In December 1949, S.G. Thompson, A. Ghiorso and G.T. Seaborg carried out an ion-exchange separation of the products formed by the irradiation of milligram quantities of Am 241 with 35 MeV helium ions. A 4.5 - hour electron-capture activity eluting just ahead of curium was assigned to an isotope of a new element with atomic number 97 and mass number 243. The new element was named berkelium for the city of Berkeley, California, where it was discovered.