Mean lifetimes of theγcascades in theCu61continuum

Abstract

The reduction of the Doppler shift for several bound-state transitions in $^{61}\mathrm{Cu}$ due to the intervening $\ensuremath{\gamma}$ cascades from the continuum was measured, using the Doppler-shift-attenuation method in a $p\ensuremath{-}\ensuremath{\gamma}$ coincidence arrangement that employed the $^{58}\mathrm{Ni}(\ensuremath{\alpha},p\ensuremath{\gamma})^{61}\mathrm{Cu}$ reaction at 19.7 MeV. It is found that the mean lifetimes of cascades that begin at the most probable excitation energy of \ensuremath{\sim} 9.5 MeV in $^{61}\mathrm{Cu}$ and populate the 2336.2-keV (9/${2}^{\ensuremath{-}}$), 2611.8-keV (9/${2}^{\ensuremath{-}}$), and 3015.6-keV (11/${2}^{\ensuremath{-}}$) states are, respectively 0.29 \ifmmode\pm\else\textpm\fi{} 0.12, 0.19 \ifmmode\pm\else\textpm\fi{} 0.07, and 0.43 \ifmmode\pm\else\textpm\fi{} 0.14 ps. The mean lifetimes of the cascades feeding the above three states were found to increase with increasing total energy available for the cascades with an approximately linear dependence with slopes of 40 \ifmmode\pm\else\textpm\fi{} 22, 29 \ifmmode\pm\else\textpm\fi{} 12, and 69 \ifmmode\pm\else\textpm\fi{} 12, and 69 \ifmmode\pm\else\textpm\fi{} 20 fs/MeV. These results are consistent with an increase in the $\ensuremath{\gamma}$ multiplicity in the cascades with increasing total available cascade energy.[NUCLEAR REACTIONS $^{58}\mathrm{Ni}(\ensuremath{\alpha},p\ensuremath{\gamma})^{61}\mathrm{Cu}$, ${E}_{\ensuremath{\alpha}}=19.7$ MeV; measured $\overline{\ensuremath{\tau}}({E}^{*})$, the energy dependence of the mean lifetimes of the $\ensuremath{\gamma}$ cascades in $^{61}\mathrm{Cu}$. Enriched targets, Ge(Li) $\ensuremath{\gamma}$ detectors, annular Si(Li) detector, Doppler-shift analysis of line shapes.]