Systematics of low-lying energy level ratios in near closed shell nuclei

Structure at low excitations in near closed shells nuclei is dominated by the excitation of a few valence particles into spherical shell model orbits. Diversely, in beyond shell closure nuclei, quadrupole collective excitations viz. vibrations near the shell closure and rotations in mid shell nuclei, dominate the nuclear structure. However, this picture of low-lying excitations in atomic nuclei is complicated by the presence of coexisting shapes as well as by the intruder configurations. Yrast band energy ratio R_4/2 ≡ E(4₁⁺)/E(2₁⁺) is a most basic signature of the nuclear shapes and has been reported widely as a base of nuclear models. As the valence nucleon number increases this ratio has also been found to increase from R_4/2 < 2 (semi/near magic) to R_4/2 ∼ 3.33 (deformed rotor) passing through the R_4/2 ∼ 2 (spherical vibrator) and transitional nuclei with R_4/2 ∼ 3