Análisis de las funciones administrativas de EIC Guanacaste

The Ni isotopic chain has long been the subject of extensive nuclear structure studies. With a magic number of protons, the chain includes doubly-magic 48Ni, 58Ni and 78Ni. As discussed in this work, 68_{Ni can be viewed as doubly-magic as well. However, following the}

unexpected discovery of shape coexistence in the latter isotope [95], considerable experi- mental efforts have been devoted to explore neighboring Ni isotopes in search of the same phenomenon. With the observation of a prolate-deformed 0+ _{state in} 70_{Ni at 1567 keV [22],}

and with 66Ni manifesting both oblate- and prolate-deformed 0+ levels at 2445 and 2965 keV, respectively, alongside a spherical one at 2664 keV [23], it became clear that shape co- existence is rather ubiquitous in this region. Furthermore, studies of high-spin states in 62Ni and 63_{Ni [28, 29] provided further evidence for collectivity. As a result of these experimental}

observations, a number of calculations have been carried out with various approaches in an attempt to identify which aspects of the nuclear force are responsible for driving the nu- clear shape. Meanwhile, additional experimental work has been initiated to investigate more thoroughly the nature of the excitations involved in the isotopes where shape coexistence is already known to exist, and to explore the Ni chain further. The present work addresses the following question: does the most neutron-rich, stable isotope, 64Ni, also exhibit shape coexistence, and, if so, at which excitation energy?

A two-pronged experimental approach was used in the present work to address these ques- tions. First, Coulomb excitation of64_{Ni was carried out at the ATLAS accelerator at Argonne}

National Laboratory, utilizing the new GRETINAγ-ray tracking array in combination with the CHICO2 particle detector in order to probe the nature of low-lying states in this nucleus

and assess the presence of collectivity. The data were analyzed with the GOSIA Coulomb excitation analysis software, in order to obtain reduced transition strengths, and quadrupole moments and compare these with those predicted by shell-model and Monte Carlo shell- model (MCSM) calculations. Secondly, data from deep-inelastic reactions collected with the Gammasphere detector array and the Fragment Mass Analyzer were examined in order to search for new, higher-energy, higher-spin levels in64Ni.

No evidence for rotational bands was found in the present data. However, the transition strengths for the de-excitation of the known excited 0+ states are found to be reproduced well by theory and, in fact, support the view that 64_{Ni closely resembles its neighbor} 66_Ni,

herewith favoring an interpretation in terms of shape coexistence. Thus, transition strengths and quadrupole moments indicate that the ground and 0+₃ states are spherical, and the 0+₂ level is characterized by a small oblate deformation. Moreover, convergence between the B(E2) strengths in experiment and calculations indicates that most excitations observed in Coulomb excitation are confined to the fp shell. Furthermore, the data on excitation energies, spins, and parity indicate that single-particle excitations dominate up to the highest energy. It is worth noting that the strength of the first-excited 2+₁ state measured in this work agrees with that observed in previous studies, herewith validating these results. Furthermore, thirteen new transitions, along with twelve new states, were discovered so that the level sequences in this nucleus now extend up to an energy of 17930 keV, with a tentative (16 or 17) spin assignment.

Lastly, while not observed in this work, a 3463-keV state was identified by Ref. [25] and unambiguously confirmed to be the 0+₄ level by the Krakov-Milano-Bucharest-UNC collaboration [101]. This 0+₄ state fits well within the pattern observed in the adjacent

66_{Ni. Indeed, if these nuclei closely mirror one another, as suggested by the data, it is}

reasonable to associate this newly identified 3463-keV, 0+₄ state with prolate deformation, a conclusion strengthened by an upper limit of 0.08 W.u. for the strength of the 0+₄ → 2+₂ transition deduced from the present data. This value can be compared with the strength

of the analogous transition of 0.2 W.u. observed in 66_{Ni. Figure 6.1 illustrates the nuclear}

deformation of each of the 0+ _{states in} 64_{Ni in terms of their spectroscopic and intrinsic}

quadrupole moments, as predicted by the MCSM calculations carried out by the Tokyo group of Otsuka et al. [100]. Here, the color scheme indicates the depth of the potential well and dots specify the location in deformation space of the basis states sampled by the calculations. It is clear from these computations that both the ground and 0+₃ levels are expected to be spherical, whereas it is anticipated that the 0+₂ and 0+₄ states are associated with oblate and prolate minima, respectively. Moreover, a comparison of low-spin excitations in terms of excitation energies is depicted in Figure 6.2. Here, the pattern exhibited by the even-even Ni isotopes as a function of mass is apparent, especially with regard to the excited 0+ _{levels. Thus, while more study is necessary in order to further assess the nature of such}

states in 64_{Ni, the agreement between experiment and theory, alongside the trends seen in}

the Ni chain, make a strong case for shape coexistence in this nucleus.