 The chemists then turned to techniques for removing light element impurities from the plutonium; but on 14 July 1944, Oppenheimer informed Kennedy that this would no longer be required.The notion of spontaneous fission had been raised by Niels Bohr and John Archibald Wheeler in their 1939 treatment of the mechanism of nuclear fission.The first attempt to discover spontaneous fission in uranium was made by Willard Libby, but he failed to detect it.