1900 Crookes and Becquerel removed the beta-active substance from uranium, while the alpha-activity remained, showing that two different chemicals were producing radiation. A year later they found that the uranium fraction had recovered its beta-activity, while the other beta-active portion had lost its activity.
1902 Rutherford and Soddy measured rates of radioactive growth and decay and calculated decay constants.
1903 Rutherford and Soddy put forward the 'spontaneous disintegration theory' of radioactivity.
1903... The problem of fitting the large number of radiation products into the periodic table, and the existence of cases where materials had the same chemical properties but different radiation properties, and Thomson's discovery of atoms of the same element having different atomic weights led to the idea of isotopes - given that name by Soddy. Soddy published further discussion of isotopes in 1913. His 1921 Nobel prize for Chemistry was "for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes"
By 1912,radioactive decay was fairly well understood.