Kinetic-theory description of isoscalar dipole modes

A semiclassical model, based on a solution of the Vlasov equation for finite systems with moving-surface, is employed to study the isoscalar dipole modes in nuclei. It is shown that, by taking into account the surface degree of freedom, it is possible to obtain an exact treatment of the centre of mass motion. It is also shown that a method often used to subtract the spurious strength in RPA calculations does not always give the correct result. An alternative analytical formula for the intrinsic strength function is derived in a simple confined-Fermi-gas model. In this model the intrinsic isoscalar dipole strength displays essentially a two-resonance structure, hence there are two relevant modes. The interaction between nucleons couples these two modes and changes the compressibility of the system. The evolution of the strength profile is then studied as a function of the compressibility of the nuclear fluid. Comparison with available data favours values of the incompressibility parameter of nuclear matter smaller than those suggested by the analysis of the monopole ``breathing'' mode.Comment: 17 pages, 4 figures, revised version to be published in Nucl. Phys.

Unified semiclassical approach to isoscalar collective modes in heavy nuclei

A semiclassical model based on the solution of the Vlasov equation for finite systems with a sharp moving surface has been used to study the isoscalar quadrupole and octupole collective modes in heavy spherical nuclei. Within this model, a unified description of both low-energy surface modes and higher-energy giant resonances has been achieved by introducing a coupling between surface vibrations and the motion of single nucleons. Analytical expressions for the collective response functions of different multipolarity can be derived by using a separable approximation for the residual interaction between nucleons. The response functions obtained in this way give a good qualitative description of the quadrupole and octupole response in heavy nuclei. Although shell effects are not explicitly included in the theory, our semiclassical response functions are very similar to the quantum ones. This happens because of the well known close relation between classical trajectories and shell structure. The role played by particular nucleon trajectories and their connection with various features of the nuclear response is displayed most clearly in the present approach, we discuss in some detail the damping of low-energy octupole vibrations and give an explicit expression showing that only nucleons moving on triangular orbits can contribute to this damping.Comment: 9 pages, 2 figures, Talk presented at the 8th International Spring Seminar on Nuclear Physics on Key Topics in Nuclear Structure, Paestum, Italy, May 23-27, 200

Dispersión Legítima De Semillas Por Aves En El Bosque Y Matorral Serrano De La Provincia De Córdoba

Birds play a fundamental role in many ecosystems because through endozoochory they can disperse lots of fruits and herefore its seeds. In the southern hemisphere, temperate forests and hills scrub of Córdoba-Argentina are characterized by great floristic diversity and an important richness of birds. However, only few studies focusing on the role of birds as dispersers of fruits and seeds have been conducted in this ecosystem. The aims of this study was to investigate if birds are dispersing fruits or seeds in the forest and hills scrub; analyse if fruits or seeds dispersion is scattered along seasons; and to assess whether seeds suffer physical damage when passing through the digestive tract of birds. During July, August, September and December 2012, fruits and seeds from birds pellets were collected while walking through two linear transects (length and width) of 1,450 x 20 and 1,200 x 5 m, respectively. Results showed that birds dispersed seeds during all seasons although the largest proportion was observed during autumn and winter when more than 70% of birds pellets contained seeds. The highest proportion of damaged seeds occurred in September (11%), but overall proportion of seeds that had physical damage was very low. Finally, our results revealed that birds have an important role in seeds dispersion on the forest and hills scrub ecosystem, and most of these dispersed seeds were exotic

Octupole response and stability of spherical shape in heavy nuclei

The isoscalar octupole response of a heavy spherical nucleus is analyzed in a semiclassical model based on the linearized Vlasov equation. The octupole strength function is evaluated with different degrees of approximation. The zero-order fixed-surface response displays a remarkable concentration of strength in the $1\hbar\omega$ and $3\hbar\omega$ regions, in excellent agreement with the quantum single-particle response. The collective fixed-surface response reproduces both the high- and low-energy octupole rsonances, but not the low-lying $3^{-}$ collective states, while the moving-surface response function gives a good qualitative description of all the main features of the octupole response in heavy nuclei. The role of triangular nucleon orbits, that have been related to a possible instability of the spherical shape with respect to octupole-type deformations, is discussed within this model. It is found that, rather than creating instability, the triangular trajectories are the only classical orbits contributing to the damping of low-energy octupole excitations.Comment: 10 pages, Latex file, 7 ps figure

Self-consistency and collective effects in semiclassical pairing theory

A simple model, in which nuclei are represented as homogeneous spheres of symmetric nuclear matter, is used to study the effects of a self-consistent pairing interaction on the nuclear response. Effects due to the finite size of nuclei are suitably taken into account. The semiclassical equations of motion derived in a previous paper for the time-dependent Hartree-Fock-Bogoliubov problem are solved in an improved (linear) approximation in which the pairing field is allowed to oscillate and to become complex. The new solutions are in good agreement with the old ones and also with the result of well-known quantum approaches. The role of the Pauli principle in eliminating one possible set of solutions is also discussed. The pairing-field fluctuations have two main effects: they restore the particle-number symmetry which is broken in the constant-$\Delta$ approximation and introduce the possibility of collective eigenfrequencies of the system due to the pairing interaction. A numerical study with values of parameters appropriate for nuclei, shows an enhancement of the density-density strength function in the region of the low-energy giant octupole resonance, while no similar effect is present in the region of the high-energy octupole resonance and for the giant monopole and quadrupole resonances.Comment: 31 pages, 6 eps figure

Rol Ecológico Del Zorro Pampeano (Lycalopex Gymnocercus) Como Dispersor De Semillas En Ambientes De Agroecosistema Y Bosques Serranos De Argentina

El zorro pampeano (Lycalopex gymnocercus) es un carnívoro de amplia distribución que posee una dieta omnívora donde los frutos/semillas conforman una parte muy importante de la misma. El objetivo de este trabajo fue conocer y describir las especies de frutos/semillas que el zorro pampeano dispersa en ambientes de agroecosistemas y bosques serranos de la provincia de CórdobaArgentina. Para ello se realizaron muestreos mensuales y se recolectaron fecas frescas de zorro en ambos ambientes. Las fecas fueron procesadas bajo lupa estereoscópica y los frutos/semillas fueran separados e identificados a nivel específico. De acuerdo con nuestros resultados se contabilizaron 46 especies diferentes de frutos/semillas en el ambiente de agroecosistemas de los cuales el 37% correspondieron a especies autóctonas. En el ambiente de bosque serrano se contabilizaron 26 especies diferentes de las cuales el 61.5% eran de especies autóctonas. El 69% y 84% de los frutos/semillas de agroecosistema y bosque serrano respectivamente estaban sanas. En ambos ambientes el zorro pampeano juega un rol esencial en la dispersión “legítima” de semillas mediante endozoocoria. Pampas fox (Lycalopex gymnocercus) is a widely distributed carnivore with an omnivorous diet, with fruits and seeds being a very important part of this diet. The aim of this research was to study and describe the fruits and seeds dispersed by the Pampas fox in the agroecosystems and hill forests of Córdoba province in Argentina. For this, monthly samplings were carried out in order to collect fresh fox feces in both environments. The feces were analyzed under a stereoscopic magnifying glass and fruits and seeds were separated and identified at the species level. In the agroecosystem environment, 46 fruit and seed species were counted, and from those, 37% corresponded to autochthonous species. Furthermore, in the hill forest environment, 26 different species were counted, of which 61.5% were autochthonous. This study showed that 69% of the fruits and seeds from the agroecosystem and 84% from the mountain forest were healthy. In both environments, the Pampas fox plays an essential role in the dispersal of seeds through endozoochory

Relativistic Approach to Superfluidity in Nuclear Matter

Pairing correlations in symmetric nuclear matter are studied within a relativistic mean-field approximation based on a field theory of nucleons coupled to neutral ($\sigma$ and $\omega$) and to charged ($\varrho$) mesons. The Hartree-Fock and the pairing fields are calculated in a self-consistent way. The energy gap is the result of a strong cancellation between the scalar and vector components of the pairing field. We find that the pair amplitude vanishes beyond a certain value of momentum of the paired nucleons. This fact determines an effective cutoff in the gap equation. The value of this cutoff gives an energy gap in agreement with the estimates of non relativistic calculations.Comment: 21 pages, REVTEX, 8 ps-figures, to appear in Phys.Rev.C. e-mail: [email protected]