Evidence for Rigid Triaxial Deformation in Ge76 from a Model-Independent Analysis

Abstract

An extensive, model-independent analysis of the nature of triaxial deformation in 76 Ge, a candidate for neutrinoless double-beta (0νββ) decay, was carried out following multi-step Coulomb excitation. Shape parameters deduced on the basis of a rotational-invariant sum-rule analysis provided considerable insight into the underlying collectivity of the ground-state and γ bands. Both sequences were determined to be characterized by the same β and γ deformation parameter values. In addition, compelling evidence for low-spin, rigid triaxial deformation in 76 Ge was obtained for the first time from the analysis of the statistical fluctuations of the quadrupole asymmetry deduced from the measured E2 matrix elements. These newly determined shape parameters are important input and constraints for calculations aimed at providing, with suitable accuracy, the nuclear matrix elements relevant to 0νββ.

An experimental view on shape coexistence in nuclei

“…The Q 2 values of the ground states in 70−76 Ge have similar values of about 0.2 e 2 b 2 , while those of the 0 + 2 states evolve considerably, from being consistent with zero in 74,76 Ge [199,200], almost identical with that of the ground state in 72 Ge [196,197]), to a value of 0.64(26) e 2 b 2 in 70 Ge [198]). The recent detailed Coulomb-excitation studies of 72 Ge [196] and 76 Ge [201] have also confirmed a rotational character of the ground-state bands in these nuclei, with the Q 2 values being remarkably constant up to spin 8. The only significant deviation from this pattern is a slight reduction observed for the 0 + 1 state in 72 Ge, which may be attributed to the strong mixing with a less deformed configuration.…”

Shape Coexistence at Zero Spin in Ni64 Driven by the Monopole Tensor Interaction

“…Transitions were detected with the GRETINA tracking array [42] in coincidence with the two reaction partners measured by the CHICO2 compact heavy ion counter [43], as described in Ref. [44]. The resulting yields were analyzed with the semiclassical Coulomb excitation code, GOSIA [45], which provided electromagnetic matrix elements for transitions from 13 states [46].…”

Low-Energy Coulomb Excitation for the Shell Model

“…On the other hand, multiple lowenergy Coulomb-excitation studies of stable Ge and Zn isotopes [14,25,28] demonstrated the importance of the triaxial degree of freedom in their structure, which was also evoked for the neighbouring 76,78 Se nuclei [29,30]. Particularly relevant is the study of 76 Ge [31], which yielded (β 2 , γ) parameters for the 0 + 1 , 2 + 1 and 2 + 2 states and their dispersions, which are consistent with rigid triaxial deformation. This is particularly important considering that 76 Ge is a candidate for searches of neutrinoless double-β decay, and the nuclear shape is predicted to play a significant role in this process [32,33].…”