Electric Monopole Transitions inGe70andZr90

Abstract

The electric monopole crossover transition from the 1.215 Mev 0+ second excited state to the ground state of ${\mathrm{Ge}}^{70}$ has been observed with an intermediate-image beta-ray spectrometer arranged for electron-electron coincidence measurements. Internal conversion electrons in delayed coincidence with ${\mathrm{Ga}}^{70}$ beta rays correspond to a 1.215-Mev transition intensity of 0.0025% per disintegration. Total beta-ray branching to the 1.215-Mev state is 0.18%, mostly followed by a (0.173\ifmmode\pm\else\textpm\fi{}0.002)-(1.042\ifmmode\pm\else\textpm\fi{}0.005) Mev gamma-ray cascade through the first excited state. Beta-ray branching to the 1.042-Mev level in ${\mathrm{Ge}}^{70}$ is 0.30%. Beta-conversion-electron delayed coincidence measurements taken at the peak of the 0.173-Mev $K$-line give a half-life value of (3.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}9}$ sec for the 1.215-Mev state. It follows that the partial half-life for $K$-conversion of the 0-0 transition is (2.4\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec which compares with 1.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ sec expected if the matrix element were the same as for the 0-0 transition in ${\mathrm{Ge}}^{72}$. Using similar techniques on ${\mathrm{Y}}^{90}$ the half-life of the 1.75 Mev 0-0 transition in ${\mathrm{Zr}}^{90}$ has been found to be (6\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ sec.