Z-dependent Barriers in Multifragmentation from Poissonian Reducibility and Thermal Scaling

We explore the natural limit of binomial reducibility in nuclear multifragmentation by constructing excitation functions for intermediate mass fragments (IMF) of a given element Z. The resulting multiplicity distributions for each window of transverse energy are Poissonian. Thermal scaling is observed in the linear Arrhenius plots made from the average multiplicity of each element. ``Emission barriers'' are extracted from the slopes of the Arrhenius plots and their possible origin is discussed.Comment: 15 pages including 4 .ps figures. Submitted to Phys. Rev. Letters. Also available at http://csa5.lbl.gov/moretto

Áreas potenciais para a criação de rã-touro-gigante (lithobates catesbeianus SHAW, 1802) na região Sul do Brasil.

A ranicultura demonstra ser promissora no Brasil, principalmente como atividade rural voltada à agricultura familiar e aos pequenos agricultores. A presente pesquisa objetivou levantar preliminarmente os ranários ativos existentes atualmente na região Sul e a sua distribuição espacial, além de delimitar áreas potenciais naturais e áreas com restrições de temperatura mínima do ar, umidade relativa do ar e altitude para a criação da rã-touro-gigante (Lithobates catesbeianus Shaw, 1802). No levantamento preliminar, foram contabilizados 16 ranários distribuídos em dois municípios do Paraná (PR), 5 em Santa Catarina (SC) e 2 no Rio Grande do Sul (RS). Entre os estados da região Sul, somente uma pequena área do PR, localizada entre as mesorregiões do noroeste paranaense e norte central paranaense, apresentou potencial natural ideal para o criatório. As áreas com restrição de temperatura mínima do ar representam uma grande área dos estados do PR e SC e a porção sudeste e nordeste do RS. As áreas com restrição de temperatura mínima e umidade relativa do ar estão localizadas na faixa sudoeste do RS e na faixa centro-norte do PR. O número reduzido de ranários na região Sul deve-se, muito provavelmente, à necessidade de instalações climatizadas ou de manejo voltado à adequação microclimática das instalações

Áreas potenciais para a criação de rã-touro-gigante (lithobates catesbeianus SHAW, 1802) na região Sul do Brasil.

A ranicultura demonstra ser promissora no Brasil, principalmente como atividade rural voltada à agricultura familiar e aos pequenos agricultores. A presente pesquisa objetivou levantar preliminarmente os ranários ativos existentes atualmente na região Sul e a sua distribuição espacial, além de delimitar áreas potenciais naturais e áreas com restrições de temperatura mínima do ar, umidade relativa do ar e altitude para a criação da rã-touro-gigante (Lithobates catesbeianus Shaw, 1802). No levantamento preliminar, foram contabilizados 16 ranários distribuídos em dois municípios do Paraná (PR), 5 em Santa Catarina (SC) e 2 no Rio Grande do Sul (RS). Entre os estados da região Sul, somente uma pequena área do PR, localizada entre as mesorregiões do noroeste paranaense e norte central paranaense, apresentou potencial natural ideal para o criatório. As áreas com restrição de temperatura mínima do ar representam uma grande área dos estados do PR e SC e a porção sudeste e nordeste do RS. As áreas com restrição de temperatura mínima e umidade relativa do ar estão localizadas na faixa sudoeste do RS e na faixa centro-norte do PR. O número reduzido de ranários na região Sul deve-se, muito provavelmente, à necessidade de instalações climatizadas ou de manejo voltado à adequação microclimática das instalações

Modelling of compound nucleus formation in fusion of heavy nuclei

A new model that includes the time-dependent dynamics of the single-particle (s.p.) motion in conjunction with the macroscopic evolution of the system is proposed for describing the compound nucleus (CN) formation in fusion of heavy nuclei. The diabaticity initially keeps the entrance system around its contact configuration, but the gradual transition from the diabatic to the adiabatic potential energy surface (PES) leads to fusion or quasifission. Direct measurements of the probability for CN formation are crucial to discriminate between the current models.Comment: 4 pages,2 figures,1 table, Submitted to PR

Stability of bubble nuclei through Shell-Effects

We investigate the shell structure of bubble nuclei in simple phenomenological shell models and study their binding energy as a function of the radii and of the number of neutron and protons using Strutinsky's method. Shell effects come about, on the one hand, by the high degeneracy of levels with large angular momentum and, on the other, by the big energy gaps between states with a different number of radial nodes. Shell energies down to -40 MeV are shown to occur for certain magic nuclei. Estimates demonstrate that the calculated shell effects for certain magic numbers of constituents are probably large enough to produce stability against fission, alpha-, and beta-decay. No bubble solutions are found for mass number A < 450.Comment: 9 pages and 9 figures in the eps format include

A statistical interpretation of the correlation between intermediate mass fragment multiplicity and transverse energy

Multifragment emission following Xe+Au collisions at 30, 40, 50 and 60 AMeV has been studied with multidetector systems covering nearly 4-pi in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system, Xe+Bi at 28 AMeV. The experimental trends are compared to statistical model predictions.Comment: 7 pages, submitted to Phys. Rev.

Statistical nature of cluster emission in nuclear liquid-vapour phase coexistence

The emission of nuclear clusters is investigated within the framework of isospin dependent lattice gas model and classical molecular dynamics model. It is found that the emission of individual cluster which is heavier than proton is almost Poissonian except near the transition temperature at which the system is leaving the liquid-vapor phase coexistence and the thermal scaling is observed by the linear Arrhenius plots which is made from the average multiplicity of each cluster versus the inverse of temperature in the liquid vapor phase coexistence. The slopes of the Arrhenius plots, {\it i.e.} the "emission barriers", are extracted as a function of the mass or charge number and fitted by the formula embodied with the contributions of the surface energy and Coulomb interaction. The good agreements are obtained in comparison with the data for low energy conditional barriers. In addition, the possible influences of the source size, Coulomb interaction and "freeze-out" density and related physical implications are discussed

Time-Dependent Hartree-Fock simulation of the expansion of abraded nuclei

A recent interpretation of the caloric curve based on the expansion of the abraded spectator nucleus is re-analysed in the framework of the Time-Dependent Hartree-Fock (TDHF) evolution. It is shown that the TDHF dynamics is more complex than a single monopolar collective motion at moderate energy. The inclusion of other important collective degrees of freedom may lead to the dynamical creation of hollow structure. Then, low density regions could be locally reached after a long time by the creation of these exotic density profiles. In particular the systematic of the minimum density reached during the expansion (the so-called turning points) appears to be different.Comment: 30 Latex pages including 9 figure

On the elliptical flow in asymmetric collisions and nuclear equation of state

We here present the results of elliptical flow for the collision of different asymmetric nuclei (10Ne20 +13 Al27, 18Ar40 +21 Sc45, 30Zn64 +28 Ni58, 36Kr86 +41 Nb93) by using the Quantum Molecular Dynamics (QMD) model. General features of elliptical flow are investigated with the help of theoretical simulations. The simulations are performed at different beam energies between 40 and 105 MeV/nucleon. A significant change can be seen from in-plane to out-of-plane elliptical flow of different fragments with incident energy. A comparison with experimental data is also made. Further, we predict, for the first time that, elliptical flow for different kind of fragments follow power law dependence ? C(Atot)? for asymmetric systems

Taxonomic resolution refinement does not improve understanding of invertebrate's role on leaf litter breakdown

Abstract The invertebrate biodiversity of subtropical streams depends on the seasonal input of organic matter, as much as the leaf decomposition process on stream system. However, one of the challenges in determining the importance of invertebrates for leaf breakdown in subtropical streams is the low taxonomic resolution applied in most studies. To overcome this limitation, here we used litter bags with senescent leaves to evaluate the impact of different taxonomic resolutions of trophic group classification to assess the seasonal importance of invertebrate community for leaf litter breakdown in a subtropical stream (Atlantic Forest in western of Paraná state, Brazil). Litterfall was quarterly measured over a year. The leaf litter accumulated in an interval of 30 days was retrieved, weighed, and used for the leaf breakdown experiments (by litter bags). We found a lower importance of invertebrate community richness and density (shredders and scrapers) in leaf breakdown process irrespective of the taxonomic resolution (family or genus level used). Hyphomycetes biomass and fungi sporulation also did not present changes among sample times, and consecutively, importance for leaf breakdown. However, the richness and density of Chironomidae taxa respond differently depending on the taxonomic resolution used. Low litter breakdown may be explained by the increase of consumption of microorganisms, due to high density of Chironomidae scrapers evaluated at the genus level. Moreover, temperature is the main factor responsible for breakdown over the year, in a positive way. Therefore, our results indicated the family level as the taxonomic resolution sufficient to assess the role of shredders and scrapers in the leaf litter breakdown process of this subtropical stream system