The discovery of radioactivity – research stimulated by Roentgen’s discovery of x rays. 

In 1903 Becquerel received half of the Nobel Prize in Physics 1903 "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity." The half was awarded to Pierre Curie and Marie Curie, née Skłodowska "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel."