Transition rates of high-spin states in /sup 56/Co and /sup 56/Fe: Possible coexistence of prolate- and oblate-like configurations in /sup 56/Fe. [Angular distributions and excitation functions for /sup 54/Fe(. cap alpha. ,pn) and (. cap alpha. ,2p) reactions at 20. 4 to 29. 5 MeV]

The decay properties of the high-spin states in $^{56}\mathrm{Co}$ and $^{56}\mathrm{Fe}$ have been investigated via the $^{54}\mathrm{Fe}(\ensuremath{\alpha},pn)^{56}\mathrm{Co}^{*}(\ensuremath{\gamma})$ and $^{54}\mathrm{Fe}(\ensuremath{\alpha},2p)^{56}\mathrm{Fe}^{*}(\ensuremath{\gamma})$ reactions in the energy range 20.4-29.5 MeV. From measurements of excitation functions and angular distributions of individual $\ensuremath{\gamma}$ rays with a Ge(Li)-NaI(T1) anti-Compton spectrometer, and from $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence experiments new levels in $^{56}\mathrm{Co}$ have been identified at 2282.6 (${7}_{1}^{+}$), 2371.9 (${6}_{1}^{+}$), 3638.0 (${8}_{1}^{+}$), 4180.2 (${9}_{1}^{+}$), and 5274.6 (${10}_{1}^{+}$) keV with the deduced ${J}^{\ensuremath{\pi}}$ values in parentheses. Similarly levels in $^{56}\mathrm{Fe}$ have been confirmed at 3388.4 (${6}_{1}^{+}$), 3755.8 (${6}_{2}^{+}$), 4700.6 (${7}_{1}^{+}$), 5255.3 (${8}_{1}^{+}$), and 5626.8 (${8}_{2}^{+}$) keV and the ${J}^{\ensuremath{\pi}}$ values in parentheses have been deduced. The angular distributions in conjunction with the associated attenuation factors were analyzed to yield multipole mixing ratios $\ensuremath{\delta}(\frac{E2}{M1})$.The mean lifetimes $\ensuremath{\tau}$ of these states were also carefully measured by extrapolation to the detection threshold of the effective mean lifetimes extracted from Doppler-broadened line shapes obtained at four bombardment energies between 20.4 and 29.5 MeV. The values obtained in $^{56}\mathrm{Co}$ are 2282.6 KeV (g2000 fs), 2371.9 keV (60 \ifmmode\pm\else\textpm\fi{} 30 fs), 3638.0 keV (${80}_{\ensuremath{-}30}^{+40}$ fs), 4180.2 keV (590 \ifmmode\pm\else\textpm\fi{} 60 fs), and 5274.6 keV (${60}_{\ensuremath{-}20}^{+40}$ fs); and those in $^{56}\mathrm{Fe}$ are 3388.4 keV (g2000 fs), 3755.8 keV (${180}_{\ensuremath{-}30}^{+40}$ fs), 4700.6 keV (120 \ifmmode\pm\else\textpm\fi{} 40 fs), 5255.3 keV (500 \ifmmode\pm\else\textpm\fi{} 50 fs), and 5626.8 keV (${100}_{\ensuremath{-}20}^{+40}$ fs). For most of the transitions from the high-spin states in $^{56}\mathrm{Co}$ and $^{56}\mathrm{Fe}B(\ensuremath{\Lambda})$ values were obtained. The results for $^{56}\mathrm{Fe}$ are compared with recent calculations based on the aligned coupling scheme indicating possible coexistence of quasiband structures with prolate- and oblate-like configurations. The results for $^{56}\mathrm{Co}$ are compared with other recent shell-model calculations and good agreement is observed.NUCLEAR REACTIONS $^{54}\mathrm{Fe}(\ensuremath{\alpha},pn)^{56}\mathrm{Co}^{*}(\ensuremath{\gamma})$ and $^{54}\mathrm{Fe}(\ensuremath{\alpha},2p)^{56}\mathrm{Fe}^{*}(\ensuremath{\gamma})$, $E=21\ensuremath{-}30$ MeV, enriched targets. Measured, ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}(\ensuremath{\theta})$, $\ensuremath{\sigma}(E)$, ${\ensuremath{\Delta}\overline{E}}_{\ensuremath{\gamma}}(E,\ensuremath{\tau})$, $\ensuremath{\gamma}\ensuremath{\gamma}$ coin., deduced levels in $^{56}\mathrm{Co}$ and $^{56}\mathrm{Fe}$, $J$, $\ensuremath{\pi}$, branching ratios, ${\ensuremath{\tau}}_{\mathrm{level}}$, $\ensuremath{\delta}(\frac{E2}{M1})$, $B(E2)$ and $B(M1)$ values; Ge(Li) detectors, Ge(Li)-NaI(T1) anti-Compton spectrometer, 2.2. keV at 1332.5 keV.