Neutron-deficient americium isotopes ${}^{233,234,235,236}\mathrm{Am}$ were produced in the bombardment of multiple thin targets of ${}^{233}\mathrm{U}$ and ${}^{235}\mathrm{U}$ with $7.6–10.5\text{MeV}∕\mathrm{nucleon}$ ${}^6\mathrm{Li}$ ions. The reaction products were effectively mass-separated using the on-line isotope separator (JAERI-ISOL) coupled to a helium gas-jet transport system. For the first time, the $\alpha$ transition of ${}^{235}\mathrm{Am}$ was observed, and some improved $\alpha$-decay data for ${}^{233,234,236}\mathrm{Am}$ and ${}^{229}\mathrm{Np}$ ($\alpha$-decay daughter of ${}^{233}\mathrm{Am}$) were accumulated. The $\alpha$-particle energies of ${}^{233}\mathrm{Am}$ and ${}^{235}\mathrm{Am}$ were determined to be $6780\pm 17\text{keV}$ and $6457\pm 14\text{keV}$ and their half-lives were $3.2\pm 0.8\min$ and $10.3\pm 0.6\min$, respectively. Measured $\alpha$-decay branching ratios of ${}^{235}\mathrm{Am}$ and ${}^{229}\mathrm{Np}$ were $\left(4.0\pm 0.5\right)\times {10}^{-3}$ and $0.68\pm 0.11$, respectively, while the lower limit for $\alpha$-decay of ${}^{233}\mathrm{Am}$ was deduced to be $3\times {10}^{-2}$. Hindrance factors and probable assignments of the levels involved in those $\alpha$ transitions are discussed.

• Received 28 September 2003

DOI:https://doi.org/10.1103/PhysRevC.69.014308