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Register a new userStructure of the lightest tin isotopes
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We link the structure of nuclei around $^{100}$Sn, the heaviest doubly magic nucleus with equal neutron and proton numbers ($N=Z=50$), to nucleon-nucleon ($NN$) and three-nucleon ($NNN$) forces constrained by data of few-nucleon systems. Our results indicate that $^{100}$Sn is doubly magic, and we predict its quadrupole collectivity. We present precise computations of $^{101}$Sn based on three-particle--two-hole excitations of $^{100}$Sn, and reproduce the small splitting between the lowest $J^pi=7/2^+$ and $5/2^+$ states. Our results are consistent with the sparse available data.
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The role of the entrance channel in fusion-fission reactions was studied by the theoretical analysis of the experimental evaporation residue excitation functions for reactions leading to the same compound nucleus. The evaporation residues cross sections for xn-channels were calculated in the frame of the combined dinuclear system concept and advanced statistical model. The revealed differences between experimental data on the evaporation residues in the ^{40}Ar+^{176}Hf, ^{86}Kr + ^{130}Xe and ^{124}Sn + ^{92}Zr reactions leading to the ^{216}Th^* compound nucleus are explained by the different spin distributions of compound nuclei which are formed. It is shown that the intrinsic fusion barrier B^*_{fus} and size of potential well are different for every entrance channel.
The shapes of neutron-rich exotic Ni isotopes are studied. Large-scale shell model calculations are performed by advanced Monte Carlo Shell Model (MCSM) for the $pf$-$g_{9/2}$-$d_{5/2}$ model space. Experimental energy levels are reproduced well by a single fixed Hamiltonian. Intrinsic shapes are analyzed for MCSM eigenstates. Intriguing interplays among spherical, oblate, prolate and gamma-unstable shapes are seen including shape fluctuations, $E$(5)-like situation, the magicity of doubly-magic $^{56,68,78}$Ni, and the coexistence of spherical and strongly deformed shapes. Regarding the last point, strong deformation and change of shell structure can take place simultaneously, being driven by the combination of the tensor force and changes of major configurations within the same nucleus.
The occupancies and vacancies of the valence neutron orbitals across the stable tin isotopic chain from $112leq Aleq 124$ have been determined. These were inferred from the cross sections of neutron-adding and -removing reactions. In each case, the reactions were chosen to have good angular-momentum matching for transfer to the low- and high-$ell$ orbitals present in this valence space. These new data are compared to older systematic studies. The effective single-neutron energies are determined by combining information from energy centroids determined from the adding and removing reactions. Two of the five orbitals are nearly degenerate, below $N=64$, and approximately two MeV more bound than the other three, which are also degenerate.
A realistic shell-model study is performed for neutron-deficient tin isotopes up to mass A=108. All shell-model ingredients, namely two-body matrix elements, single-particle energies, and effective charges for electric quadrupole transition operators, have been calculated by way of the many-body perturbation theory, starting from a low-momentum interaction derived from the high-precision CD-Bonn free nucleon-nucleon potential. The focus has been put on the enhanced quadrupole collectivity of these nuclei, which is testified by the observed large B(E2;0+ -> 2+)s. Our results evidence the crucial role played by the Z=50 cross-shell excitations that need to be taken into account explicitly to obtain a satisfactory theoretical description of light tin isotopes. We find also that a relevant contribution comes from the calculated neutron effective charges, whose magnitudes exceed the standard empirical values. An original double-step procedure has been introduced to reduce effectively the model space in order to overcome the computational problem.
The mass region with A~100 and Z~40 is known to experience a sudden onset of deformation. The presence of the subshell closure $Z=40$ makes feasible to create particle-hole excitations at a moderate excitation energy and, therefore, likely intruder states could be present in the low-lying spectrum. In other words, shape coexistence is expected to be a key ingredient to understand this mass region. The aim of this work is to describe excitation energies, transition rates, radii, and two-neutron separation energies for the even-even 94-110Zr nuclei and, moreover, to obtain information about wave functions and deformation. The interacting boson model with configuration mixing will be the framework to study the even-even Zr nuclei, considering only two types of configurations: 0particle-0hole and 2p-2h excitations. On one hand, the parameters appearing in the Hamiltonian and in the E2 transition operator are fixed trough a least-squares fit to the whole available experimental information. On the other hand, once the parameters have been fixed, the calculations allow to obtain a complete set of observables for the whole even-even Zr chain of isotopes. Spectra, transition rates, radii, $rho^2(E0)$, and two-neutron separation energies have been calculated and a good agreement with the experimental information has been obtained. Moreover, a detailed study of the wave function has been conducted and mean-field energy surfaces and deformation have been computed too. The importance of shape coexistence has been shown to correctly describe the A~100 mass area for even-even Zr nuclei. This work confirmed the rather spherical nature of the ground state of 94-98Zr and its deformed nature for 100-110Zr isotopes. The sudden onset of deformation in 100Zr is owing to the rapid lowering of a deformed (intruder) configuration which is high-lying in lighter isotopes.
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