Marie Curie first chose uranium rays as her thesis research topic. In 1897, she began investigating the mysterious rays that Henri Becquerel had found were emitted by uranium compounds.
Curie measured the rays by testing how they ionized air, using a highly sensitive electrometer associated with Pierre Curie and his brother Jacques. Her early measurements showed that the intensity depended on the amount of uranium present, suggesting that the effect came from the atoms themselves rather than from a chemical arrangement or a special physical condition.
She soon broadened the investigation to other substances. In 1898, Curie found that thorium compounds also emitted similar radiation and introduced the term “radioactivity.” When uranium-rich pitchblende proved more active than purified uranium, she and Pierre Curie reasoned that it must contain undiscovered radioactive elements.
That line of research led to the announcement of polonium in July 1898 and radium in December 1898. The common mix-up is to name radium or polonium, since those elements became famous discoveries of the Curies; however, uranium rays were her original thesis subject.