Marguerite Catherine Perey was born on 19 October 1909 in Villemomble, on the eastern edge of Paris, the youngest of five children of Émile Louis Perey and Anne Jeanne Ruissel. Her father, who owned a mill, died in March 1914, and the family was left in financial difficulty. Perey did not follow the conventional path of the French scientific elite. She trained as a laboratory technician at the École d’enseignement technique féminine and, in 1929, at the age of twenty, entered the Institut du Radium as a personal assistant to Marie Curie. Curie interviewed her and hired her on the spot. For the next few years Perey worked closely with the woman who had already discovered polonium and radium, learning the exacting habits of radiochemical separation at a time when protective standards were still rudimentary.
After Curie’s death in 1934, Perey remained at the laboratory under André Debierne and Irène Joliot-Curie. Her principal task was the purification of actinium-227, a natural radioelement that always accompanied the rare earths and was extraordinarily difficult to isolate. Fractional crystallization, the only practical method then available, was slow, tedious, and hazardous. Years of that work gave her an intimate knowledge of actinium’s behavior and of the small anomalies that others might have dismissed as contamination.
By the mid-1930s the periodic table still lacked a confirmed element 87, the alkali metal that Dmitri Mendeleev had called eka-caesium. Claims of its discovery had been made and withdrawn. In 1935 Perey read a report by American researchers who believed they had detected energetic beta particles from actinium itself. She was skeptical: the energies did not fit what was known of actinium’s decay. She suspected that the radiation came from a short-lived daughter formed by a rare side branch. To test the idea she prepared actinium of exceptional purity and measured its radioactivity within minutes of the final separation. She found a beta activity that rose toward a limit in about two hours and belonged to a species with a half-life of roughly twenty-one minutes. The main decay of actinium-227 leads by beta emission to thorium-227; only about one percent of the disintegrations proceed by alpha emission. That minor branch removes two protons and two neutrons and produces an atom of atomic number 87. Perey identified the new nuclide, later known as francium-223, and showed by co-precipitation with caesium perchlorate that it behaved chemically as an alkali metal.
Because she held no university degree, she could not present the result herself. On 9 January 1939 Jean Perrin communicated her note to the Académie des Sciences under the title announcing an element 87 derived from actinium. She first called the substance actinium K. She later named it francium, after her country, in deliberate homage to the way Curie had named polonium after Poland. Francium was the last element discovered in nature rather than by artificial transmutation, and it remains one of the rarest: only a few hundred grams exist in the Earth’s crust at any moment, continually formed and decaying. Its longest-lived isotope lasts only twenty-two minutes.
The discovery transformed Perey’s status, but formal recognition still required credentials she did not possess. While continuing her research she completed a licence and, in March 1946, defended a doctoral thesis at the Sorbonne on element 87. She was appointed maître de recherches at the Centre National de la Recherche Scientifique and, in 1949, was given a newly created chair of nuclear chemistry at the University of Strasbourg, the only such chair outside Paris. There she built a teaching and research program in radiochemistry and nuclear chemistry and directed the laboratory that in 1958 became the Laboratory of Nuclear Chemistry of the Centre de Recherches Nucléaires. From 1950 to 1963 she also served on the Atomic Weights Commission. She was nominated several times for the Nobel Prize in Chemistry and never received it.
On 12 March 1962 she was elected the first woman corresponding member of the Académie des Sciences, an institution founded in 1666 that had never admitted Marie Curie. The honor arrived as her health collapsed. Decades of unprotected work with actinium and other emitters had produced bone cancer. She underwent repeated operations and spent her final years in pain, largely unable to exercise the academic role she had earned. She died on 13 May 1975 in Louveciennes at the age of sixty-five. She had never married.
Perey’s later career was shaped by that illness. From the moment she arrived in Strasbourg she pressed for stricter radioprotection in the laboratory, arguing that the practices she had inherited were no longer acceptable. Colleagues remembered her as meticulous, intuitive, and insistent on clean separations. The same patience that let her catch a one-percent branching ratio also made her a demanding teacher. Francium itself proved too short-lived and too scarce for the medical uses she had once hoped it might serve; the element that made her name was also part of the exposure that shortened her life.
Her story sits between two eras of radioactivity research. She began as Curie’s technician in a laboratory where women could prepare samples and measure decay curves but could not easily claim authorship or academic rank. She ended as a professor and the first woman inside the Académie des Sciences, yet the radiation that had been the laboratory’s working medium had already marked her body. The 1939 note remains a model of careful radiochemistry: purify the parent, measure immediately, distinguish a side branch from contamination, and confirm the chemistry by carrier precipitation. Element 87 entered the periodic table not through a cyclotron but through the older craft of fractional crystallization and rapid counting, practiced by someone who, at the moment of discovery, was still officially a laboratory assistant.