Resonance Fluorescence inRe187

Abstract

The excited states of ${\mathrm{Re}}^{187}$, populated in the decay of ${\mathrm{W}}^{187}$, have been investigated by the resonance fluorescence method. Using the centrifuge technique, five resonantly scattered $\gamma$ lines of 773-, 686-, 620-, 552-, and 479-keV energy, originating from the excitation of levels at 773, 686, and very probably 618 keV, have been observed. The measured angular distributions of these $\gamma$ radiations are in agreement with spin assignments and multipole orders deduced from studies of the radioactive decay of ${\mathrm{W}}^{187}$. Combining relative intensities given by Gallagher et al. with the observed cross sections for resonance scattering, the following mean lifetimes were computed: 773-keV level: $\tau =(2.5\mathrm{}\pm 0.3)\times {10}^{-13\mathrm{}}$ sec, 686-keV level: $\tau =(8.5\mathrm{}\pm 0.7)\times {10}^{-12\mathrm{}}$ sec, and 618-keV level: $\tau =(1.6\mathrm{}\pm 0.2)\times {10}^{-11\mathrm{}}$ sec. These results agree with the theoretical interpretation in terms of the Nilsson model. The experiments give a lower limit of $\tau >1.0\mathrm{}\times {10}^{-12\mathrm{}}$ sec for the lifetime of the 864-keV level, assuming spin $\frac{3}{2}$ or $\frac{5}{2}$.