The Time Involved in the Process of Nuclear Fission

Abstract

BOHR¹ has discussed the process of nuclear fission in uranium and thorium under neutron bombardment on the assumption that a neutron is first captured by the nucleus and that later the energy of excitation of the compound nucleus is concentrated in motion of some special type which leads to deformation and ultimately to division of the system. From this point of view, it is of interest to try to obtain an estimate of the life-time of the compound nucleus before fission, and Green and Alvarez² have reported upon experiments with a modulated beam of neutrons produced in the Berkeley cyclotron which show that this life-time must be smaller than 3 × 10^–3 sec. I have recently obtained evidence of an asymmetry in the spatial distribution of the fission fragments, in the case of uranium bombarded by fast neutrons, from which a limiting time very much smaller than this may be deduced. On the basis of considerations given below, it would in fact appear that the mean life of the intermediate nucleus cannot be longer than about 5 × 10^–13 sec. in this case.