Abstract
It is notable that along the N = Z line in the nuclear chart, extremely large collectivity emerges suddenly in the mass-80 region. By applying the Monte Carlo shell model (MCSM) and the Hartree-Fock-Bogolyubov plus generator coordinate method (HFB+gcm), we study this problem to find the origin. On the basis that both calculations reproduce the experimental data of the N ≈ Z nuclei with A = 64 ∼ 88 , we identify the backbone from full shell-model calculations that can explain the strong prolate deformation. We find that inclusion of the 2 d 5 / 2 orbit in the model space to cooperate with 1 g 9 / 2 is the key ingredient to describe the rapid increase of collectivity from 70 Se to 76 Sr and to produce the observed large B ( E 2 ) values in 76 Sr, 78 Sr and 80 Zr. The quadrupole-quadrupole ( QQ ) interaction acting between the quasi-SU(3) partner orbits, 1 g 9 / 2 − 2 d 5 / 2 , is the driving force that changes the nuclear shape from oblate- to prolate-deformed. We further suggest that the quasi-SU(3) effect is particularly amplified in the N ≈ Z nuclei because these are the unique examples where quasi-SU(3) partner orbits can be formed, like the nuclear pairing, simultaneously in three different types: neutron-neutron (n-n), proton-proton (p-p), and neutron-proton (n-p), which respectively interact through the n-n, p-p, and n-p components of the QQ force to enhance the quadrupole collectivity coherently.
Published Version
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