Directed and elliptic flows in 40Ca + 40Ca and 112Sn + 112Sn collisions

The directed and elliptic flows for different light particles and fragments in collisions of ⁴⁰Ca + ⁴⁰Ca and ¹¹²Sn + ¹¹²Sn at energies from 30MeV/nucleon to 100MeV/nucleon were studied in the isospin-dependent quantum molecule dynamics model (IQMD). With increasing incident energy, the directed flow rises from negative to positive, while the elliptic flow decreases with increasing the incident energies. The directed flow for the ⁴⁰Ca + ⁴⁰Ca system is not sensitive to the nuclear equation of states (EOS), but the directed flow for the ¹¹²Sn + ¹¹²Sn system is sensitive to the EOS. However, the elliptic flows for both ⁴⁰Ca + ⁴⁰Ca and ¹¹²Sn + ¹¹²Sn systems are not sensitive to EOS. A study of the dependence of directed and elliptic flows on the fragment charge (mass) is also performed. Received: 15 March 2002 / Accepted: 14 June 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: Zhanghy@sinr.ac.cn; Permanent address: Shanghai Institute of Nuclear Research, Chinese Academy of Sciences, P.O. Box 800-204(2), Shanghai 201800, China. Communicated by W. Henning     

Nuclear matter approach to the heavy-ion optical potential: (II). Imaginary part

The heavy-ion optical potentials are constructed in a nuclear matter approach, for the ¹⁶O + ¹⁶O, ⁴⁰Ca + ¹⁶O and ⁴⁰Ca + ⁴⁰Ca elastic scattering at the incident energies per nucleon E^lab/A ? 45 MeV. The energy density formalism is employed assuming that the complex energy density of colliding heavy ions is a functional of the nucleon density ?(r), the intrinsic kinetic energy density τ⁽²⁾(r) and the average momentum of relative motion per nucleon K_r(≦ 1.5 fm^?1). The complex energy density is numerically evaluated for the two units of colliding nuclear matter with the same values of ρ, τ⁽²⁾ and K_r. The Bethe-Goldstone equation is solved for the corresponding Fermi distribution in momentum space using the Reid soft-core interaction. The “self-consistent” single-particle potential for unoccupied states which is continuous at the Fermi surface plays a crucial role to produce the imaginary part. It is found that the calculated optical potentials become more attractive and absorptive with increasing incident energy. The elastic scattering and the reaction cross sections are in fair agreement with the experimental data.     

核子-核子碰撞截面对同位素标度参数α的介质效应

核子能量在40—60MeV能区范围,对在两对重离子中心碰撞系统⁴⁰Ca+⁴⁸Ca和⁶⁰Ca+⁴⁸Ca以及¹¹²Sn+¹¹²Sn和¹²⁴Sn+¹²⁴Sn 的反应中就同位素标度参数α对于核子-核子碰撞截面σ^med_NN(α_m)的介质 关键词： 同位素标度行为 介质效应 核子-核子碰撞截面 重离子碰撞     

Light charged particles associated with subthreshold neutral pion emission in the16O+27Al reaction

The production ofZ=1 andZ=2 particles associated with neutral pion emission in the¹⁶O+²⁷Al reaction at 94 MeV/nucleon has been studied. Results are compared with previous findings obtained by charged pions in the same collision at the same bombarding energy and with the prediction of a dynamical model based on a numerical solution of the Boltzmann-Nordheim-Vlasov equation.     

Evidence for the temporary existence of a “composite” system in deep inelastic nuclear interactions

Masses and charges have been identified for the light fragments (640Ar+⁵⁸Ni), (⁴⁰Ar+⁶⁴Ni), (⁴⁰Ca+⁵⁸Ni) and (⁴⁰Ca+⁶⁴Ni). From the study ofN and Z distributions, it has been shown that the ratioN/Z of thecomposite system has a dominant role in the determination of theN/Z of the product. Then there should be at least a charge equilibration during the damped collision.     

Evaporation residue cross sections for 40Ca + 48Ti at 25.0 and 33.9 MeV per nucleon

The fusion and subsequent decay of the system ⁴⁰Ca+⁴⁸Ti has been studied at 25.0 and 33.9 MeV per nucleon bombarding energies. Heavy residues were detected in coincidence with light particles, and evaporation residue cross sections have been extracted. Light particle distributions are well described by a simple kinematical model, and from this the average excitation energies of the system were determined.     

Target fragmentation in proton-nucleus and16O-nucleus reactions at 60 and 200 GeV/nucleon

Target remnants withZ<3 from proton-nucleus and¹⁶O-nucleus reactions at 60 and 200 GeV/nucleon were measured in the angular range from 30° to 160° (?1.7<η<1.3) employing the Plastic Ball detector. The excitation energy of the target spectator matter in central oxygen-induced collisions is found to be high enough to allow for complete disintegration of the target nucleus into fragments withZ<3. The average longitudinal momentum transfer per proton to the target in central collisions is considerably higher in the case of¹⁶O-induced reactions (≈300 MeV/c) than in proton-induced reactions (≈130 MeV/c). The baryon rapidity distributions are roughly in agreement with one-fluid hydrodynamical calculations at 60 GeV/nucleon¹⁶O+Au but are in disagreement at 200 GeV/nucleon, indicating the higher degree of transparency at the higher bombarding energy. Both, the transverse momenta of target spectators and the entropy produced in the target fragmentation region are compared to those attained in head-on collisions of two heavy nuclei at Bevalac energies. They are found to be comparable or do even exceed the values for the participant matter at beam energies of about 1–2 GeV/nucleon.     

Calculation of resonance reduced neutron widths by semiclassical description

Peripheral interactions of¹²C nuclei with protons and carbon nuclei have been investigated in a 2m propane bubble chamber at 4.2 GeV/c per nucleon. Energetic and angular characteristics ofγ-quanta accompanying the production of multicharged fragments withZ _f =5 and 6 have been analyzed. It is shown that in the fragment rest system there is an excess of γ-quanta with ε_γ<40 MeV over the background from known sources (π ⁰→2γ decays, bremsstrahlung and nuclear excitation).     

Temperature and excitation energy of hot nuclei in the reaction of 40Ar+197Au at 25 MeV/nucleon

The coincidence measurements between heavy fission fragments and light charged particles with Z ⩽ 2 were carried out for the ⁴⁰Ar+¹⁹⁷Au reaction at 25 MeV/nucleon, to study the properties of hot nuclei in heavy ion induced reactions. The linear momentum transfers (LMTs) were deduced from the folding angle and the time-of-flight difference between two fission fragments of heavy residues. The relationship of the nuclear temperature (slope parameter of the energy spectrum) and the excitation energy was determined independently from the measurement of the kinetic energy spectra in the frames of the emitting sources and from the LMT analysis. Both the temperature and the excitation energy increase with decreasing impact parameter, which suggests that a plateau temperature of 5.5 MeV is reached at an excitation energy of 3.1 MeV/nucleon. The result was also compared with various statistical models that explain the plateau by the multifragmentation process, where the excitation energy is assumed to be stored in compression and expansion effects.     

Directed and elliptic flow in 197Au+197Au at intermediate energies

Directed and elliptic flow for the ¹⁹⁷Au+¹⁹⁷Au system at incident energies between 40 and 150 MeV per nucleon has been measured using the INDRA 4π multi-detector. For semi-central collisions, the excitation function of elliptic flow shows a transition from in-plane to out-of-plane emission at around 100 MeV per nucleon. The directed flow changes sign at a bombarding energy between 50 and 60 MeV per nucleon and remains negative at lower energies. Molecular dynamics calculations (CHIMERA) indicate sensitivity of the global squeeze-out transition on the σ _NN and demonstrate the importance of angular momentum conservation in transport codes at low energies.     

Emission of X-ray continua by bombardment of thick Al,Si and Ti targets with protons and14N ions

Using thick targets the continuous X-ray spectra arising in the collision systems Al+p (0.5–7 MeV), Si+p (0.6–6 MeV), Si+¹⁴N (6 MeV, 14 MeV), Ti+p(1 MeV) and Ti+¹⁴N (14 MeV) were measured by means of a Si(Li) detector. Total yields were derived and compared with numerical results of calculations for secondary electron and nuclear bremsstrahlung. For Si and Ti the continuous spectra from one and the same target bombarded with protons and¹⁴N ions of equal velocity were investigated. Drastic deviations from theZ ₁ ² scaling have been found. They could be attributed to contributions of MO radiation originating by recoil of target atoms. The influence of characteristic line profiles on the measured spectra is also discussed.     

The effect of the entrance channel on the fission of 216Ra

We have studied mass-energy distributions (MED) of fission fragments using two projectile-target combinations, ¹²C + ²⁰⁴Pb and ⁴⁸Ca + ¹⁶⁸Er, leading to the same compound nucleus ²¹⁶Ra at the excitation energy E*～40 MeV. It has been found that the contribution of the asymmetric mode in the case of the former reaction is 1.5%, and it is 30% in the case of the latter one. We connect such a sharp increase in the yield of asymmetric products in the ⁴⁸Ca + ¹⁶⁸Er reaction with the quasifission process, the MED of which have a clearly expressed shell structure. The characteristics of the fission fragment MED are of such a kind that they can be interpreted by analogy with the low-energy fission of heavy nuclei as a manifestation of an independent mode of nuclear decay which competes with the classical fusion-fission process.     

A three-body calculation of πd scattering

We present a theoretical treatment of the pion-deuteron system, meant specifically for the energy region below 100 MeV, and based on the Faddeev method for three-body scattering. This includes all orders of multiple scattering, two- and three-body unitarity (to a good approximation), nucleon recoil, deuteron d-state and a correct treatment of spin and isospin. For consistency with nuclear physics we treat the nucleons non-relativistically. However, relativistic kinematics are used for the pion. In order to obtain one-dimensional integral equations in the three-body system, we have constructed a set of separable πN t-matrices (with analytic form factors), which fit selected data up to 300 MeV. A comparison is made with existing π⁺d data at 48 MeV. This data tends to favour the Faddeev type of energy dependence for the πN t-matrix in the πd system. This could also be important in low-energy pion-nucleus scattering.     

Synthesis of 292116 in the 248Cm + 48Ca reaction

We report on the observation of the first decay event of the new nuclide ²⁹²116 produced in an experiment devoted to the synthesis of Z=116 nuclei in the ²⁴⁸Cm + ⁴⁸Ca reaction. The implantation of a heavy recoil in the focal-plane detector was followed in 46.9 ms by an α particle with E _α=10.56MeV. The energies and decay times of the descendant nuclei are in agreement with those observed in the decay chains of the even-even isotope ²⁸⁸114, previously produced in the ²⁴⁴Pu + ⁴⁸Ca reaction. Thus, the first α decay should be attributed to the parent nuclide ²⁹²116 produced via the evaporation of four neutrons in the complete fusion of ²⁴⁸Cm and ⁴⁸Ca. The experiment is in progress at Flerov Laboratory for Nuclear Reactions (FLNR, JINR, Dubna).     

Light charged particle emission in40Ar induced reactions on68Zn at 14.6, 19.6 and 35 MeV/nucleon

The light charged particles emitted in the⁴⁰Ar+⁶⁸Zn reaction performed at 14.6, 19.6 and 35 MeV/nucleon have been studied inclusively. The energy spectra have been analysed in terms of preequilibrium emission, moving source and coalescence models. A complete coherence between the present data and those obtained separately from heavy fragment studies is achieved. These data are consistent with the onset of projectile fragmentation between 20 and 35 MeV/nucleon correlated with the formation of a highly excited region of the nuclear system inducing light particle emission. The other sources of light particles can be interpreted as statistical and sequential decay of the quasi projectile.     

Predictions for cross sections of light proton-rich isotopes in the ~(40)Ca + ~9Be reaction

The cross sections for Z=10–19 with isotopes T_z=-3/2 to-5 in the 140A Me V~(40)Ca+~9Be projectile fragmentation reaction have been predicted.An empirical formula based on the correlation between the cross section and average binding energy of an isotope has been adopted to predict the cross section.The binding energies in the AME16,WS4,and the theoretical prediction by the spherical relativistic continuum Hartree-Bogoliubov theory have been used.Meanwhile,the fracs parametrization and the modified statistical abrasion-ablation model are also used to predict the cross sections for the proton-rich isotopes.The predicted cross sections for the T_z=-3 isotopes are close to 10~(-10)mb,which hopefully can be studied experimentally.In addition,based on the predicted cross sections,Z=14 is suggested to be a new magic number in the light proton-rich nuclei with T_z-3/2,for which the phenomenon is much more evident than it is from the average binding energy per nucleon.     

Origin of projectile-like fragments from40Ar +68Zn collisions between 15 and 35 MeV/nucleon: Direct transfer of nucleons and projectile fragmentation

The projectile-like fragments emitted in the⁴⁰Ar +⁶⁸Zn reaction performed at 14.6, 19.6, 27.6 and 35 MeV/nucleon are studied. Their energy spectra and angular distributions have been measured. Velocities, widths of the linear momentum distributions and cross sections have been deduced. The results are discussedi) in terms of transfer of a few nucleons and analysed with a diffractional model. They are consistent, for stripping reactions, with a direct transfer of nucleons and a target excitation yielding multiparticlemultihole configurations,ii) In terms of projectile fragmentation-like process with a modified abrasion model taking into account the energy separation of the participant nucleons. The projectile fragmentation-like process appears above 20 MeV/nucleon and strongly competes with transfer of nucleons at 35 MeV/nucleon. The evolution of the mechanisms with incident energy is discussed on the basis of the reduced widths of the linear momentum distributions and on those of velocities and cross sections.     

Rotation-like behavior as a function of projectile energy and target mass

The azimuthal distributions produced in the intermediate energy⁴⁰Ar-induced reactions are calculated via the nuclear transport theory. The rotational collective motion is observed from the mid-rapidity particles emission. In addition to the investigation on particle emission in the coordinate space, the momentum distributions of emitted particles as a function of azimuthal angle are also discussed. The azimuthal distributions are fitted by performing Legendre polynomial expansion to second order. By incorporating the fluctuation between the estimated and true reaction plane into our calculations, quantitative agreements with the data are obtained for⁴⁰Ar+²⁷Al reactions below 85 MeV/nucleon. It is found that the rotation-like behavior becomes stronger with the increasing of the impact parameter and weaker with the increasing of the projectile energy. For⁴⁰Ar+²⁷Al system at about 85 MeV/nucleon the rotation-like behavior almost vanishes and the mid-rapidity azimuthal distribution tends to be isotropic. For 35 MeV/nucleon⁴⁰Ar-induced reactions the rotation-like behavior becomes weaker and the out-of-plane enhancement of particle emission appears with the increasing of target mass. The possible origins of these target mass dependent azimuthal distributions at mid-rapidity are discussed.     

Isospin splitting of nucleon effective mass and symmetry energy in isotopic nuclear reactions

Within an isospin and momentum dependent transport model, the dynamics of isospin particles(nucleons and light clusters) in Fermi-energy heavy-ion collisions are investigated for constraining the isospin splitting of nucleon effective mass and the symmetry energy at subsaturation densities. The impacts of the isoscalar and isovector parts of the momentum dependent interaction on the emissions of isospin particles are explored, i.e., the mass splittings of m_n~*=m_p~* and m_n~* m_p~*(m_n~* m_p~*). The single and double neutron to proton ratios of free nucleons and light particles are thoroughly investigated in the isotopic nuclear reactions of ~(112)Sn+~(112)Sn and ~(124)Sn+~(124)Sn at incident energies of 50 and 120 MeV/nucleon, respectively. It is found that both the effective mass splitting and symmetry energy impact the kinetic energy spectra of the single ratios, in particular at the high energy tail(larger than 20 Me V). The isospin splitting of nucleon effective mass slightly impacts the double ratio spectra at the energy of 50 MeV/nucleon. A soft symmetry energy with stiffness coefficient of γ_s =0.5 is constrained from the experimental data with the Fermi-energy heavy-ion collisions.     

Isospin effect in peripheral heavy-ion collisions at Fermi energies

The isospin effect in peripheral heavy-ion collisions was thoroughly investigated within the framework of the Lanzhou quantum molecular dynamics (LQMD) transport model. A coalescence approach was used to recognize the primary fragments formed in nucleus-nucleus collisions. The secondary decay process of these fragments was described using the statistical code GEMINI. The production mechanism and isospin effect of the projectile-like and target-like fragments were analyzed using the combined approach. It was found that the isospin migration from the high-isospin density to the low-density matter occurred in the neutron-rich nuclear reactions, i.e., ⁴⁸Ca+²⁰⁸Pb, ⁸⁶Kr+⁴⁸Ca/²⁰⁸Pb/¹²⁴Sn, ¹³⁶Xe+²⁰⁸Pb, ¹²⁴Sn+¹²⁴Sn, and ¹³⁶Xe+¹³⁶Xe. A hard symmetry energy was available for creating the neutron-rich fragments, particularly in the medium-mass region. The isospin effect of the neutron-to-proton (n/p) ratio of the complex fragments was reduced when the secondary decay process was included. However, a soft symmetry energy enhanced the n/p ratio of the light particles, particularly at kinetic energies greater than 15 MeV/nucleon.