By the time their first daughter, Irene, was born in 1897, Marie had her teaching diploma and resolved to go on studying for a doctorate. As a thesis topic she chose to further investigate the "uranium rays" that Henri Becquerel had reported on just a year earlier.
First, Marie Curie used a methodical approach to determine which elements emitted this radiation; the results indicated uranium and thorium to be the only candidates. Minerals and ores which contain these elements were her next targets. After exhaustive investigation, using the mineral pitchblende as starting material, the new elements polonium and radium were detected. This work was reported in December of 1898.
Marie and Pierre continued to teach and pursue their research in grim conditions; a large, unheated shed served as the laboratory in which they prepared a decigram sample of very pure radium chloride. This effort culminated in Marie's doctoral thesis, which she submitted in 1903. The doctorate was awarded amid general acclaim concerning the scientific value of this breakthrough.
While awed by their discovery, the Curies did not realize the unfavorable medical consequences of the glowing radiation from the small samples they constantly handled. They were frequently tired, the skin of their fingers was scarred and cracked, and illnesses beset them.