C. Lebrun

A plastic multidetector for light nuclei identification at GANIL

Abstract A multidetector consisting of 96 plastic scintillators is now in operation at GANIL. It can identify in charge all fast light nuclei emitted between 3° and 30°.

Influence of angular momentum on the mass distribution width of heavy ion induced fission: What is the frontier between fission and quasi-fission?

Abstract The 124 and 206 MeV 20 Ne+ nat Re and the 192 and 297 MeV 40 Ar+ 165 Ho systems have been used to form the 205 At nucleus with different angular momentum populations. The mass distributions of the fission fragments have been measured. Removing the effect of the temperature, it has been found that their width strongly increases with angular momentum especially when the fission barrier has vanished.

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.

Tonneau: a multidetector array for charged-particle and light-fragment 4π detection☆

Abstract The central part of the light-particle detection unit for the GANIL multidetector is a spherical (80 cm radius) shell of 2 mm thick NE 102A plastic scintillator. It spans the range of polar angles from θ = 30° − 150° and the azimuthal range θ = 0° − 360°, and it is segment into 72 individual Δθ = 60° and Δθ = 10° wide detection modules. Each module is viewed by a photomultiplier at each end which provide information on the particle impact position, atomic number Z and velocity. The performance of the modules was studied with an α-source and during several experiments with heavy-ion beams from 25 up to 85 MeV/amu.

Nucleon-induced reactions at intermediate energies: New data at 96 MeV and theoretical status

Double-differential cross sections for light charged particle production (up to A=4) were measured in 96 MeV neutron-induced reactions, at the TSL Laboratory Cyclotron in Uppsala (Sweden). Measurements for three targets, Fe, Pb, and U, were performed using two independent devices, SCANDAL and MEDLEY. The data were recorded with low-energy thresholds and for a wide angular range (20 deg. -160 deg. ). The normalization procedure used to extract the cross sections is based on the np elastic scattering reaction that we measured and for which we present experimental results. A good control of the systematic uncertainties affecting the results is achieved. Calculations using the exciton model are reported. Two different theoretical approaches proposed to improve its predictive power regarding the complex particle emission are tested. The capabilities of each approach is illustrated by comparison with the 96 MeV data that we measured, and with other experimental results available in the literature.

A facility for measurements of nuclear cross sections for fast neutron cancer therapy

A facility for measurements of neutron-induced double-differential light-ion production cross-sections, for application within, e.g., fast neutron cancer therapy, is described. The central detectio ...

Charged-particle calorimetry of 40Ar + 27Al reactions from 36 to 65 MeV/u☆

Abstract Temperatures and excitation energies have been independently determined for hot nuclei formed in the 40Ar + 27Al reaction at energies ranging from 36 to 65 MeV/u. Charged products have been measured in a geometry close to 4π in the center of mass with the multidetectors MUR and TONNEAU. Events were sorted as a function of the impact parameter and the products emitted from equilibrated nuclei were separated from both pre-equilibrium and target-like products. Temperature (slope parameter) and excitation energy values were deduced from the kinetic energies of particles in the frame of the reconstructed equilibrated nucleus. Both have been found to increase with decreasing impact parameter. For the most central collisions impact parameters — less than 2 fm — the temperature was seen to increase with incident energy (slowly above 45 MeV/u) and reach a value of 7 MeV at 65 MeV/u. The excitation energy per nucleon exhibited a similar behaviour. The correlation between excitation energy and temperature was found to remain compatible with the statistical theory.

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.