Fission of Heavy Nuclei: a New Type of Nuclear Disintegration

Abstract

THE first indication that the transmutation of heavy nuclei could be effected in a laboratory experiment was obtained by Fermi in March 1934. Curie and Joliot had just discovered that shortlived radioactive species are produced as the result of α-particle bombardment of certain light elements, and Fermi, accepting the appearance of such ‘induced’ radioactivity as proof of transmutation, very soon showed that the nuclei of almost all elements, even those of highest atomic weight, undergo transformation when neutrons are used. From his early experiments Fermi concluded that in general the neutron is simply captured by the nucleus, and he went on to show that this process of capture is usually more efficient-and sometimes very much more efficient-when the neutron is moving with a small (‘thermal’) velocity, before the collision, than when its energy is large. Eventually he found that negative electrons were emitted in the disintegration of the radioactive products obtained in all these capture transformations, and thus the final result of the combined process of neutron capture and β disintegration was in every case shown to be the production of a nucleus having both mass and charge numbers greater by one unit than the mass and charge numbers of the nucleus bombarded.