Spontaneously fissioning isomers in U, Pu, Am and Cm isotopes

Abstract

Spontaneously fissioning isomers with microsecond half-lives have been observed in 237Pu, 239Pu, 241Pu and 243Am using a pulsed beam technique. Isomers having nanosecond half-lives have been observed in 237Am, 241Cm and 243Cm using a DC beam and a fission-inflight detection method. The isomers have been studied through p and d induced reactions on thin targets of U, Np, Pu and Am at energies varying from 9–14.2 MeV. The assignments are based on measured excitation functions and the important reactions are found to be (p, 2n), (d, 2n), (d, p) and (d, pn). The reaction 238Pu+p shows threshold behaviour typical for a (p, 2n) reaction in the interval from 12 to 13 MeV. The difference E II in the thresholds for the ground state and the isomeric state is 3.0 ± 0.3 MeV. A 300 ns isomer earlier assigned to 239Np has been reassigned to 238U. A half-life systematics for fission isomers based on isomers reported in the literature and on isomers measured in the present work is presented. Odd-even effects on the spontaneous fission half-lives and the existence of excited states in the second potential well are discussed.