Lifetimes of Excited States inCa40by the Doppler-Shift Attenuation Method

Lifetimes of levels in ${\mathrm{Ca}}^{40}$ below an excitation energy of 6.3 MeV have been studied through the ${\mathrm{Ca}}^{40}(p, p^{\prime }\gamma ){\mathrm{Ca}}^{40}$ 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 ${\theta }_{\gamma }=0\mathrm{}°$ and ${\theta }_{\gamma }=120\mathrm{}°$ using targets of thick metallic calcium as well as calcium foils prepared by evaporation. From these measurements, accurate excitation energies and $\gamma$-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 $\gamma$-ray Doppler shifts. The existence of two close-lying levels with excitation energies 6025.2±0.4 and 6029.0±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); ${\tau }_m\left(\mathrm{psec}\right)$] for a number of levels of ${\mathrm{Ca}}^{40}$: 5.212(1.9±0.3), 5.249(0.18±0.03), 5.279(0.33±0.08), 5.615(0.85±0.20), 6.025(0.22±0.03), 6.029(0.47±0.13), and 6.285(0.45±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 $J^{\pi }=3^{+}$ for the 6.029-MeV level. The measured $M1\mathrm{}$ and $E2\mathrm{}$ radiative widths for the transitions among the $5^{-}$, $4^{-}$, $3^{-}$, and $2^{-}$ 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 ${d_{\frac{3}{2}}}^{-1\mathrm{}}{f}_{\frac{7}{2}}$ configuration. Calculated decay modes of the $4^{-}$, $T=1\mathrm{}$ level at 7.659 MeV are compared to experiment.