Radioactivity, Discovery of

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RADIOACTIVITY, DISCOVERY OF

Radioactivity was discovered by Henri Becquerel in early 1896. At the time it was considered only a moderately interesting phenomenon. Before long, however, it was recognized as a key to the study of the atom, and it led within two decades to the creation of nuclear physics. After the discovery of nuclear fission in 1938, the subject was regarded as the most significant in all of science, not only for the nuclear reactors and bombs that emerged from World War II, but also for the remarkable new particles and the ories of matter that filled the rest of the twentieth century.

Because newly discovered X rays (1895) seemed to stream from a luminescent spot on the cathode ray tube in which they were produced, Henri Poincaré suggested that all glowing bodies, not just those at high voltage, might be sources of these rays. Becquerel, with much experience examining minerals that glowed upon stimulation by light, was well suited to look for the invisible X rays. Among the minerals he tested in his physics laboratory in the Museum of Natural History in Paris was a compound of uranium that responded well. He wrapped a photographic plate in black paper to make it light-tight and placed a piece of the compound on it. Then he set this arrangement on his window sill, where sunlight stimulated the uranium salt for a few hours (had the mineral been moved quickly into a dark closet, it would have glowed a while). When Becquerel developed the plate, he saw a darkened area beneath the rock. Soon, he inserted coins and keys under a crystalline layer of the salt and was rewarded with silhouettes of their patterns. Since one of the striking properties of X rays was the sharp pictures they made, he believed merely that he had confirmed their production from sources other than cathode ray tubes.

Further experiments showed Becquerel that the intensity of the rays was attenuated as they passed through thin sheets of metal and that diffuse, reflected, and refracted light worked equally well in stimulating his uranium source. Other tests at the end of February 1896 were interrupted by almost a week of overcast skies. Since he felt that sunlight was required to excite his crystal, he put away the experimental arrangement in a dark drawer. When cloudy weather persisted, Becquerel developed the plate anyway, so he could give a report at the Monday meeting of the Academy of Sciences. He thought that he could perhaps show a weak exposure from light in his laboratory or no exposure at all—a "control" experiment to confirm his working hypothesis. He was astounded, however, to find his plate blackened.

This called for a reassessment of his ideas. Was it possible that X rays could be emitted without the necessity of first exposing the uranium crystal to sunlight? Thus began a long series of tests in which he kept a crystal in a dark box and periodically checked its activity with photographic plates, finding no detectable diminution. Becquerel also conducted investigations similar to those done with other forms of radiation. When he placed a lump of uranium next to a charged electroscope, he saw the gold leaves fall: the rays emerging from the uranium crystal made air a conductor of electricity. He concluded (incorrectly) that uranium rays were reflected, refracted, and polarized, confirming for him their similarity to X rays and their electromagnetic nature.

Yet other puzzles appeared. When Becquerel tested uranium compounds that did not phosphoresce, and when he destroyed a source's ability to phosphoresce by melting it and allowing it to recrystallize in darkness, he nonetheless got intense images on the photographic plates. By May 1896, Becquerel learned that uranium metal was more active than any compound and concluded that the element itself was the source of activity; it was an atomic phenomenon. Still, he could not abandon his original line of reasoning and proclaimed the discovery of a new property of metals: invisible phosphorescence.

X rays dominated scientific and popular attention, for they yielded sharper pictures more quickly and were useful in diagnostic medicine. Moreover, the equipment for X rays was more common in physics laboratories than uranium crystals. More than 1,000 papers on these penetrating rays were published in 1896 alone, compared with just a handful on uranium rays. Becquerel himself seems to have abandoned his discovery: he wrote seven papers in 1896, two in 1897, and none in 1898. Several other prominent physicists added another dozen papers in the first two years. Becquerel rays, as they were called, were not especially interesting.

The year 1898 saw a resurrection of interest. Gerhard C. Schmidt of the University of Erlangen tested other materials and found thorium compounds emitted somewhat similar rays. Because of Becquerel's errors in determining some of the radiation's properties, Schmidt could not be certain of an exact match. Soon after, and independently, Marie Curie in Paris also pointed to thorium. It is unclear why she chose to investigate these rays after some very able scientists seemed to have exhausted the subject. Perhaps she sensed it remained important, or possibly she wanted a doctoral dissertation topic without any likely competition.

A careful and thorough investigator, Marie Curie used a sensitive electrometer to measure the intensity of the radiation. Designed by her physicist husband, Pierre, and his brother, Jacques, this instrument provided quantitative data, compared with Becquerel's largely qualitative results, and was in the more modern tradition of seeking numerical results from experiments. Like Becquerel and his invisible phosphorescence of metals, Marie Curie had a guiding idea: space was filled with rays similar to X rays but more penetrating. When they struck elements of high atomic weight, such as uranium and thorium, they caused Becquerel rays to be emitted as secondaries.

She too was faced with a puzzle. Her uranium ore was more active than its uranium content should allow. Faced with the possibility of another active element, Pierre Curie dropped his own research to join that of his wife. In the summer of 1898 they announced discovery of a new element, named polonium in honor of Poland, her native country. Marie also gave the phenomenon the name radioactivity. Before the end of the year, they and a chemist colleague named Gustave Bémont revealed yet another constituent of the ore: radium, named for its outpouring of rays. The quantities of these new substances were so small they were at first invisible, yet the Curies persisted in declaring them to be new elements. Eventually, the spectrum and atomic weight of radium were measured, providing the proof.

Also in 1898, Ernest Rutherford began to investigate radioactivity. A graduate student in J. J. Thomson's Cavendish Laboratory at Cambridge University, he quickly became the central figure in radioactivity and in nuclear physics. He showed, by their different abilities to penetrate thin sheets of foil, that the radiation consisted of two components, which for convenience he named alpha and beta, names that have endured. Paul Villard in 1900 revealed a third component, the electromagnetic gamma rays. A year earlier, Friedrich Giesel in Braunschweig deflected beta rays in a magnetic field, showing they were charged particles. Becquerel, rejoining a now-exciting field, showed that beta particles were identical to the recently discovered electron. In 1903, Rutherford, now a professor at McGill University, bent alpha rays in a magnetic field, proving they were positively charged particles.

Several new radioelements also were discovered around the turn of the century, some that seemed to maintain a constant level of activity and others that lost activity over time, and there was need of a concept to organize and explain them. With the chemist Frederick Soddy, Rutherford in 1902–1903 advanced the transformation theory of radioactivity. The radioelements were placed in just a few series, with uranium and thorium heading their own. All decayed, with different half-lives, until an as-yet unknown, stable, end product was reached (lead). With this insight, research largely shifted from studies of the radiations to investigations of the bodies that emitted the alphas, betas, and gammas. The goal was to ascertain the identity and position of each radioelement in each decay series.

Rutherford and Soddy's explanation of radioactivity overcame the interpretations of Becquerel and the Curies. By placing the energy source within the atom itself, they opened a four-decade-long debate over whether atomic energy could be harnessed. The nuclear weapons and reactors constructed in World War II answered that question in the affirmative.