Shape coexistence and the role of axial asymmetry in 72Ge

Abstract

The quadrupole collectivity of low-lying states and the anomalous behavior of the 0 + 2 and 2 + 3 levels in 72 Ge are investigated via projectile multi-step Coulomb excitation with GRETINA and CHICO-2. A total of forty six E2 and M 1 matrix elements connecting fourteen low-lying levels were determined using the least-squares search code, gosia. Evidence for triaxiality and shape coexistence, based on the model-independent shape invariants deduced from the Kumar-Cline sum rule, is presented. These are interpreted using a simple two-state mixing model as well as multistate mixing calculations carried out within the framework of the triaxial rotor model. The results represent a significant milestone towards the understanding of the unusual structure of this nucleus.The structure of low-lying states in even-even Ge isotopes has been the subject of intense scrutiny for many years due to the inherent challenge of interpreting their systematics as a function of mass A. These nuclei possess at least one excited 0 + state in their low-energy spectrum that differs from the ground state in its properties. Systematically, the energy of the 0 + 2 level varies parabolically with A and reaches a minimum in 72 Ge, where it becomes the first excited state. The existence of even-mass nuclei with a J π = 0 + first excited state is an uncommon phenomenon which, to date, has been observed in only a few nuclei located near or at closed shells: 16 O [1], 40 Ca [2, 3], 68 Ni [4, 5], 90 Zr [6], 180,182 Hg [7-9], 184,186,188,190,192,. There are also examples of such nuclei where a subshell appears to play a role similar to a closed shell such as 96,98 Zr [16,17]. These cases have all been explained as resulting from shape coexistence due to the presence of intruder configurations; i.e., configurations involving the excitation of at least one pair of nucleons across a shell or subshell energy gap [18].The structure of 72 Ge is highly unusual in that this nucleus is far from closed shells and, yet, possesses a 0 + first excited state. It shares this distinction with only two other known nuclei: 72 Kr [19] and 98 Mo [20]. It should be noted that while 68 Ni might be doubly magic [4,21] with a presumed subshell closure at N = 40 [22], there is no evidence to date of spherical ground-state configurations in any of the other N = 40 isotones [23]. There are, however, strong experimental indications of enhanced collective behavior in or near the ground states of the N = 40 neutron-rich Fe and Cr isotones [24]. In the Ge isotopes, the absence of a subshell closure at N = 40 can be inferred from the fact that the 2 + 1 level with the highest energy appears in 70 Ge rather than 72 Ge. Thus, understanding the nature and origin of this anomalous 0 + 2 state in 72 Ge has been a major challenge for collective model descriptions.The theory of collectivity in nuclei is predominantly focused on models with quadrupole degrees of freedom. The simplest of these models, based on the quantization of a liquid drop [25,26], describes quadrupole collectivity as ei...

Novel ΔJ=1 Sequence in Ge78 : Possible Evidence for Triaxiali

“…Other, more recent calculations [21] support the view of triaxiality in 76 Ge. Decades after the Larsson work, experimental confirmation of predictions by their approach were reported in the numerous examples of triaxiality near the ground states of the even-even 108−114 44 Ru nuclei [22,23] as well as in the presence of two coexisting triaxially deformed shapes in 72 32 Ge [24]. In this Letter, a markedly different sequence of levels with spins 2 + -6 + built upon the 2 + 2 state is reported in 78 Ge.…”

Nuclear structure of Ge76 from inelastic neutron scattering measurements and shell model calculations

“…The nuclei in the Ge region exhibit many interesting structural features. The low-lying 0 + states in the stable Ge nuclei have been interpreted as evidence for shape coexistence [2], which was established in 72 Ge with multistep Coulomb excitation [3] and was recently extended to 80 Ge [4]. Moreover, Toh et al [5] proposed that 76 Ge may be a rare example of a nucleus exhibiting rigid triaxial deformation in its low-lying states, i.e., that it follows the rigid triaxial rotor model of Davydov and Filipov [6] with a well-defined potential minimum at a non-zero value of γ.…”

Shell-model-based deformation analysis of light cadmium isotopes

“…These methods have been used in a large number of recent papers (in particular in the region of the Ge, Kr, Mo, Ru and Pd nuclei [90][91][92][93][94][95], as well as in the much heavier Pb mass region (W, Os, Pt, Hg, Po isotopes) [96][97][98][99][100][101].…”