Coulomb excitation of 67Zn and core-quasiparticle coupling in [formula omitted] nuclei

Abstract

A qualitative explanation of the low-lying levels of g 9 2 shell nuclei using a recent theoretical improvement upon quasiparticle-phonon coupling theories has motivated the investigation of 67Zn, which belongs in the f 5 2 neutron shell. A Coulomb excitation experiment was performed with 3.2 to 4.86 MeV α-particles. De-excitation γ-rays were observed with a 45 cm 3 Ge(Li) detector. In addition to excitation of previously known states at 184.6 keV ( 3 2 − ) and 880.0 keV ( 5 2 − ) , strong excitation of states at 815.2 keV ( 5 2 − , 7 2 − ) and 870.9 keV ( 3 2 − , 5 2 − , 7 2 − ) was observed. The latter state also appears to have been observed in high resolution (d, p) stripping work. Angular distribution data were taken at E α = 4.0 and 4.86 MeV, and were used to obtain branching ratios, reduced upward transition probabilities, mixing ratios and spin assignments. The B(E2)↑ associated with the 67Zn levels populated in this experiment are (in units of e 2 · b 2) 184.6 keV (0.0190 ± 0.0014), 393.6 keV (0.00049 ± 0.00003), 815.2 keV (0.029 ± 0.002), 870.9 keV (0.04 ± 0.02), and 888.0 keV (0.0086 ± 0.0006). An extensive diagonalization using three single-quasiparticle states ( f 5 2 , p 3 2 , and P 1 2 ) in 67Zn, each coupled to a core phonon, was carried out and detailed predictions were made for level energies, reduced transition probabilities, magnetic dipole and electric quadrupole moments, relative γ-ray intensities, and mixing ratios. Generally, the predictions are in good agreement with 67Zn experimental data. Data for other f 5 2 shell nuclei lend further qualitative support to a quasiparticle-phonon coupling picture for odd- A nuclei in the f 5 2 shell. It is suggested that an extra 3 2 − low-lying level present in several f 5 2 shell nuclei has characteristics similar to those of the three-quasiparticle “intruder” state described by Kisslinger.