Search for one-phonon mixed-symmetry states in the radioactive nucleusNd140

Abstract

Low-spin excited states of $^{140}\mathrm{Nd}$ have been studied via the $^{140}\mathrm{Ce}$($^{3}\mathrm{He}$,$3n$)$^{140}\mathrm{Nd}$ reaction. The data show that one of the candidates for the one-phonon mixed-symmetry state (MSS) of $^{140}\mathrm{Nd}$, namely, the ${2}_{3}^{+}$ state at 2140 keV with an effective lifetime of 220(90) fs, exhibits a fast $M1$ decay to the ${2}_{1}^{+}$ state. Thus, this state can be considered, at least, as a fragment of the one-phonon MSS of $^{140}\mathrm{Nd}$. This is the first example where mixed symmetry character is tentatively assigned to a state of an unstable nucleus from the mass $A\ensuremath{\approx}140$ region based on the data on absolute $M1$ transition rates. However, the data are not conclusive on whether this decay exhausts the total $M1$ strength or whether the one-phonon MSS of $^{140}\mathrm{Nd}$ is fragmented.