Research Breakthroughs

X-rays and Uranium Rays

MARIE CURIE'S CHOICE of a thesis topic was influenced by two recent discoveries by other scientists. In December 1895, about six months after the Curies married, German physicist Wilhelm Roentgen discovered a kind of ray that could travel through solid wood or flesh and yield photographs of living people's bones. Roentgen dubbed these mysterious rays X-rays, with X standing for unknown. In recognition of his discovery, Roentgen in 1901 became the first Nobel laureate in physics.

early X-ray photograph
One of Roentgen's first X-ray photographs -- a colleague's hand (note the wedding ring). The revelation of X-rays fascinated the public and deeply puzzled scientists
Henri Becquerel
Henri Becquerel, discoverer of uranium radiation. Although he tried to help the Curies solve their financial problems and advance their careers, the relationship eventually soured--as sometimes happens with scientists touchy about sharing credit for discoveries.

In early 1896, only a few of months after Roentgen's discovery, French physicist Henri Becquerel reported to the French Academy of Sciences that uranium compounds, even if they were kept in the dark, emitted rays that would fog a photographic plate. He had come upon this discovery accidentally. Despite Becquerel's intriguing finding, the scientific community continued to focus its attention on Roentgen's X-rays, neglecting the much weaker Becquerel rays or uranium rays.

THE IGNORED URANIUM RAYS appealed to Marie Curie. Since she would not have a long bibliography of published papers to read, she could begin experimental work on them immediately. The director of the Paris Municipal School of Industrial Physics and Chemistry, where Pierre was professor of physics, permitted her to use a crowded, damp storeroom there as a lab.

A clever technique was her key to success. About 15 years earlier, Pierre and his older brother, Jacques, had invented a new kind of electrometer, a device for measuring extremely low electrical currents. Marie now put the Curie electrometer to use in measuring the faint currents that can pass through air that has been bombarded with uranium rays. The moist air in the storeroom tended to dissipate the electric charge, but she managed to make reproducible measurements.

“Instead of making these bodies act upon photographic plates, I preferred to determine the intensity of their radiation by measuring the conductivity of the air exposed to the action of the rays.”

quartz balance

This device for precise electrical measurement, invented by Pierre Curie and his brother Jacques, was essential for Marie's work. (Photo ACJC)

You can exit this site to an exhibit on the discovery of the electron

With numerous experiments Marie confirmed Becquerel's observations that the electrical effects of uranium rays are constant, regardless of whether the uranium was solid or pulverized, pure or in a compound, wet or dry, or whether exposed to light or heat. Likewise, her study of the rays emitted by different uranium compounds validated Becquerel's conclusion that the minerals with a higher proportion of uranium emitted the most intense rays. She went beyond Becquerel's work, however, in forming a crucial hypothesis: the emission of rays by uranium compounds could be an atomic property of the element uranium--something built into the very structure of its atoms.

MARIE'S SIMPLE HYPOTHESIS would prove revolutionary. It would ultimately contribute to a fundamental shift in scientific understanding. At the time scientists regarded the atom--a word meaning undivided or indivisible -- as the most elementary particle. A hint that this ancient idea was false came from the discovery of the electron by other scientists around this same time. But nobody grasped the complex inner structure or the immense energy stored in atoms. Marie and Pierre Curie themselves were not convinced that radioactive energy came from within atoms--maybe, for example, the earth was bathed in cosmic rays, whose energy certain atoms somehow caught and radiated? Marie's real achievement was to cut through the complicated and obscure observations with a crystal-clear analysis of the set of conclusions that, however unexpected, were logically possible.

Marie tested all the known elements in order to determine if other elements or minerals would make air conduct electricity better, or if uranium alone could do this. In this task she was assisted by a number of chemists who donated a variety of mineral samples, including some containing very rare elements. In April 1898 her research revealed that thorium compounds, like those of uranium, emit Becquerel rays. Again the emission appeared to be an atomic property. To describe the behavior of uranium and thorium she invented the word “radioactivity” --based on the Latin word for ray.

The Discovery of Polonium and Radium

Radioactivity: The Unstable Nucleus

� 2000 - American Institute of Physics