Discovery of the actinium, thorium, protactinium, and uranium isotopes
  Currently, 31 actinium, 31 thorium, 28 protactinium, and 23 uranium isotopes have so far been observed; the discovery of these isotopes is discussed. For each isotope a brief summary of the first refereed publication, including the production and identification method, is presented. 
C. Fry and M. Thoennessen
nucl-ex