Emission order of light particles and light fragments from hot nuclei followed by binary fission

Emission orders of light charged particles and Li fragments from highly excited fissioning nuclei in the reaction of ⁴⁰Ar+¹⁹⁷Au at E _beam /A=25 MeV have been studied by measuring difference velocity distributions of two correlated particles at small relative angles in coincidence with two fission fragments. By comparing the data with three-body trajectory calculations, we found that high velocity deuterons are emitted prior to high velocity Li fragments but low velocity deuterons are emitted after low velocity Li fragments. On the other hand, no preferential emission was observed among light particles, such as protons and deuterons. Furthermore, the emission orders are found to depend only weakly on the mass asymmetry of fission fragments. Received: 3 June 1997 / Revised version: 23 July 1997     

Simplified universal screened Coulomb potential for ion scattering

Coulomb's interaction of ions is complicated by the presence of electronic shells. A major variety of more or less complex screening functions has been proposed over a time span of about 80 years. A simple universal screening function with only one fitting parameter is presented here.     

Momentum transfer theory of ion transport under the influence of resonant charge transfer collisions: the case of argon and neon ions in parent gases

Transport properties of ion swarms in presence of Resonant Charge Transfer (RCT) collisions are studied using Momentum Transfer Theory (MTT). It was shown that, not surprisingly, RCT collisions may be represented as a special case of elastic scattering. Using the developed MTT we tested a previously available anisotropic set of cross-sections for Ar+Ar ⁺ collisions by making the comparisons with the available data for the transverse diffusion coefficient. We also developed an anisotropic set of Ne+Ne ⁺ integral cross-sections based on the available data for mobility, longitudinal and transverse diffusion. Anisotropic sets of cross-sections are needed for Monte Carlo simulations of ion transport and plasma models. Received 16 June 2002 / Received in final form 2nd August 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: vrhovac@phy.bg.ac.yu RID="b" ID="b"e-mail: zoran@phy.bg.ac.yu     

Collisional damping of giant monopole and quadrupole resonances

Collisional damping widths of giant monopole and quadrupole excitations for ¹²⁰Sn and ²⁰⁸Pb at zero and finite temperatures are calculated within Thomas-Fermi approximation by employing the microscopic in-medium cross-sections of Li and Machleidt and the phenomenological Skyrme and Gogny forces, and are compared with each other. The results for the collisional widths of giant monopole and quadrupole vibrations at zero temperature as a function of the mass number show that the collisional damping of giant monopole vibrations accounts for about 30 - 40% of the observed widths at zero temperature, while for giant quadrupole vibrations it accounts for only 20 - 30% of the observed widths at zero temperature. Received: 9 January 2001 / Accepted: 29 March 2001     

Mössbauer Investigation of Electrodeposited Sn–Zn,Sn–Cr,Sn–Cr–Zn and Fe–Ni–Cr Coatings

⁵⁷Fe and ¹¹⁹Sn CEMS, XRD and electrochemical measurements were used to investigate the effect of the preparation parameters and the components on the structure and phase composition of electrodeposited Fe-Ni-Cr alloys in connection with their corrosion behavior. XRD of the electrodeposits reflect an amorphous-like character. ⁵⁷Fe CEM spectra of Fe-Ni-Cr electrodeposited samples, prepared in a continuous flow plating plastic circulation cell with variation of current density, electrolyte velocity and temperature, can be evaluated as a doublet associated with a highly disordered paramagnetic solid solution phase. This phase was identified earlier in Fe-Ni-Cr electrodeposits that were prepared by another plating method and contained both ferromagnetic and paramagnetic metastable phases [1]. This is the first time that we have succeeded to prepare Fe-Ni-Cr alloys containing only the metastable paramagnetic phase. The effect of the plating parameters on the structure is also analysed by the quadrupole splitting distribution method. ¹¹⁹Sn CEM spectra of all Sn-containing plated alloys show a broad line envelop which can be decomposed at least into two components. One can be associated with β-tin. The other one can be assigned to an alloy phase. The structure and distribution of microenvironments of these phases depends on the plating parameters especially on the parameters of the reverse pulse applied. This revised version was published online in August 2006 with corrections to the Cover Date.     

The directed flow maximum near cs = 0

We investigate the excitation function of quark-gluon plasma formation and of directed in-plane flow of nucleons in the energy range of the BNL-AGS and for the E ^kin _Lab = 40A GeV Pb + Pb collisions performed recently at the CERN-SPS. We employ the three-fluid model with dynamical unification of kinetically equilibrated fluid elements. Within our model with first-order phase transition at high density, droplets of QGP coexisting with hadronic matter are produced already at BNL-AGS energies, E ^kin _Lab≃ 10A GeV. A substantial decrease of the isentropic velocity of sound, however, requires higher energies, E ^kin _Lab≃ 40A GeV. We show the effect on the flow of nucleons in the reaction plane. According to our model calculations, kinematic requirements and EoS effects work hand-in-hand at E ^kin _Lab = 40A GeV to allow the observation of the dropping velocity of sound via an increase of the directed flow around midrapidity as compared to top BNL-AGS energy. Received: 7 May 2000 / Accepted: 2 August 2000     

Gamma-ray spectroscopy of the neutron-rich Ni region through heavy-ion deep-inelastic collisions

Nuclei in the neutron-rich Ni region have been studied by γ-ray spectroscopy. Gamma-rays emitted from isomers, with T _1/2 > 1 ns, produced in heavy-ion deep-inelastic collisions were measured with an isomer-scope. The nuclear structure of the doubly magic ⁶⁸Ni and its neighbor ^69,71Cu is discussed on the basis of the shell model. Future experiments for more neutron-rich Ni nuclei are also viewed. Received: 1 May 2001 / Accepted: 4 December 2001     

Simulation of energy scan of pion interferometry in central Au+Au collisions at relativistic energies

We present a systematic analysis of two-pion interferometry for the central Au+Au collisions at √s_NN=3, 5, 7, 11, 17, 27, 39, 62, 130 and 200 GeV/c with the help of a multiphase transport (AMPT) model. Emission source-size radius parameters R_long, R_out, R_side and the chaotic parameter λ are extracted and compared with the experimental data. Transverse momentum and azimuthal angle dependencies of the HBT radii are also discussed for central Au+Au collisions at 200 GeV/c. The results show that the HBT radii in central collisions do not change much above 7 GeV/c. For central collisions at 200 GeV/c, the radii decrease with the increasing of transverse momentum p_T but are not sensitive to the azimuthal angle. These results provide a theoretical reference for the energy scan program of the RHIC-STAR experiment.     

I. The properties of hot Ca-like fragments from the 40Ca + 40Ca reaction at 35AMeV

The creation of hot Ca-like fragments was investigated in the ⁴⁰Ca + ⁴⁰Ca reaction at 35 AMeV. Using the AMPHORA 4π detector system, the primary projectile-like fragment was reconstructed and its properties were determined. Both primary and secondary distributions are compared with the predictions of a Monte Carlo code describing a heavy-ion collision as a two-step process. Some of the nucleons which are identified as participants in the first step are transferred in the second step to these final states, which correspond on the average to the maximum value of entropy (thermodynamic probability). The model allows for competition between mean-field effects and nucleon-nucleon interactions in the overlap zone of the interacting nuclei. The analysis presented here suggests a thermalized source picture of the decay of the projectile-like fragment. The validity of the reconstruction procedure for projectile-like fragments is discussed. Received: 12 March 2001 / Accepted: 20 June 2001     

(<Emphasis Type="Italic">p</Emphasis>,π<Superscript>±</Superscript>) correlations in central heavy-ion collisions at 1 ÷ 2 AGeV

The proton–charged pion correlated emission is studied in the reactions Au (1.06 AGeV) + Au, Ni (1.06 and 1.93 AGeV) + Ni and Ni (1.97 AGeV) + Cu within the BUU approach. The associated invariant mass distributions are shifted to smaller energies with respect to the free Δ(1232) mass distribution due to kinematical reasons. We find that the existing and partly conflicting experimental data do not allow to draw definite conclusions on the in-medium modification of the Δ(1232). Received: 18 October 1999 / Revised version: 4 February 2000     

Resonant Ionization Laser Ion Source Project at TRIUMF

Resonant laser excitation and ionisation is one of the most successful tools for the selective production of radioactive ion beams (RIB) at on-line mass separator facilities. TRIUMF plans to augment the current ion sources with a resonant ionisation laser ion source (RILIS), to use the high production yields from the target, as shown by the delivery of 3*10⁴/s ¹¹Li ions from a standard target ion source with surface ionisation. The development and installation of TRIUMF's RILIS (TRILIS) is necessary to provide beams of short lived isotopes that conventional ion sources could not produce in sufficient intensity and purity for nuclear-, and nuclear astrophysics- experiments. A laser system consisting of three tunable titanium–sapphire (TiSa) lasers with frequency doubling and tripling was employed to demonstrate first off-line resonance ionisation of Ga, and is being installed for first on-line test and a run on ⁶²Ga in December 2004.     

The effect of hadronization on the instabilities in an expanding parton plasma

The growth rate for instabilities in an expanding parton plasma is investigated by using a quasiparticle transport model including hadronization. The coupled Boltzmann equations for partons and pions with time dependent mean field masses and source terms are solved in the Bjorken boost invariant picture. Hadronization modifies the known instability in the parton plasma created by the mean field in two ways: In the beginning, hadronization increases the rate Γ of instability, but then Γ→ 0 when the hadronization is dominating the time evolution. Received: 11 January 1999     

Weak Interaction Studies Using a Paul Trap

New developments in ion cooling and ion bunching allow the trapping of radioactive ions from a low energy beam, in a very short time scale and with very good efficiency. The performances of Radio Frequency Quadrupole (RFQ) coolers, recently developed in several laboratories in Europe and USA, are now better known. In the present study, a RFQ device will be used to inject ⁶He⁺ ions in a transparent Paul trap. The ion beam will be delivered by the SPIRAL facility at GANIL. The careful measurement of the β-recoil ion coincidence spectrum is sensitive to the angular correlation parameter a, which depends on the coupling constants of the weak hamiltonian. It should be equal to −1/3 if the interaction is only of axial vector type (V-A theory). Deviation from this value would imply a new tensor-like interaction involving a new exchange boson thus introducing new physics beyond the Standard Model. This revised version was published online in July 2006 with corrections to the Cover Date.     

欧洲核子研究中心发现新物质形态——夸克胶子等离子体存在的证据

介绍和评述了欧洲核子研究中心（CERN）的相对论重离子碰撞的重要实验结果及其物理解释。碰撞初期形成了高温高密系统。能量密度达到了产生夸克胶子等离子体（QGP）的阈值,低质量双轻子增长、J／ψ反常压低和奇异粒子增长等实验现象可以用形成了QGP来解释。但目前的实验还不足以证明已经产生了QGP。     

Matter-induced ω→ππ decay

We calculate the width for the ω→ππ decay in nuclear matter in a hadronic model including mesons, nucleons and Δ isobars. We find a substantial width of the longitudinally polarized ω modes, reaching ∼100 MeV for mesons moving suitably fast with respect to the nuclear medium. Received: 7 June 1999 / Revised version: 14 October 1999     

Chiral phase transition at finite temperature in the linear sigma model

We study the chiral phase transition at finite temperature in the linear sigma model by employing a self-consistent Hartree approximation. This approximation is introduced by imposing self-consistency conditions on the effective meson mass equations which are derived from the finite temperature one-loop effective potential. It is shown that in the limit of vanishing pion mass, namely when the chiral symmetry is exact, the phase transition becomes a weak first order accompanying a gap in the order parameter as a function of temperature. This is caused by the long range fluctuations of meson fields whose effective masses become small in the transition region. It is shown, however, that with an explicit chiral symmetry breaking term in the Lagrangian which generates the realistic finite pion mass the transition is smoothed out irrespective of the choice of coupling strength. Recieved: 19 September 1997 / Revised version: 30 October 1997     

Systematic study of mechanical deformation on Fe3Al x Si1−x powders by Mössbauer spectroscopy

The Mössbauer technique has been used to measure hyperfine magnetic fields, isomer shifts and relative areas of ⁵⁷Fe atoms located at various sites in Fe₃Al _x Si_1?x series with x?=?0, 0.3, 0.5, 0.7. Four samples were crushed; then they were annealed for 10 h at 1,023 K and cooled down at 3°/min in order to recover the DO₃ stable phase. Mössbauer studies revealed that annealed samples have a DO₃ structure, whereas deformed samples are partially disordered, with both ordered DO₃ and disordered A2 structures, even though X-rays measurements do not show superstructure peaks. The amount of disordered structure decreases with Si content.     

Present and future experiments with stored exotic nuclei at the FRS-ESR facility

High accuracy mass and lifetime measurements are performed with the combination of the in-flight separator FRS and the cooler-storage ring ESR at GSI. Exotic nuclei are produced via projectile fragmentation and fission at (400–900) MeV/u. This energy range is the basis for unique experimental conditions with bare and few-electron fragments and allows for the first time investigations for decay channels which are rare or strictly forbidden in neutral atoms. New experimental developments and data including the discovery of new isotopes are presented. The comparison of the experimental results with theoretical predictions indicates the potential for improvements of nuclear models. A new generation of experiments will be possible with the advent of the proposed international Facility for Antiproton and Ion Research (FAIR).     

Dipolar interactions in ferromagnetic thin films

We introduce a discrete model describing the motion of a zigzag domain wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field. The main ingredients are dipolar interactions and anisotropy. We investigate the dynamic hysteresis by analyzing the effects of external field frequency on the coercive field by Monte Carlo simulations. Our results are in good agreement with experiments on Fe/GaAs films reported in literature, and we conclude that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation.     

Testing Dirac-Brueckner models in collective flow of heavy-ion collisions

We investigate differential in-plane and out-of-plane flow observables in heavy-ion reactions at intermediate energies from 0.2-2 AGeV within the framework of relativistic BUU transport calculations. The mean field is based on microscopic Dirac-Brueckner-Hartree-Fock (DB) calculations. We apply two different sets of DB predictions, those of ter Haar and Malfliet and more recent ones from the Tübingen group, which are similar in general but differ in details. The latter DB calculations exclude spurious contributions from the negative-energy sector to the mean field which results in a slightly softer equation of state and a less repulsive momentum dependence of the nucleon-nucleus potential at high densities and high momenta. For the application to heavy-ion collisions in both cases non-equilibrium features of the phase space are taken into account on the level of the effective interaction. The systematic comparison to experimental data favours the less repulsive and softer model. Relative to non-relativistic approaches one obtains larger values of the effective nucleon mass. This produces a sufficient amount of repulsion to describe the differential flow data reasonably well. Received: 8 January 2001 / Accepted: 12 November 2001