Transition rates of high-spin states inCo56andFe56: Possible coexistence of prolate- and oblate-like configurations inFe56

Abstract
The decay properties of the high-spin states in Co56 and Fe56 have been investigated via the Fe54(α,pn)Co*56(γ) and Fe54(α,2p)Fe*56(γ) reactions in the energy range 20.4-29.5 MeV. From measurements of excitation functions and angular distributions of individual γ rays with a Ge(Li)-NaI(T1) anti-Compton spectrometer, and from γγ-coincidence experiments new levels in Co56 have been identified at 2282.6 (71+), 2371.9 (61+), 3638.0 (81+), 4180.2 (91+), and 5274.6 (101+) keV with the deduced Jπ values in parentheses. Similarly levels in Fe56 have been confirmed at 3388.4 (61+), 3755.8 (62+), 4700.6 (71+), 5255.3 (81+), and 5626.8 (82+) keV and the Jπ values in parentheses have been deduced. The angular distributions in conjunction with the associated attenuation factors were analyzed to yield multipole mixing ratios δ(E2M1).The mean lifetimes τ 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 Co56 are 2282.6 KeV (>2000 fs), 2371.9 keV (60 ± 30 fs), 3638.0 keV (8030+40 fs), 4180.2 keV (590 ± 60 fs), and 5274.6 keV (6020+40 fs); and those in Fe56 are 3388.4 keV (>2000 fs), 3755.8 keV (18030+40 fs), 4700.6 keV (120 ± 40 fs), 5255.3 keV (500 ± 50 fs), and 5626.8 keV (10020+40 fs). For most of the transitions from the high-spin states in Co56 and Fe56B(Λ) values were obtained. The results for Fe56 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 Co56 are compared with other recent shell-model calculations and good agreement is observed.