J. Kasagi

Disappearance of flow as a function of impact parameter and energy in nucleus-nucleus collisions☆

Abstract The dependence of collective nuclear flow on impact parameter and beam energy from 25 to 85 MeV/u has been measured for Ar+Al with the 4 π array Mur+Tonneau. The flow of the particles emitted from the interaction region falls to zero at a beam energy in the range 70–80 MeV/u for impact parameters below 2 fm. This energy increases with the impact parameter. This study as a function of the impact parameter and the energy should allow information both on the nucleon-nucleon crosss section in medium and the EOS of nuclear matter to be obtained.

Intruder deformed bands in 110Sn and 112Sn

Abstract Results from gamma-ray spectroscopy studies using five BGO anti-Compton spectrometers reveal the existence of distinct deformed bands on the high-spin region of 110Sn and 112Sn. The bands are identified as those of the proton intruder configuration. A striking similarity of the band structure between 112Sn and 116Xe is shown not only for the g.s. band but also for the v(h 11 2 ) 2 aligned band and for the negative-parity band.

Excitation energy dependence of the giant dipole resonance in hot nuclei

Abstract High energy γ-rays were measured with a large BaF2 detector in coincidence with fusion residues produced in the 92 Mo+ 40 Ar reactions at E A=21 and 26 MeV . The obtained γ-ray spectra were compared with statistical calculations for γ-ray transitions from highly excited nuclei. Contributions of bremsstrahlung in the early stages of the reaction were subtracted. The evolution of the GDR γ-ray yields and the shape of the γ-ray spectra are well reproduced by calculations in which the width of the GDR increases with the excitation energy.

Global variables and impact parameter determination in nucleus-nucleus collisions below 100 MeV/u

Abstract Several global variables are tested for determining the impact parameter in 4π measurements of charged products. For the system Ar + Al from 25 to 85 MeV/u, simulated reactions are used to study the correlation between the real impact parameter value and the experimentally determined one. Even with a perfect detector, the total multiplicity and the mid-rapidity charge give a poor correlation. Filtering through the acceptance of an actual detector (Mur + Tonneau at GANIL) provides a poor correlation with the total charge, and an acceptable one with the total perpendicular momentum. The average (mass weighted) parallel velocity V av gives the best correlation and is less sensitive to the incident energy and to the mechanisms used in the simulation. Measurements extending to very forward angles are needed to accurately sort events by impact parameter. Real data are used to study the variation of other global variables versus V av : they exhibit the expected average trend, with broad fluctuations. The eccentricity and flow angle allow the evolution of central reactions with the incident energy to be followed. The proportion of relative motion transformed into energy perpendicular to the beam direction decreases when the incident energy increases; its absolute value, however, doubles from 25 to 65 MeV/u. In experimental studies where good separation between participant (pre-equilibrium) and spectator (equilibrium) particles is wanted in addition to good impact parameter sorting, the projectile mass should be in the range 1.2–2.5 times the target mass.

Components of collective flow and azimuthal distributions in 40Ar + 27Al and 40Ar + 58Ni collisions below 85 MeV/u☆

The transverse and longitudinal components of collective flow have been measured for the nuclear reactions 27Al(40Ar, X), E = 25–85 MeV/nucleon and Ni(40Ar, X), E = 36 and 65 MeV/u with the 4π array Mur+Tonneau. Monte Carlo simulations show that, even for peripheral reactions, the transverse-momentum analysis method gives a better determination of the reaction plane than the method which uses only the projectile-like fragments. The measured in-plane transverse-flow values reach −30 MeV/c at low energies for Z = 2 particles and tends to zero around 85 MeV/u. Flow-angle values are in the range −65° to −35°. The azimuthal distributions of the measured charged products show that there is no squeeze-out of nuclear matter, but the reaction system has a rotation-like behaviour which increases with the impact-parameter value.

Strong impact parameter dependence of pre-equilibrium particle emission in nucleus-nucleus reactions at intermediate energies☆

Abstract Charged particles and fragments emitted in reactions between 40 Ar at 45 and 65 MeV/u and an 27 Al target have been detected in a geometry close to 4π in the center of mass. A new global variable, the average parallel velocity, has been used to sort the events as a function of the impact parameter value. For particles with Z = 1 and 2, a pre-equilibrium component is present. Its multiplicity increases strongly when the impact parameter value decreases, and reaches 7 in head-on reactions.

Excitation-energy and Spin Dependence of the Giant-dipole Resonance At Finite Temperature

In order to investigate the evolution of the GDR from low to high excitation energies, a series of experiments, in which high energy gamma-rays were measured in heavy ion fusion reactions, have been performed at JAERI and at RIKEN. It has been confirmed that the width of the GDR increases rapidly for Ex greater-than-or-equal-to 100 MeV and our previously introduced GAMMA(E(x)) can explain the data well for 80 less-than-or-equal-to E(x) less-than-or-equal-to 120 MeV. The limiting temperature of the nucleus for existing of the GDR has been deduced for A approximately 70, 105 and 130. The GDR seems to disappear at the temperature of about a quarter of the resonance energy.

Incomplete fusion in nucleus-nucleus central collisions. Study of 40Ar on 27Al from 25 to 85 MeV/u☆

Abstract Light charged particles and fragments emitted in collisions between 40Ar between 25 and 85 MeV/u and 27Al target have been detected in a geometry close to 4π in the center of mass. An event by event analysis based on the use of the momentum sphericity tensor allowed to identify a significant fraction of the central collisions. The data show that an incomplete fusion nucleus is formed and decays via isotropic emission at all incident energies, although no heavy residue is left above 36 MeV/u. Fusion is less and less complete as the incident energy increases. Nevertheless, an increase in the multiplicity of charged particles shows that the average excitation energy increases.

Exclusive study of nucleus-nucleus reactions at intermediate energies: Impact parameter dependence of pre-equilibrium emission, Collective flow and hot nuclei formation

Abstract Charged particles and fragments emitted in reactions between 40 Ar at energies ranging from 25 to 85 MeV/u and an 27 Al target have been detected in a geometry close to 4π in the center of mass with the 4π array MUR + TONNEAU. A new global variable, the average parallel velocity, has been used to sort the events as a function of the impact parameter value. The multiplicity of particles emitted from the interaction region increases strongly when the impact parameter value decreases, and reaches 7 in head-on reactions. The flow of these particles is attributed to scattering at negative angles. When the energy increases, compression gradually opposes this negative scattering, until the flow falls to zero. This is obtained at a beam energy in the range 70–80 MeV/u for impact parameters below 2 fm and increases with the impact parameter. This study as a function of the impact parameter and the energy should allow information both on the nucleon-nucleon cross section in medium and the EOS of nuclear matter to be obtained. In central reactions, incomplete fusion nuclei are formed at all incident energies. Their excitation energy increases with the incident energy. Above 36 MeV/u no heavy residue is left, the multiplicity of final products increases as well as the emission probability of several heavy fragments.

Excitation of discrete particle-unbound states in heavy-ion collisions

Abstract Neutron spectra in coincidence with fragments have been observed in 14 N- 165 Ho collisions at 490 MeV. Contributions to the spectra from excited light fragments are very pronounced when the fragments detector and the neutron detector are in colinear geomwtry. Relative velocity spectra then reveal that neutron decay of a single, well-known excited state dominates. In inclusive spectra of neutrons or protons these effects are still present and may not be disregarded.