Identification of the slow<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>E</mml:mi><mml:mn>3</mml:mn></mml:mrow></mml:math>transition<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>136</mml:mn></mml:msup></mml:math>Cs<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mi>m</mml:mi></mml:msup></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mo>→</mml:mo><mml:mn>136</mml:mn></mml:msup></mml:math>Cs with conversion electrons

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.

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