What was the name of the French chemist who helped Marie Curie with radium experiments?

The French chemist who helped Marie Curie with radium experiments was André Debierne.

His full name was André-Louis Debierne, but “André Debierne” is the standard short form and the single canonical answer intended by the question. He was a close friend and scientific associate of Pierre and Marie Curie.

Debierne discovered the radioactive element actinium in 1899 while continuing research connected with the Curies’ work on pitchblende. After Pierre Curie died in 1906, Debierne helped Marie continue teaching and laboratory research. Their collaboration included demanding work on radium chemistry.

By 1911, Debierne and Marie Curie had prepared radium in metallic form in visible quantities. This achievement was chemically difficult because radium is closely related to barium and occurred only in tiny amounts in uranium ores. Debierne is therefore more directly connected with Curie’s radium experiments than the other names listed: Rutherford studied radioactivity, Langevin was a physicist, and Becquerel discovered radioactivity.