Coulomb excitation of aAm242isomeric target:E2andE3strengths, rotational alignment, and collective enhancement

Abstract

A $98%$ pure ${}^{242\mathrm{m}}$Am ($K={5}^{\ensuremath{-}}$, ${t}_{1/2}=141$ 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 $18\ensuremath{\hbar}$ in at least four rotational bands; $11$ of these new states lie in the isomer band, $13$ in a previously unknown yrast ${K}^{\ensuremath{\pi}}={6}^{\ensuremath{-}}$ rotational band, and $13$ in a band tentatively identified as the predicted yrast ${K}^{\ensuremath{\pi}}={5}^{+}$ band. The rotational bands based on the ${K}^{\ensuremath{\pi}}={5}^{\ensuremath{-}}$ isomer and the ${6}^{\ensuremath{-}}$ bandhead were populated by Coulomb excitation with unexpectedly equal cross sections. The $\ensuremath{\gamma}$-ray yields are reproduced by Coulomb excitation calculations using a two-particle plus rotor model (PRM), implying nearly complete $\ensuremath{\Delta}K=1$ mixing of the two almost-degenerate rotational bands, but recovering the Alaga rule for the unperturbed states. The degeneracy of the ${5}^{\ensuremath{-}}$ and ${6}^{\ensuremath{-}}$ 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 ${I}_{K}^{\ensuremath{\pi}}={6}_{6}^{\ensuremath{-}}$ state in the nominal ${6}_{5}^{\ensuremath{-}}$ isomer band state is measured within the PRM as $45.{6}_{\ensuremath{-}1.1}^{+0.3}%$. The $E$2 and $M$$1$ strengths coupling the ${5}^{\ensuremath{-}}$ and ${6}^{\ensuremath{-}}$ 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 $\ensuremath{\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 $^{241}\mathrm{Am}$.