Lifetimes of Rotational Levels in Some Odd-ANuclei Measured by a Microwave Method

Abstract

Measurements of mean lives of the first excited states of a number of odd-$A$ nuclei have been carried out with the intention of obtaining more accurate information on magnetic transition probabilities than is at present available. Most of these mean lives are in the range of 2-20\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}11}$ sec, and for this measurement a special timing apparatus has been devised which consists of a microwave beam pulsing system and a beta-ray spectrometer and cavity combination which acts as a microwave shutter. In four cases the exponential decay could be determined directly; these were the first excited states of ${\mathrm{Lu}}^{175}$, ${\mathrm{Hf}}^{179}$, and ${\mathrm{Ta}}^{181}$, and the 118-kev level of ${\mathrm{Tm}}^{169}$. The results for these levels are: $\ensuremath{\tau}({\mathrm{Tm}}^{169})=(9.0\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec, $\ensuremath{\tau}({\mathrm{Lu}}^{175})=(14.6\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec, $\ensuremath{\tau}({\mathrm{Hf}}^{179})=(5.4\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec, and $\ensuremath{\tau}({\mathrm{Ta}}^{181})=(6.0\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}$ sec. Lifetime measurements were also carried out for the first excited states of ${\mathrm{Hf}}^{177}$, ${\mathrm{Re}}^{185}$, and ${\mathrm{Re}}^{187}$. The limitations and scope of this method of measuring lifetimes are discussed.