Repositório RCAAP
The transition temperature of the nuclear caloric curve
Experimental studies have obtained the caloric curve of nuclear matter from heavy ion collisions as well as its dependence on the size of the fragmenting source. In particular it has been determined that smaller systems have caloric curves with higher plateau temperatures than larger systems. This work uses molecular dynamics simulations to study the thermodynamics of heavy ion collisions and to identify the main factors that determine the caloric curve. The simulations indicate that the reaction is composed of three stages: 1) an initial collision that transforms the nuclei from normal density and zero temperature and entropy, to a hot and dense blob of matter with higher values of density, temperature and entropy; 2) this is followed by a constant-entropy expansion that takes the system to the spinodal of the phase diagram; 3) where the system rapidly disassembles into fragments by the process of spinodal decomposition, and not by nucleation. These findings indicate that the plateau temperature of the caloric curve is nothing more than the temperature of the phase change and it is set by the intersection of the isentropic expansion and the spinodal. In other words, the plateau temperature is simply the temperature at which the system breaks as it enters the spinodal. This transition temperature is thus set by the entropy generated during the initial part of the collision.
2004
Barrañón,A. Escamilla Roa,J. López,J. A.
Study of the effect of the breakup on the fusion cross section of the systems 6,7Li+59Co
Fusion cross section excitation functions were measured for the 6;7Li+59Co systems (12 < Elab < 26 MeV)aiming the investigation of the effect of the breakup process on the fusion cross section. The experimental method consisted on the detection of gamma-rays from the evaporation residues. Coupled channels calculations have been performed for both systems. The comparison to the experimental results support the conclusion that there is neither supression nor enhancement at energies above the coulomb barrier. A slight enhancement at energies below the barrier is observed for the 6Li+59Co cross section over the one for 7Li+59Co. In order to better quantify the effect of the breakup process, coincidence measurements for the breakup products are being performed.
2004
Souza,F.A. Toledo,A. Szanto de Munhoz,M.G. Takahashi,J. Carlin,N. Suaide,A.A.P. Moura,M.M. de Szanto,E.M. Beck,C. Sanders,S.J.
JLab: probing hadronic physics with electrons and photons
Precision measurements of the structure of nucleons and nuclei in the regime of strong interaction QCD are now possible with the availability of high current polarized electron beams, polarized targets, and recoil polarimeters, in conjunction with modern spectrometers and detector instrumentation. The physics at JLab will be highlighted using two recent measurements of general interest. The ratio of the proton electric to magnetic form factors indicates the importance of the role of angular momentum in the structure of the nucleon. The existence of 5-quark configurations in the ground state wavefunctions of hadrons is confirmed by a narrow peak attributed to an exotic baryon with strangeness S=+1. These and other examples will be used to illustrate the capabilities and focus of the experimental program at JLab.
2004
Smith,Elton S.
A low-mass neutron flux monitor for the n _TOF facility at CERN
A small-mass system has been developed for monitoring the flux of neutrons with energy up to 1 MeV at the new time-of-flight facility at CERN, n_TOF. The monitor is based on a thin Mylar foil with a 6Li deposit, placed in the neutron beam, and an array of Silicon detectors, placed outside the beam, for detecting the products of the 6Li(n, alpha)³H reaction. The small amount of material on the beam ensures a minimal perturbation of the flux and minimizes the background related to scattered neutrons. Moreover, a further reduction of the gamma-ray background has been obtained by constructing the scattering chamber hosting the device in carbon fibre. A detailed description of the flux monitor is here presented, together with the characteristics of the device, in terms of efficiency, resolution and induced background. The use of the monitor in the measurement of neutron capture cross-sections at n_TOF is discussed
2004
Mastinu,P. F. Abbondanno,U. Aerts,G. Alvarez,H. Andriamonje,S. Angelopoulos,A. Assimakopoulos,P. Bacri,C.O. Badurek,G. Baumann,P. Becvár,F. Beer,H. Benlliure,J. Berthier,B. Berthomieux,E. Boffi,S. Borcea,C. Boscolo-Marchi,E. Bustreo,N. Calviño,P. Cano-Ott,D. Capote,R. Carlson,P. Cennini,P. Chepel,V. Chiaveri,E. Coceva,C. Colonna,N. Cortes,G. Cortina,D. Couture,A. Cox,J. Dababneh,S. Dahlfors,M. David,S. Dolfini,R. Domingo-Pardo,C. Duran,I. Eleftheriadis,C. Embid-Segura,M. Ferrant,L. Ferrari,A. Ferreira-Lourenco,L. Ferreira-Marques,R. Frais-Koelbl,H. Furman,W.I. Giomataris,Y. Goncalves,I.F. Gonzalez-Romero,E. Goverdovski,A. Gramegna,F. Griesmayer,E. Gunsing,F. Haight,R. Heil,M. Herrera-Martinez,A. Ioannides,K.G. Janeva,N. Jericha,E. Käppeler,F. Kadi,Y. Karamanis,D. Kelic,A. Ketlerov,V. Kitis,G. Koehler,P.E. Konovalov,V. Kossionides,E. Lacoste,V. Leeb,H. Lindote,A. Lopes,M.I. Loriggiola,M. Lozano,M. Lukic,S. Markov,S. Marigo,S. Marrone,S. Martinez-Val,J. Mengoni,A. Milazzo,P.M. Minguez,E. Molina-Coballes,A. Moreau,C. Neves,F. Oberhummer,H. O'Brien,S. Pancin,J. Papaevangelou,T. Paradela,C. Pavlik,A. Pavlopoulos,P. Perez-Parra,A. Perlado,J.M. Perrot,L. Peskov,V. Plag,R. Plompen,A. Plukis,A. Poch,A. Policarpo,A. Pretel,C. Quesada,J.M. Radici,M. Raman,S. Rapp,W. Rauscher,T. Reifarth,R. Rejmund,F. Rosetti,M. Rubbia,C. Rudolf,G. Rullhusen,P. Salgado,J. Savvidis,E. Soares,J.C. Stephan,C. Tagliente,G. Tain,J.L. Tapia,C. Tassan-Got,L. Tavora,L.M.N. Terlizzi,R. Terrani,M. Tsangas,N. Vannini,G. Vaz,P. Ventura,A. Villamarin-Fernandez,D. Vincente-Vincente,M. Vlachoudis,V. Vlastou,R. Voss,F. Wendler,H. Wiescher,M. Wisshak,K. Zanini,L.
Hot hypernucleus formation in high-energy photonuclear reactions
The probability of a compound residual hot hypernucleus formation at the end of pre-equilibrium phase of high energy photonuclear reactions is calculated with a time dependent Monte Carlo Multicollisional Cascade (MCMC) approach. The emission of hadrons during the rapid phase of the reaction is discussed and characteristics of the residual nucleus are obtained. Results for mass and charge distributions of the formed hot hypernucleus are shown and its excitation energy is discussed.
2004
Gonçalves,M. Oliveira,E.C. de Medeiros,E.L. Pina,S. de Duarte,S.B.
New approach to nuclear photofission reactions above 0.15 GeV
A simple approach to evaluate nuclear photofissilities at energies above the pion photoproduction threshold has been developed. It is based on the current, two-step model for intermediate-energy photonuclear reactions, i.e. a photon-induced intranuclear cascade followed by a fission-evaporation competition process for the excited, post-cascade residual nucleus. The calculation method (semiempirical by nature) shows that fissility (i.e., total fission probability) is governed by two basic quantities, namely, the first-chance fission probability for the average cascade residual, and a parameter which defines an evaporative sequence of residuals in which the average, equivalent chance-fission probabilities of nuclides belonging to the same generation are located. The natPb photofissility data measured recently in the range ~ 0.2 - 3.8 GeV at the Thomas Jefferson Laboratory could be explained very satisfactorily by the present approach.
2004
Tavares,O. A. P. Duarte,S. B. Deppman,A. Likhachev,V. P.
The reaction entrance channel and isotopic composition of fragments in a dynamical and statistical multifragmentation regime
From the study of the Ni+Ni,Ag 30 MeV/nucleon dissipative midperipheral collisions, it has been possible to detect events in which Intermediate Mass Fragments (IMF) are emitted by two different sources with different mechanisms. The sources are: a) a quasi-projectile (QP) (and a quasi-target (QT)), with an excitation energy that leads to multifragmentation totally described in terms of a statistical disassembly of a thermalized system (T <IMG SRC="/img/revistas/bjp/v34n3a/a59img01.gif">4. MeV, E*<IMG SRC="/img/revistas/bjp/v34n3a/a59img01.gif"> 4 MeV/nucleon); b) an intermediate source, emitting both light particles and IMF. In this second case, fragments are more neutron rich than the average matter of the overall system, and have a very different charge distribution, with respect to those statistically emitted from the QP. The above features can be considered as a signature of the dynamical origin of the midvelocity emission. The results of this analysis show that IMF can be produced via different mechanisms simultaneously present within the same collision. They also show that the probability of IMF production via dynamical reaction increase with the size of the target nucleus.
2004
Milazzo,P. M. Margagliotti,G. V. Rui,R. Sisto,M. Vannini,G. Bruno,M. D'Agostino,M. Agodi,C. Alba,R. Bellia,G. Coniglione,R. Del Zoppo,A. Finocchiaro,P. Maiolino,C. Migneco,E. Piattelli,P. Santonocito,D. Sapienza,P. Colonna,N. Gramegna,F. Mastinu,P.F. Iori,I. Moroni,A.
Spectroscopy of 6Li using the ³He(7Li, alpha)6Li reaction
The spectroscopic study of unbound states in the 6Li nucleus was performed by measuring the energy spectrum of the alpha-particles emitted from the transfer reaction ³He(7Li, alpha)6Li. The 7Li beam at E Lab = 31.2MeV was produced at the São Paulo Pelletron accelerator. A ³He gas cell was used as target. alpha-Particle spectra were measured from thetaLab = 8º up to thetaLab = 20º with steps of 1º. Due to its positive Q = +13.3MeV, the ³He(7Li, alpha)6Li reaction favours the population of states in 6Li at high excitations energies up to about 20MeV. We observed resonances at 17.29MeV, 15.31MeV, and a new state at 12.45MeV, in addition to all previously known states of 6Li. An <FONT FACE=Symbol>Â</FONT>-Matrix analysis was performed and the positions and widths of these states were extracted.
2004
Kuramoto,R. Lichtenthäler,R. Lépine-Szily,A. Guimarães,V. Lima,G. F. Benjamim,E. Faria,P. N. de
On the origin of the satellite peaks in alpha particle spectra
In this work we describe the results obtained with S3590-06 and ion-implanted diodes for alpha particle spectrometry. Satellite peaks were also investigated in order to identify not only the origin of these phenomena, but also the importance of the guard ring region on their relative intensities. The preliminary results indicate that the true cause of the spurious satellite peaks might be associated with the weak electric fields around the guard rings region and changes on the entrance window absorption near the edge of the diodes. Further steps are needed to clear up this problem.
2004
Camargo,F. Bueno,Carmen C. Gonçalves,Josemary A. C. Mendes,P. F. P. Rato Pinto,J. K. C. Souza,J. P. Santos,M. D. S.
Standardization of Ca-45 radioactive solution by tracing method
The procedure followed by the Laboratório de Metrologia Nuclear (LMN) at the IPEN, in São Paulo, for the standardization of the 45Ca is described. The activity measurement was carried out in a 4<FONT FACE=Symbol>pb</FONT>-gamma coincidence system, by the tracing method. The radionuclide chosen as the beta-gamma emitting tracer nuclide was 60Co because of its end-point beta-ray energy which is close to 45Ca. Six sources were prepared using a 1:1 ratio (beta-pure and beta-gamma) dropped directly on the Collodion film, and other two solutions of 45Ca + 60Co were mixed previously using a 1:1 and 1:2 ratio before making the radioactive sources. The activity of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer.
2004
Ponge-Ferreira,Cláudia Regina Ponte Koskinas,Marina Fallone Dias,Mauro da Silva
Nuclear fragmentation in protontherapy
The effect of nuclear fragmentation in the passage of 180MeV protons through the human body tissue is discussed. Prostate cancer protontherapy with these intermediate-energy protons is discussed in light of model calculation.
2004
Rebello,Patrícia Hussein,Mahir S.
Inclusive photoproduction of lepton pairs in the parton model
In the framework of the QCD parton model, we study unpolarized scattering of high energy real photons from a proton target into lepton pairs and a system of hadrons. For a given parametrization of parton distributions in the proton, we calculate the cross section of this process and show the cancellation of the interference terms.
2004
Psaker,A.
Fluoropolymer studies for radiation dosymetry
The polymers Tetrafluoroethylene- hexa- fluoropropylene (FEP) and Tetrafluoroethylene- per- fluoromethoxyethelene (PFA) are normally used as anti-adherent coatings and can also be applied for several applications in research. For example, they can be used as radiation dosimeters for X-ray and gamma photons, electrons, protons and other ionazing particles. In order to determine radiation induced damage, that can compromise applications in dosimetry, FEP and PFA films were bombarded with protons of 1 MeV at fluences from 1 × 10(11) protons/cm² to 1 × 10(16) protons/cm². During the bombardment, the chemical species emission was monitored with a Residual Gas Analyzer (RGA), and results show that the CF3 radical is the specie preferentially emitted. The bombarded films were also analyzed with Optical Absorption Photospectrometry (OAP) which shows quantitative chemically specific evidence of the damage caused by the proton bombardment. Our results show that damage to polymers is detectable for all fluences used in this work, but damage that can compromise applications in dosimetry occurs only for fluences greater than 1 × 10(14) protons/cm².
2004
Parada,Marco Aurélio Minamisawa,Renato Amaral Almeida,Adelaide de Muntele,Claudiu Zimmerman,Robert Lee Muntele,Iulia Ila,Daryush
The IFUSP microtron: status and perspectives
In this paper we summarize the status of the racetrack microtron that is under construction at IFUSP, showing the achievements obtained in the transport, RF, control and vacuum systems. A description of the experimental equipment that will be available to the users is also presented.
2004
Martins,M.N. Takahashi,J. Malafronte,A.A. Rios,P.B. Lopes,M.L. Bonini,A.L. Lima,R.R.
Production cross sections of super heavy elements
This report describes a method for calculating fusion and decay probabilities in reactions leading to the production of transfermium elements. The competition between quasi-fission and fusion is described on the basis of the Dinuclear System Concept (DNSC). Both the competition between fusion and quasi-fission and statistical decay of heavy highly fissionable excited compound nuclei is described in an approach based on the Monte-Carlo method.
2004
Cherepanov,Evgeni A.
Radiation interaction with DNA
We developed a model to describe the radiation-DNA interaction in water solution. The model is based on some important aspects of the Bethe Theory for the ionizing process, and on the assumption that the direct interaction between the incident particle and the DNA molecule is negligible in comparison with the indirect effect. This assumption can always be true under controlled conditions. The results show that the number of damages induced by radiation depends on the accumulated dose, on parameters related only to the medium properties, and on the DNA structure, but not on the incident particle. A few applications of the model are discussed, and results are successfully compared with experimental data. New experiments are proposed.
2004
Deppman,A. Echeimberg,J.O. Gouveia,A.N. Arruda-Neto,J.D.T. Milian,F.M. Added,N. Camargo,M.E. Guzman,F. Helene,O.A.M. Likhachev,V.P. Rodriguez,O. Schenberg,A.C.G. Vanin,V. Vicente,E.J.
SU(3) model description of Be isotopes
Experimental data on light nuclei close to dripline suggests that as the nucleon number asymmetry increases, the shell structure from stability line is not preserved. In contrast with spherical shell model, Elliott's SU(3) model, uses a deformed multi-nucleon basis to describe nuclear states. The SU(3) symmetry is a strong feature of 1p shell nuclei, where symmetry breaking spin-orbit force is rather weak. We have calculated the binding energies and low-lying energy spectra of Be isotopes (A=6 to A=14), within the framework of Elliott's SU(3) model, with special emphasis on effects due to the presence of intruder orbit 1S1/2 in the region. The model space includes SU(3) basis states that maximize the quadrupole-quadrupole interaction (-chiQ.Q). An extended model space includes, in addition, a set of 2p-2h excitations, with excited nucleons occupying lowest energy SU(3) states in harmonic oscillator shell N=2. Group theoretical methods have been used to classify the states and calculate the nuclear interaction matrix elements. Good angular momentum states are projected out from intrinsic deformed SU(3) states by using standard angular momentum projection techniques. The interaction used contains the monopole-monopole, quadrupole-quadrupole and isospin dependent terms. Interaction parameters are fixed so as to reproduce the binding of 4 nucleons in N=1 orbit for the N=Z isotope-8Be in ground state and first excited 2+ state, along with the known systematics of single neutron separation energies for Be isotopes. The calculated energy espectra are compared with available experimental data. The calculated excitation energies of intruder states in 8Be and 10Be, are discussed in the light of available experimental evidence and theoretical results from shell model as well as deformed oscillator model calculations of Fayache et al..
2004
Almeida,Eduardo de Sharma,S. Shelly
Developement of crisp package for spallation reaction studies and its utilization in ADS (Accelerator Driven System)
The progress of nuclear physics had allowed a better understanding of nuclear structure and reaction mechanisms, but also enables important applications of nuclear energy in our modern way of life. We can point out those ones related to power generation from nuclear reactors, providing an almost inexhaustive power source due to the huge quantities of nuclear fuel existent in our planet, which guaranties its utilization for thousands of years. The main interest is the so-called Hybrid Reactors (ADS - Accelerator Driven System), in which the reactor has a sub critical core and the chain reaction is sustained by an external source that comes from spallation reactions induced by high energy protons (about 1 GeV) from an accelerator, producing a large number of secondary particles (mainly neutrons). For those applications, precise knowledge about nuclear structure and nuclear reaction characteristics is needed.
2004
Anéfalos,S. Deppman,A. Arruda-Neto,J. D. T. Likhachev,V.P. Santos,A. dos Coelho,P. R. P. Duarte,S. B. Tavares,O. A. P.
SR-TXRF detection limit reduction using thin polymer film substrates
Three different substrates for the analysis of liquid samples by Total Reflection X-Ray Fluorescence with Synchrotron Radiation (SR-TXRF) were investigated and compared: Lucite (Perspex), Kimfoil and Mylar. Dry targets were prepared by pipetting 5 mul aliquotes of the liquid samples (synthetic standards and fresh water samples) on the different substrates. A five fold reduction of the continuous background and a corresponding reduction of the elementary detection limits were observed when thin polymer film substrates were used instead of the common thick Perspex substrate.
2004
Poli,V.S. Tabacniks,M.H. Rizzutto,M.A. Added,N. Espinoza-Quiñones,F.R. Palácio,S.M.
Manufactured silicon diode used as an internal conversion electrons detector
In this work we have studied the direct detection and spectrometric capabilities for internal conversion electrons of an ion-implanted diode, developed in the framework of R&D programs at CERN, envisaging its use in an electron-gamma coincidence system for nuclear parameters measurements. The best energy resolution obtained until now, 12.6 keV (FWHM) for the 133Ba 320.32 keV electron emission, is sufficiently good to justify the use of this diode for spectrometry of low energy electrons.
2004
Corrêa,A. A. S. Bueno,Carmen C. Gonçalves,Josemary A.C. Mendes,P. F. P. Rato Pinto,J. K. C. Souza,J. P. de Santos,M. D. de S.