Electron-Capture Ratio and Internal Bremsstrahlung inW181Decay

A capture ratio $\frac{P_L}{P_K}=0.27\mathrm{}\pm 0.05$ has been determined for the decay of ${\mathrm{W}}^{181}$ to the ground state and 6-keV level of ${\mathrm{Ta}}^{181}$, on the basis of an x-ray intensity ratio $\frac{I_L}{I_K}=0.311\mathrm{}\pm 0.006$ and a measured partial $L$-shell fluorescence yield ${\omega }_{\mathrm{LL}}=0.225\mathrm{}\pm 0.010$. This value of ${\omega }_{\mathrm{LL}}$, determined from coincidences between $L$ x rays and $\gamma$ rays, is shown to apply for $L$ capture to the 0- and 6-keV levels as well as to the 136- and 159-keV levels. The decay energy of ${\mathrm{W}}^{181}$ is found to be 187±10 keV from an analysis of the internal bremsstrahlung associated with $1s$-electron capture, observed in coincidence with $K$ x rays, and the internal bremsstrahlung spectrum produced in $2s$-electron capture, measured with a Ge(Li) spectrometer. For this transition energy, the theoretical capture ratio is somewhat higher than the measured value, but lies within the error limits of the measurement.