Decay ofCs128

Samples of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Cs}$(3.9 min) in equilibrium with its parent ${}_{}{}^{128}{}_{}{}^{}\mathrm{}(2.4\mathrm{d}\mathrm{a}\mathrm{y})\mathrm{}$ have been produced by ∼800-MeV proton-induced spallation in praseodymium metal foils. Chemical purification and isotope separation have been performed to produce pure samples. Measurements of $\gamma$-ray singles and $\gamma -\gamma$ coincidence spectra have been made. A total of 66 $\gamma$ rays between 442 and 3104 keV are assigned to the decay of ${}_{}{}^{128}{}_{}{}^{}\mathrm{Cs}$, 57 of which have been placed in a proposed level scheme for ${}_{}{}^{128}{}_{}{}^{}\mathrm{Xe}$ involving 22 established and 3 tentative excited states. Absolute $\gamma$-ray intensities have been computed based on the positron and x-ray intensities. Spin and parity assignments are proposed based on the $log\mathrm{ft}$ values and the existence of $\gamma$ rays depopulating the states. The first excited $0^{+}$ state is found to occur at 1582 keV. Evidence is presented which suggests that it is of predominantly "three-phonon" character, in agreement with some theoretical predictions for transitional nuclei in this mass region.