A $98\%$ pure ${}^{242\mathrm{m}}$Am ($K=5^{-}$, $t_{1/2}=141$ years) isomeric target was Coulomb excited with a $170.5$-MeV ${}^{40}\mathrm{Ar}$ beam. The selectivity of Coulomb excitation, coupled with the sensitivity of Gammasphere plus CHICO, was sufficient to identify $46$ new states up to spin $18\hslash$ in at least four rotational bands; $11$ of these new states lie in the isomer band, $13$ in a previously unknown yrast $K^{\pi }=6^{-}$ rotational band, and $13$ in a band tentatively identified as the predicted yrast $K^{\pi }=5^{+}$ band. The rotational bands based on the $K^{\pi }=5^{-}$ isomer and the $6^{-}$ bandhead were populated by Coulomb excitation with unexpectedly equal cross sections. The $\gamma$-ray yields are reproduced by Coulomb excitation calculations using a two-particle plus rotor model (PRM), implying nearly complete $\Delta K=1$ mixing of the two almost-degenerate rotational bands, but recovering the Alaga rule for the unperturbed states. The degeneracy of the $5^{-}$ and $6^{-}$ bands allows for precise determination of the mixing interaction strength $V$, which approaches the strong-mixing limit; this agrees with the $50\%$ attenuation of the Coriolis matrix element assumed in the model calculations. The fractional admixture of the $I_K^{\pi }=6_6^{-}$ state in the nominal $6_5^{-}$ isomer band state is measured within the PRM as $45.6_{-1.1}^{+0.3}\%$. The $E$2 and $M$$1$ strengths coupling the $5^{-}$ and $6^{-}$ bands are enhanced significantly by the mixing, while $E1$ and $E2$ couplings to other low-$K$ bands are not measurably enhanced. The yields of the $5^{+}$ band are reproduced by an $E3$ strength of $\approx$15 W.u., competitive with the interband $E2$ strength. Alignments of the identified two-particle Nilsson states in ${}^{242}\mathrm{Am}$ are compared with the single-particle alignments in ${}^{241}\mathrm{Am}$.

• Received 2 August 2010

DOI:https://doi.org/10.1103/PhysRevC.82.044319