429 research outputs found
Systematics of 2+ states in semi-magic nuclei
We propose a simple systematics of low lying 2+ energy levels and
electromagnetic transitions in semi-magic isotopic chains Z=28,50,82 and
isotonic chains N=28,50,82,126. To this purpose we use a two-level pairing plus
quadrupole Hamiltonian, within the spherical Quasiparticle Random Phase
Approximation (QRPA). We derive a simple relation connecting the 2+ energy with
the pairing gap and quadrupole-quadupole (QQ) interaction strength. It turns
out that the systematics of energy levels and B(E2) values predicted by this
simple model is fulfilled with a reasonable accuracy by all available
experimental data. Both systematics suggest that not only active nucleons but
also those filling closed shells play an important role
Molecular Polarizability in Open Ensemble Simulations of Aqueous Nanoconfinements under Electric Field
Molecular polarization at aqueous interfaces involves fast degrees of freedom that are often averaged-out in atomistic-modeling approaches. The resulting effective interactions depend on a specific environment, making explicit account of molecular polarizability particularly important in solutions with pronounced anisotropic perturbations, including solid/liquid interfaces and external fields. Our work concerns polarizability effects in nanoscale confinements under electric field, open to an unperturbed bulk environment. We model aqueous molecules and ions in hydrophobic pores using the Gaussian-charge-on-spring BK3-AH representation. This involves nontrivial methodology devel- opments in expanded ensemble Monte Carlo simulations for open systems with long-ranged multibody interactions and necessitates further improvements for efficient modeling of polarizable ions. Structural differences between fixed-charge and polarizable models were captured in molecular dynamics simulations for a set of closed systems. Our open ensemble results with the BK3 model in neat-aqueous systems capture the ∼10% reduction of molecular dipoles within the surface layer near the hydrophobic pore walls in analogy to reported quantum mechanical calculations at water/vapor interfaces. The polarizability affects the interfacial dielectric behavior and weakens the electric-field dependence of water absorption at pragmatically relevant porosities. We observe moderate changes in thermodynamic properties and atom and charged-site spatial distributions; the Gaussian distribution of mobile charges on water and ions in the polarizable model shifts the density amplitudes and blurs the charge-layering effects associated with increased ion absorption. The use of polarizable force field indicates an enhanced response of interfacial ion distributions to applied electric field, a feature potentially important for in silico modeling of electric double layer capacitors
Band structure from random interactions
The anharmonic vibrator and rotor regions in nuclei are investigated in the
framework of the interacting boson model using an ensemble of random one- and
two-body interactions. We find a predominance of L(P)=0(+) ground states, as
well as strong evidence for the occurrence of both vibrational and rotational
band structures. This remarkable result suggests that such band structures
represent a far more general (robust) property of the collective model space
than is generally thought.Comment: 5 pages, 4 figures, Phys. Rev. Lett., in pres
Possible experimental signature of octupole correlations in the 0 states of the actinides
= 0 states have been investigated in the actinide nucleus
Pu up to an excitation energy of 3 MeV with a high-resolution (p,t)
experiment at = 24 MeV. To test the recently proposed = 0
double-octupole structure, the phenomenological approach of the
spdf-interacting boson model has been chosen. In addition, the total 0
strength distribution and the strength fragmentation have been compared
to the model predictions as well as to the previously studied (p,t) reactions
in the actinides. The results suggest that the structure of the 0 states
in the actinides might be more complex than the usually discussed pairing
isomers. Instead, the octupole degree of freedom might contribute
significantly. The signature of two close-lying 0 states below the
2-quasiparticle energy is presented as a possible manifestation of strong
octupole correlations in the structure of the 0 states in the actinides.Comment: 6 pages, 5 figures, published in Phys. Rev. C 88, 041303(R) (2013
Land degradation and mitigation policies in the mediterranean region. A brief commentary
Land degradation is more evident where conditions of environmental vulnerability already exist because of arid climate and unsustainable forms of land exploitation. Consequently, semi-arid and dry areas have been identified as vulnerable land, requiring attention from both science and policy perspectives. In some regions, such as the Mediterranean region, land degradation is particularly intense, although there are no extreme ecological conditions. In these contexts, a wide range of formal and informal responses is necessary to face particularly complex and spatially differentiated territorial processes. However, the fit of responses has been demonstrated to be different over time and space according to the underlying socioeconomic context and the specific ecological conditions. The present commentary discusses this sort of \u201centropy\u201d in the policy response to land degradation in Southern Europe, outlining the intrinsic complexity of human\u2013nature dynamics at the base of such processes. Reflecting the need of differentiated regional strategies and more specific national measures to combat desertification, three policy frameworks (agro-environmental, economic, social) with an indirect impact on fighting land degradation have been considered, delineating the importance of policy assemblages. Finally, the importance of policy impact assessment methodologies was highlighted, focusing on the possible responses reinforcing a continental strategy against land degradation. By evidencing the role of participatory planning, developmental policies indirectly addressing land degradation reveal to be an important vector of more specific measures abating desertification risk, creating, in turn, a favorable context for direct interventions of mitigation or adaptation to climate change
Phase Transitions in Finite Nuclei and the Integer Nucleon Number Problem
The study of spherical-deformed ground--state phase transitions in finite
nuclei as a function of N and Z is hindered by the discrete values of the
nucleon number. A resolution of the integer nucleon number problem, and
evidence relating to phase transitions in finite nuclei, are discussed from the
experimental point of view and interpreted within the framework of the
interacting boson model.Comment: 8 pages Latex + 8 figs (postscript). In Phys Rev Lett, June 199
Octupole correlations in positive-parity states of rare-earth and actinide nuclei
In this contribution, further evidence of the importance of multiphonon-octupole excitations to describe experimental data in the rare earths and actinides will be presented. First, new results of a (p, t) experiment at the Q3D magnetic spectrograph in Munich will be discussed, which was performed to selectively excite J(pi) = 0(+) states in Pu-240. spd f interacting boson model (IBM) calculations suggest that the previously proposed double-octupole phonon nature of the J(pi) = 0(2)(+) state is not in conflict with its strong (p, t) population. Second, the framework of the IBM has been adopted for the description of experimental observables related to octupole excitations in the rare earths. Here, the IBM is able to describe the signature splitting for positive- and negative-parity states when multi-dipole and multi-octupole bosons are included. The present study might support the idea of octupole-phonon condensation at intermediate spin (J(pi) = 10(+)) leading to the change in yrast structure observed in Nd-146
Octupole correlations in positive-parity states of rare-earth and actinide nuclei
In this contribution, further evidence of the importance of multiphonon-octupole excitations to describe experimental data in the rare earths and actinides will be presented. First, new results of a (p, t) experiment at the Q3D magnetic spectrograph in Munich will be discussed, which was performed to selectively excite J(pi) = 0(+) states in Pu-240. spd f interacting boson model (IBM) calculations suggest that the previously proposed double-octupole phonon nature of the J(pi) = 0(2)(+) state is not in conflict with its strong (p, t) population. Second, the framework of the IBM has been adopted for the description of experimental observables related to octupole excitations in the rare earths. Here, the IBM is able to describe the signature splitting for positive- and negative-parity states when multi-dipole and multi-octupole bosons are included. The present study might support the idea of octupole-phonon condensation at intermediate spin (J(pi) = 10(+)) leading to the change in yrast structure observed in Nd-146
F-spin as a Partial Symmetry
We use the empirical evidence that F-spin multiplets exist in nuclei for only
selected states as an indication that F-spin can be regarded as a partial
symmetry. We show that there is a class of non-F-scalar IBM-2 Hamiltonians with
partial F-spin symmetry, which reproduce the known systematics of collective
bands in nuclei. These Hamiltonians predict that the scissors states have good
F-spin and form F-spin multiplets, which is supported by the existing data.Comment: 14 pages, 1 figur
Mean-field analysis of interacting boson models with random interactions
We investigate the origin of the regular features observed in numerical
studies of the interacting boson model with random interactions, in particular
the dominance of L=0 ground states and the occurrence of vibrational and
rotational band structures. It is shown that all of these properties can be
interpreted and explained in terms of a Hartree-Bose mean-field analysis, in
which different regions of the parameter space are associated with geometric
shapes. The same conclusions hold for the vibron model.Comment: 8 pages, 4 figures, 2 tables. Physical Review C, in pres
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