Spontaneous Fission Isomerism in Uranium Isotopes

A search for spontaneous fission isomerism in uranium and neptunium isotopes has resulted in the identification of a new isomer in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ with a half-life of 195 ± 30 nsec. A measurement of the half-life of previously reported ${}_{}{}^{236m}{}_{}{}^{}\mathrm{U}$ gives a value of 105 ± 20 nsec. Excitation functions have been determined for the production of the spontaneous fission isomers of ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ by deuteron bombardment of ${}_{}{}^{236}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ targets. Isomer ratios have also been estimated. The low values of the isomer ratios and the failure to observe delayed fission in lighter uranium isotopes and in neptunium isotopes suggest that $\gamma$ decay may be the dominant mode of decay of shape isomers in this mass region.