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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...
Measurements of the excitation function for the fusion of (24)Mg+(30)Si (Q=17.89 MeV)have been extended toward lower energies with respect to previous experimental data. The S-factor maximum observed in this large, positive-Q-value system is the most pronounced among such systems studied thus far. The significance and the systematics of an S-factor maximum in systems with positive fusion Q values are discussed. This result would strongly impact the extrapolated cross sections and reaction rates in the carbon and oxygen burnings and, thus, the study of the history of stellar evolution.
A B S T R A C TA large number of α p ( , ) and α n ( , ) reactions are known to play a fundamental role in nuclear astrophysics. This work presents a novel technique to study these reactions with the active target system MUSIC whose segmented anode allows the investigation of a large energy range of the excitation function with a single beam energy. In order to verify the method, we performed direct measurements of the previously measured reactions 17 O α n ( , ) 20 Ne, 23 Na α p ( , ) 26 Mg, and 23 Na α n ( , ) 26 Al. These reactions were investigated in inverse kinematics using 4 He gas in the detector to study the excitation functions in the energy range of about 2-6 MeV in the center of mass. We found good agreement between the cross sections of the 17 O α n ( , ) 20 Ne reaction measured in this work and previous measurements. Furthermore we have successfully performed a simultaneous measurement of the 23 Na α p ( , ) 26 Mg and 23 Na α n ( , ) 26 Al reactions.
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