L to K Ratios in the Electron Capture Decay of W181 and Ta179

Abstract

The $L$ to $K$ ratios in the electron capture decay of the isotopes ${\mathrm{Ta}}^{179}$ and ${\mathrm{W}}^{181}$ have been redetermined. Thin (0.060 in.) NaI crystals with thin (0.002 in.) Be windows were used to detect the $L$ and $K$ x rays. The partial fluorescence yields of the $L$ subshells of Hf and Ta were also measured by determining the coincidence rates between the $L$ and the $K$ x rays emitted by the sources, It is necessary to know the fluorescence yields if the $L$ to $K$ ratios are to be determined from the measured intensities of the $L$ and the $K$ x rays. The $L$ to $K$ ratio of ${\mathrm{Ta}}^{179}$ was found to be 0.63±0.06, which implies a total decay energy of approximately 115±5 kev for this isotope. This energy is consistent with the observation that no gamma rays accompany the decay of ${\mathrm{Ta}}^{179}$ since the first excited level of ${\mathrm{Hf}}^{179}$ has an energy (122 kev) exceeding the total decay energy of ${\mathrm{Ta}}^{179}$. The $L$ to $K$ ratio of ${\mathrm{W}}^{181}$ was found to be 0.23±0.05, from which a decay energy of approximately 260 kev is computed. This result is in agreement with the fact that two weak gamma rays are emitted by the source, one at 137 kev and the other at 152 kev. These gamma rays correspond to excited levels in ${\mathrm{Ta}}^{181}$. These gamma rays are in coincidence with the $L$ x rays emitted by the source but not with the $K$ x rays, which means that the total decay energy of ${\mathrm{W}}^{181}$ must exceed 166 kev. The $L$ to $K$ capture ratios reported here are not in good agreement with previously reported values.