On the nature of 0$^+$ states in $^{64}$Ni from Coulomb excitation

Recent experimental work on the doubly-magic nucleus $^{68}$Ni has shown that shape coexistence occurs despite its rigidly spherical ground state [1]. Several low-lying $0^+$ states have been discovered and attributed to different minima in the nuclear potential associated with oblate and prolate deformations. Consequently, it is important to question if such $0^+$ excitations can also occur in the most neutron-rich, stable Ni isotope, $^{64}$Ni, and whether or not they can be tied to shape coexistence. Two low-lying $0^+$ states have previously been observed in $^{64}$Ni, but additional information beyond their excitation energy and spin is needed in order to investigate their properties. A high-statistics Coulomb excitation experiment was performed at the ATLAS facility at ANL, where a $^{208}$Pb target was bombarded by a $^{64}$Ni beam at an energy of 272 MeV. The experimental setup involved the new GRETINA tracking array in conjunction with the Compact Heavy Ion COunter, CHICO2. Thirteen transitions were observed in $^{64}$Ni, including the 1521- and 1680-keV $\gamma$-rays associated with the de-excitation of the $0_2^+$ and $0_3^+$ states, respectively. B(E2) reduced transition probabilities were obtained for all observed states. [1] S. Leoni et al., PRL 118, 162502 (2017).