Identification of the slowE3transition136Csm→136Cs with conversion electrons

Abstract

We performed at ISOLDE the spectroscopy of the decay of the ${8}^{\ensuremath{-}}$ isomer in ${}^{136}$Cs by $\ensuremath{\gamma}$ and conversion-electron detection. For the first time the excitation energy of the isomer and the multipolarity of its decay have been measured. The half-life of the isomeric state was remeasured to ${T}_{1/2}=17.5(2)$ s. This isomer decays via a very slow 518-keV $E3$ transition to the ground state. In addition to this, a much weaker decay branch via a 413-keV $M4$ and a subsequent 105-keV $E2$ transition has been found. Thus we have found a new level at 105 keV with spin ${4}^{+}$ between the isomeric and the ground state. The results are discussed in comparison to shell-model calculations.