Static electric quadrupole moments of the first excited states of 56Fe and the even Titanium nuclei

Abstract

The reorientation effect in Coulomb excitation was used to measure the static electric quadrupole moments of the first excited (2 +) states in 48Ti and 56Fe. These measurements yielded the following values: Q 2 + ( 48Ti) = −13.5 ± 8.8 e · fm 2 and Q 2 + ( 56Fe) = −24.9 ± 5.8 e · fm 2 . Comparison with other measurements indicates that nuclear interaction effects produce spurious quadrupole moments if the distance of closest approach for 180° scattering is much less than 1.25(A P 1 3 + A T 1 3 ) + 6.0 fm . In the present measurements, the 2 + states were excited by means of 32S projectiles and the de-excitation γ-rays were observed in coincidence with scattered particles, using a multiple detector system. The angular distribution of de-excitation γ-rays was found to be significantly attenuated (deorientation effect) and it was crucial to include this effect in analysing the reorientation data. Measurements of the attenuation were consistent with a magnetic-dipole hyperfine interaction between the excited nuclear state and a randomly fluctuating atomic environment for ions recoiling into vacuum. The effective magnetic hyperfine field was observed to have a velocity dependence consistent with the form H ∞ β x , where β = recoil velocity/ c and x = 0.66±0.13. The static quadrupole moments and other E2 properties of 56Fe are characteristic of a relatively stiff prolate rotor, while 46, 48Ti exhibit properties of nuclei soft to collective quadrupole motion. Pure (f 7 2 ) n shell-model calculations for the even titanium isotopes cannot explain the E2 properties of the low-lying levels, and predict static quadrupole moments of the wrong sign. The inclusion of configurations with one nucleon excited into the upper (1f−2p) shell yields a dramatic improvement, compared with the pure (f 7 2 ) n shell-model results, in explaining the experimental E2 properties of 48Ti. A similar, though not so striking, improvement holds for 46Ti. The E2 properties of the ground state band in 56Fe are adequately described by a 2p−2h shell-model configuration, assuming an inert 56Ni core. However, the calculation fails to reproduce the magnetic moment of the first excited state, as well as the electromagnetic properties of other low-lying levels, indicating the presence of appreciable admixtures of core-excited configurations in 56Fe.