A. Péghaire

Neutron production in bombardments of thin and thick W, Hg, Pb targets by 0.4, 0.8, 1.2, 1.8 and 2.5 GeV protons

Abstract Neutron experimental data relevant to the design of the target of neutron spallation sources are presented and discussed. The data include the reaction cross-sections for W, Hg and Pb investigated with 0.4, 0.8, 1.2, 1.8 and 2.5 GeV proton beams as well as the neutron production, neutron multiplicity distribution, as determined event per event using a high-efficiency detector. The production as a function of target material is investigated for both thin (with a single reaction) and thick targets (multiple reactions). Comparisons are made with the predictions of a high-energy transport code.

The decay of primary products in binary highly damped 208Pb + 197Au collisions at 29 MeV/u☆

Abstract Events with fragment multiplicities up to eight have been detected with a large detector array in Pb + Au reactions at 29 MeV/u. All collisions show a binary character irrespective of a possible further disassembly of the two highly excited primary partners. For the most violent collisions, dissipative orbiting is observed as well as full energy damping corresponding to excitation energies as high as 6 MeV/u.

Evolution of a spallation reaction: experiment and Monte Carlo simulation

Abstract Reaction cross sections and production cross sections for neutrons, hydrogen, and helium have been measured for 1.2, 1.8 GeV p+Fe, Ni, Ag, Ta, W, Au, Pb and U and are compared with different intra-nuclear-cascade- combined with evaporation-models. Agreement for neutrons and considerable differences for light charged particles are observed between experiment and calculation as well as between different models. The discrepancies are associated with specific deficiencies in the models. The exclusive data measured with two 4π-detectors for neutron and charged particle detection allowed furthermore a systematic comparison of observables characteristic of different stages of the temporal evolution of a spallation reaction: inelastic collision probability, excitation energy distribution, pre-equilibrium emission, and inclusive production cross sections.

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.

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.

Transfer reactions and sequential decays of the projectile-like fragments in the 60 MeV/nucleon 40Ar + natAg, 197Au Reactions☆

Light charged particles and nuclei (1⩽Z⩽8) have been detected in a large forward plastic multidetector (3° < θ <30°) in coincidence with projectile-like fragments (5⩽Z⩽ 19) in the 40Ar+ natAg and 40Ar + 197Au reactions at 60 MeV/nucleon bombarding energy. It is shown that transfer reactions from the projectile to the target are still present at such a high incident energy. Most of the coincident Z = 2 particles are found to be sequentially emitted by excited primary projectile-like fragments. The sequential decay pattern is also observed with the Z = 1 particles, but the main part of those particles probably arise from other mechanisms such as direct and/or pre-equilibrium emission. An attempt is made to derive the excitation energies of the primary projectile-like fragments from these coincident events. The data are not reproduced by the usual models such as the high-energy-abrasion model, the equal-temperature-limit assumption or the equal partition of the excitation energy.

Helium production for 0.8–2.5 GeV proton induced spallation reactions, damage induced in metallic window materials

Production cross-sections for neutrons and charged particles as well as excitation energy distributions in spallation reactions were measured recently by the NESSI-collaboration and have been employed to test different intra nuclear cascade models and the subsequent evaporation. The INCL/GEMINI code, which describes best the experimental data has been employed to calculate the damage cross-sections in Fe and Ta as well as the He/dpa ratio as a function of proton energy. For the same amount of neutron production in a typical target of a spallation neutron source the proton beam induced radiation damage in an Fe window is shown to decrease almost linearly with proton energy. For heavier materials such as Ta a similar decrease of the radiation damage is found only for energies above about 3 GeV.