Collectivity in light neutron-rich nuclei near N = 20: Intermediate-energy Coulomb excitation of magnesium-32,34, aluminum-35,36 and silicon-37.

摘要

Collectivity in the neutron-rich nuclei 32,34Mg, 35,36Al and 37Si has been studied via intermediate-energy Coulomb excitation in an early experiment at the National Superconducting Cyclotron Laboratory's Coupled Cyclotron Facility. Reduced transition probabilities and quadrupole deformation parameters were extracted from the measurements.; 32,34Mg are members of a group of nuclei near N = 20 which exhibit collective characteristics unexpected for nuclei at or near a shell closure. The observations are successfully reproduced by shell model calculations that allow a 2ħω intruder configuration as the ground state, in which 2 neutrons are promoted from the ν1 d_3/2 level to the ν1f_7/2 level. Models based on valence spaces limited to the 0ħω configuration which describe the stable nuclei well are unsuccessful in calculating observables for these nuclei. Because of this inversion of the 0ħω and 2ħω configurations in energy, the group has been named the “Island of Inversion,” and is predicted by the 2ħω shell model to extend to 10 ≤ Z ≤ 12 and 20 ≤ N ≤ 22. The possibility for static shape deformation has also been considered, and models utilizing a deformed potential are successful in calculating excited state energies and reduced quadrupole transition probabilities for this group. Recently, the neutron boundary has been predicted to extend to N = 24 by the sd − pf Monte Carlo Shell Model, and several mean-field calculations predict shape coexistence for 32Mg.; The energy of the first 2+ state in 32Mg was measured to be 885(18) keV, and found to be in agreement with the adopted value of 885.5(7) keV. A second gamma-ray at 1436 keV was also observed with low statistics. The reduced electric quadrupole transition probability, B(E2;$0+g.s.\to 2+1$), of 447(57) e2fm4 and a |β ₂| value of 0.51(3) were extracted from the measurements without consideration of feeding from the 2321 keV state. A B(E2;$0+g.s.\to 2+1$) value of 328(48) e2fm4 and a resulting quadrupole deformation parameter |β₂| = 0.42(3) were obtained after consideration of a minimum correction for feeding of the 885 keV state by the 2321 keV state via the 1436 keV gamma-ray. Both values are in agreement with the adopted value, and in contrast with the measurement by Chisté et al. which yielded 622(90) e2fm4 for the excitation strength. Our measurement agrees with the calculations of the 2ħω shell model.; A first excited-state energy of 659(14) keV for 34Mg was also measured via Coulomb excitation. (Abstract shortened by UMI.)