Spontaneous fission in the neutron bombardment of uranium isotopes

A search is being made for spontaneously fissioning isomers with half-lives in the range of 10-1000 ns from the neutron bombardment of the various isotopes of uranium. The time distribution of the pulses due to the detection of fission fragments are studied after bombardment by 0.1-77 MeV neutrons produced in reactions initiated by the pulsed proton and deuteron beams from the IBIS electrostatic accelerator. With the present technique the intensity of the residual beam between beam bursts is less than 10-5 of the main beam intensity so that the time distribution of the pulses in any delayed fission process can be observed quite cleanly. Preliminary results obtained with solid-state detectors indicate the existence of weakly excited events having a half-life of about 100 ns in the bombardment of 235U by 2 MeV neutrons. Further studies are in progress to determine the half-life more accurately, as well as to obtain a measure of the cross-section for the formation of a possible spontaneously fissioning mode. (author)