γ-ray multipolarimetry between low-spin states ofCe136: Search for the21,ms+one-phonon mixed-symmetry state

Abstract

An experiment was performed on $^{136}\mathrm{Ce}$ to measure $E2/M1$ multipole mixing ratios using the method of $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlations. The low-spin states of interest were populated by \ensuremath{\beta} decay from the ${2}^{+}$ ground state of $^{136}\mathrm{Pr}$ that has a half-life of 13.1 min. Emitted \ensuremath{\gamma} rays were detected with the Stony Brook cube germanium detector array. Five $E2/M1$ multipole mixing ratios were measured for the first time and two spin and parity quantum numbers were assigned. Candidates for the ${2}_{1,\mathrm{ms}}^{+}$ state of $^{136}\mathrm{Ce}$ were identified from large $M1$ fractions in the $E2/M1$ multipole mixing ratios. The ${2}_{1,\mathrm{ms}}^{+}$ state is suggested to be fragmented between the closely lying ${2}_{3}^{+}$ and ${2}_{4}^{+}$ states. A two state mixing scenario is employed for deducing an $F$-spin mixing matrix element of 43(5) keV.