930s the field of radioactivity gradually turned into nuclear physics, which later produced the subfield of particle physics.

Advances in nuclear physics brought new ethical, social, and political concerns. Was it right to use, or even to create, nuclear weapons? Would the human race eventually destroy itself in a nuclear war? Could we deter such a war by stockpiling nuclear weapons?

Privileged by hindsight, it seems easy to look back and trace many modern developments to Becquerel's experiments with uranium. Yet no one in the nineteenth century, certainly not Becquerel himself, could have foreseen the consequences of those experiments. Nineteenth-century researchers worked with theories, instruments, laboratories, and expectations quite different from those of today. By 1900, radioactivity was only a minor subspecialty of physics, and nearly everything for which we recognize it today was yet to materialize.

MARJORIE C. MALLEY