Repositório RCAAP

In this work we study an extension of a light-cone QCD-inspired model, where the interacting part of the mass squared operator consists of a harmonic oscillator potential as confinement, a Coulomb-like interaction and a zero-range spin interaction acting on the ³S0 states. The renormalization of the model is performed by using as a input the pion mass to fix the strength of the spin interaction. We apply the extended model to study the splitting of the ³S1 and ³S0 ground state mesons and the spectrum in the light meson sector. We show that the experimental values of the splitting between the masses of ³S1 and ³S0 ground state mesons as a function of the ground state pseudoscalar mass is reproduced by the model. In the botonium case our result is consistent with other theoretical estimates.

In order to obtain the properties of compact stellar objects, appropriate equations of state have to be used. In the literature, strange meson fields, namely the scalar meson field sigma*(975) and the vector meson field phi(1020), had to be considered in order to reproduce the observed strongly attractive <FONT FACE=Symbol>LL</FONT> interaction. The introduction of these strange mesons makes the equations of state harder (EOS) due to the repulsive effect of the phi(1020) meson. In this work the inclusion of these mesons in the equation of state and their influence on the properties of the neutron stars are investigated.

Short range correlations are introduced using unitary correlation method in a relativistic approach to the equation of state of the infinite nuclear matter in the framework of the Hartree-Fock approximation. The effect of the correlations in the ground state properties of the nuclear matter is studied.

In this communication we present results of a study of chiral symmetry in quark matter using an effective Coulomb gauge QCD Hamiltonian. QCD in Coulomb gauge is convenient for a variational approach based on a quasiparticle picture for the transverse gluons, in which a confining Coulomb potential arises naturally. We show that such an effective Hamiltonian predicts chiral restoration at too low quark densities. Possible reasons for such deficiency are discussed.

A recursive subtractive renormalization of the scattering equation is applied to the nucleon-nucleon ¹S0 channel with one-pion-exchange plus derivative contact interactions. This method can be easily extended to any derivative order of the singular interaction. Although we limit this work to the singlet partial wave, the method can be used as well in higher waves and coupled channels. The ¹S0 renormalization parameters are fitted to the data.

We obtain upper limits for the contributions of the incomplete fusion and sequential complete fusion processes to the total fusion cross section. Through those upper bounds we find that these processes are negligible in reactions induced by projectiles such as 6He and 11Li, which break up into neutrons and one fragment containing the full projectile charge.

In order to better understand the breakup contribution to fusion, we perfomed light particle coincidence measurements for the systems 6,7Li+12C,59Co at energies near and above the barrier. A three body kinematics analysis is performed with the objective of separating the contributions of different reaction mechanisms. For small angular differences between the light particles, the sequential and direct breakup seem to dominate. On the other hand, for large detector separations, a sequential decay following transfer is likely to dominate.

The method of fusion barrier distribution has been widely used to interpret the effect of nuclear structure on heavy-ion fusion reactions around the Coulomb barrier. We discuss a similar, but less well known, barrier distribution extracted from large-angle quasi-elastic scattering. We argue that this method has several advantages over the fusion barrier distribution, and offers an interesting tool for investigating unstable nuclei.

This paper reviews the physics of the spallation which is a nuclear reaction in which a particle (e.g. proton) interacts with a nucleus. Given to the high energy of the incident proton, in a first stage it interacts with the individual nucleons in an intranuclear cascade which leads to the emission of secondary particles (neutrons, protons, mesons, etc.). In a secondary stage the nucleus is left in an excited state and can de-excite by evaporation and/or fission. Given to the high number of secondary neutrons produced (~ 30 n/p for proton energy of 1 GeV), this reaction can be used as a source of neutrons, for example for ADS systems as external source to drive the sub critical reactor. The main codes used in the ADS target design and an example on the utilization of one of these codes (the LAHET code) for typical ADS target are given.

Two nuclear reactions, 11B+100Mo, (Eb=43 MeV) and 16O+51V, (Eb=70 MeV) were studied with a gamma spectrometer with a charged particle array in order to measure the correlation between gamma-rays emitted by residual nuclei produced by heavy-ion reactions in coincidence with a charged particles detected at different angles. Ratios of gamma-transition intensities in coincidence with charged particles were measured for the main products of these reactions, providing a differentiation between the exit channels, even when complete fusion is the only reaction mechanism.

The detection of delayed X-rays produced by the decay that follows electron capture in the residual nuclei has been used in the past for the determination of fusion cross sections of tightly bound nuclei. In this work we applied this technique to study the effect of the break-up of a weakly bound projectile, like in the case of the 9Be + 144Sm system. Preliminary results of the complete fusion in this system at near barrier energies are presented.

We obtained theoretical fusion cross sections based on a new and accurate procedure to calculate the Coulomb and nuclear interactions between deformed nuclei. The calculations were performed within the context of the barrier penetration model with zero point motion for the 64Ni + 64Ni system.

We have used a parameter - free potential, obtained from data analysis at sub-barrier energies, to explain our measurements of elastic and several inelastic scattering cross sections for the 16,18O + 58Ni systems at an energy above the barrier. The data were analyzed through extensive coupled-channel calculations. Transfer cross sections could also be explained by the same interaction, which is consistent with the more fundamental São Paulo potential.

One of the main applications of the Hybrid Reactors (ADS - Accelerator Driven System) is the incineration of transuranics (TRU) by fast neutrons that emerge from a spallation source in a sub critical reactor waste burner [1, 2]. For this application, an accurate description and prediction of spallation reaction is necessary, including all the characteristics concerning spatial and energetic angular distributions, spallation products and neutron multiplicity. To describe the nuclear reactions at intermediate and high energies, Monte Carlo calculations have been used. The CRISP package considers the intranuclear cascade (INC) that occurs during the spallation process in a realistic time-sequence approach in which all particles inside the nucleus can participate in the cascade and the nuclear density fluctuations are naturally taken into account during the process. The occupation number of each single particle level is considered as a function of time and a more realistic Pauli blocking mechanism can be performed. None of the existing models have effectively used all those features. The evaporation of protons and alpha particles are taken into account making possible the correct prediction of fissilities of actinides and pre-actinides [3]. Another implementation is the NN single-pion production reaction. This reaction is especially relevant if one is interested in neutron or proton multiplicities, since the creation/emission of pions is directly related with the excitation energy of the residual nucleus. We will present some results obtained with the CRISP package for proton-nucleus reaction at intermediate and high energies. This package was obtained by the coupling of the MCMC [4] and MCEF [5] codes, with the introduction of some improvements, such as better Pauli blocking mechanism, which constrains the residual nucleus energetic evolution to the Pauli Principle from the ground-state to the final compound-nucleus formed at the end of the intranuclear cascade process, and introduction of the most relevant resonant excitation and the NN single pion production channel. The results of interest for ADS development are consistent with the experimental data at different proton energies. More detailed calculations are being performed for studying other features of proton-nucleus reactions and with different targets.

A new approach to evaluate nuclear photofissilities at incident photon energies above the pion photoproduction threshold has been recently developed and proved to describe successfully the fissilities of natPb and 232Th target nuclei at energies ~0.2-4.0 GeV. The method is merely a simple, semiempirical description of the photofission reactions in which fissility, f, is governed by two basic quantities, namely, i) the first-chance fission probability, $\bar{f}_1$, for the average cascade residual, and ii) the slope, $\bar{s}$, of the chance-fission probability associated with the average evaporative sequence of fissionable residuals. In the present work we have extended this approach to analyse photofissity data that have been accumulated over the past fourty years or so, measured at 1 GeV, for nearly fourty target nuclei extending from Ti up to Np. Results have shown that the variation of fissility with Z²/A could be described quite satisfactorily by the proposed model.

The decay of the compound nucleus is traditionally calculated using one of two types of statistical models, either a sequential or a simultaneous emission one. The best known sequential emission models are the Weisskopf-Ewing one and the Hauser-Feshbach one. Both sequential emission models emit only one particle at a time. A well know simultaneous emission model is the Fermi breakup one, which takes into account the fragmentation of the compound nuclei into two, three or more residual nuclei/particles. We have compared the particle and residual nucleus distributions of the Weisskopf-Ewing and Fermi breakup models in the case in which only stable nuclei/particles are emitted. We find that the Weisskopf-Ewing results in larger yields of light particles and heavy residues than the Fermi breakup model, while the latter results in larger yields of intermediate mass fragments.

A model based on the nuclear track potential is described and used to predict trajectories and the energy distribution of secondary ions emitted by insulating targets. In this model, the electric field generated by each track pushes away the secondary ions formed on the target surface. The effects on the potential due to target thickness, track charge density, projectile incidence angle and secondary ion mass are analyzed. Predictions are compared with experimental data existing in the literature. It is found that the proposed model describes partially the behavior observed in the angular distribution of the emitted ions and new processes are proposed to be included in the model.

The importance of fragmentation of water molecules by heavy ions is discussed for several different physical environments, including tumor treatment, planetary sciences and fission reactors. The characteristics of the fragmentation yields by electrons, protons and photons are presented and compared with fragmentation yields of water molecules by C3+ and O5+ ions at energies corresponding to the Bragg peak. It is shown that for low energy electron loss and high energy electron capture, the water molecules essentially blow-out, releasing a much larger fraction of Oq+ ions as compared with electrons, protons and photons.

We compare the Amplified Spontaneous Emission (ASE) of the proton transfer dyes 2-(2'-hydroxyphenyl) benzimidazole (HPBI) and 2-(2'-hydroxy-5'-chlorphenyl)benzimidazole (Cl-HPBI) in three different polymeric hosts under UV pulsed laser excitation. Pulseform and degradation of the dyes in the solid polymers were analysed. Spectroscopic data were used in a simple four-level kinetic rate equation system, in the attempt to explain the observed results, i.e. first order exponential decay of maximum output energy and pulse shortening. The model includes transitions for the normal and tautomer form of the molecule as well as intersystem crossing and triplet-triplet transitions.

Ionic fragmentation of the sublimated amino acid DL-proline has been studied using time-of-flight mass spectrometry and synchrotron radiation. Total ion yield and mass spectra were recorded in the 13 to 21.6 eV energy range. Partial ion yields have been calculated for the produced fragments and the results analyzed in a comparative way. Mass spectrum of proline previously obtained at 21.21 eV using photons from a discharge lamp (He I), was used as reference in the comparison to the synchrotron radiation based spectra. The loss of the COOH fragment represents the most probable dissociation pathway following the photoionization of DL-proline in the valence region. These are the first results of total and partial ion yields spectra for this molecule in its gas phase in the valence region using time-of-flight spectrometry.