After discovery of uranium fission by Hahn and Strassmann in Berlin-Dahlem at the end of 1938, nuclides regarded as isotopes of transuranium elements were shown to be those of lighter elements. Only the uranium isotope 239 was confirmed. As beta emitter it was likely to be the mother substance of at least one transuranium isotope. Separation had failed, possibly because incorrect analytical behaviour had been assumed. Radioactive recoil, being independent of chemical properties, served successfully to isolate the neptunium isotope 239.