Lifetimes of Excited States inCa40by the Doppler-Shift Attenuation Method

Abstract

Lifetimes of levels in ${\mathrm{Ca}}^{40}$ below an excitation energy of 6.3 MeV have been studied through the ${\mathrm{Ca}}^{40}(p, {p}^{\ensuremath{'}}\ensuremath{\gamma}){\mathrm{Ca}}^{40}$ reaction. Bombarding energies between 8.5 and 9.0 MeV were used. Gamma-ray spectra were recorded with a Ge(Li) spectrometer at angles between ${\ensuremath{\theta}}_{\ensuremath{\gamma}}=0\ifmmode^\circ\else\textdegree\fi{}$ and ${\ensuremath{\theta}}_{\ensuremath{\gamma}}=120\ifmmode^\circ\else\textdegree\fi{}$ using targets of thick metallic calcium as well as calcium foils prepared by evaporation. From these measurements, accurate excitation energies and $\ensuremath{\gamma}$-ray decay modes were determined for the levels below 6.3-MeV excitation energy, information on level spin parity was deduced, and values or limits for the mean lifetimes of the levels were extracted from observations of $\ensuremath{\gamma}$-ray Doppler shifts. The existence of two close-lying levels with excitation energies 6025.2\ifmmode\pm\else\textpm\fi{}0.4 and 6029.0\ifmmode\pm\else\textpm\fi{}0.5 keV was established. Recent results obtained elsewhere confirm the existence of this doublet. Upon combining our results with previously available information, we deduce the following lifetimes [${E}_{\mathrm{expt}}$ (MeV); ${\ensuremath{\tau}}_{m}(\mathrm{psec})$] for a number of levels of ${\mathrm{Ca}}^{40}$: 5.212(1.9\ifmmode\pm\else\textpm\fi{}0.3), 5.249(0.18\ifmmode\pm\else\textpm\fi{}0.03), 5.279(0.33\ifmmode\pm\else\textpm\fi{}0.08), 5.615(0.85\ifmmode\pm\else\textpm\fi{}0.20), 6.025(0.22\ifmmode\pm\else\textpm\fi{}0.03), 6.029(0.47\ifmmode\pm\else\textpm\fi{}0.13), and 6.285(0.45\ifmmode\pm\else\textpm\fi{}0.10). In view of the decay modes established for the doublet at 6.027 MeV, we have reanalyzed previously published work to obtain, in conjunction with the lifetime measurement, a rigorous spin-parity assignment of ${J}^{\ensuremath{\pi}}={3}^{+}$ for the 6.029-MeV level. The measured $M1$ and $E2$ radiative widths for the transitions among the ${5}^{\ensuremath{-}}$, ${4}^{\ensuremath{-}}$, ${3}^{\ensuremath{-}}$, and ${2}^{\ensuremath{-}}$ states at 4.492, 5.615, 3.737, and 6.025 MeV are compared to calculations in which these levels are identified as belonging to the ${{d}_{\frac{3}{2}}}^{\ensuremath{-}1}{f}_{\frac{7}{2}}$ configuration. Calculated decay modes of the ${4}^{\ensuremath{-}}$, $T=1$ level at 7.659 MeV are compared to experiment.