Search

The charge, velocity, and angular distributions of three coincident fragments measured for the system [sup 40]Ar on [sup 232]Th at an incident energy of 31 MeV/nucleon evidence a deep inleastic collision process followed by fission. The estimated total kinetic energy loss is about 600--800 MeV which represents roughly 60--80% damping of the initial kinetic energy.

Angular correlations between fission fragments induced by an 40Ar bombardment of 232Th targets at four energies, 31, 35, 39 and 44 MeV/u, have been measured as well as the energies, velocities and masses of both fragments. The characteristics of peripheral collisions remain about constant in the range studied, whereas the component attributed to the central collisions which shows a saturation in linear momentum transfer at 180 MeV/c per incident nucleon, has a cross section decreasing with bombarding energy which is correlated to an increase of the excitation energy of the fissioning nuclei.

Correlations between target fragments were measured in α- and 14N-induced reactions at 70, 250 and 800 MeV/u incident energies. The reaction mechanism is characterized by the linear momentum transfer and the excitation energy which were deduced from the kinematics and the mass distribution of the fission fragments. By selecting targets lighter than Th (Au and Ho) the yield from peripheral collisions is reduced by the increase in the fission barrier thus allowing events with the highest linear momentum transfer and excitation energy to be favoured. The results show that up to an incident energy of 800 MeV/u hot nuclei are formed which decay via normal binary fission. The linear momentum transfer is essentially constant over the covered energy range, but the excitation energy increases until the total incident energy is greater than 3 GeV. At this energy, independent of the projectile mass the fission probability of the heavy nuclei drops below 50%, while the emission of intermediate-mass fragments increases. The relative velocities between two intermediate-mass fragments exceed strongly the values of binary fission. Monte Carlo calculations show that the relative velocities between these fragments exclude a sequential emission from the recoil nucleus and support a simultaneous breakup mechanism.

Fragment-fragment correlations have been studied in α -induced reactions at incident energies from 70 to 800 MeV/u on Ag, Ho and Au targets. Even at the highest energy binary thermal fission is observed with excitation energies up to 800–900 MeV. In addition, events with mass number around 30 and relative velocities up to twice the values expected for thermal fission occur and are attributed to multifragmentation. The energy deposit rather than the projectile velocity seems to be parameter governing the appearance of the multifragmentation process which competes strongly with fission for imparted energies beyond 800 MeV.

Cross sections for evaporation residues following the complete fusion of the 12 C + 12 C, 14 N + 12 C and 15 N + 12 C systems have been measured with a E − ΔE counter telescope in a wide range of incident energies. They are fairly well reproduced by evaporation calculations based on the statistical theory. The total fusion excitation function of the 12 C + 12 C system shows strong structure, which is compared to the predictions of the reaction cross sections derived from coupled channel calculations and to the integrated inelastic cross sections. Critical angular momenta have been obtained from the fusion cross-section data and these values are discussed in the framework of compound nucleus and entrance channel effects. A striking difference is observed between the fusion cross sections of the 14 N + 12 C and 15 N + 12 C systems and shows the importance of the valence nucleons of colliding ions in the fusion process. A possible interpretation might be the influence of the yrast line of the compound nuclei.

The fusion cross sections of systems leading to s-d shell compound nuclei reveal a strong dependence upon the number of valence nucleons of the colliding ions. This striking feature is discussed in terms of entrance channel and compound nucleus effects.

Angular distributions of levels observed in the 18O(12C, 10Be)20Ne, 26Mg(12C, 11B)27Al, 26Mg(12C, 10Be)28Si and 26Mg(12C, 9Be)29Si reactions have been measured at 46 MeV incident energy; some of them show a strong oscillating pattern. Spectroscopic factors extracted from a DWBA analysis of the (12C, 11B) 1p transfer with the LOLA code, are consistent with those derived from (3He, d). The influence of recoil effects on both the shapes of the angular distributions and the absolute magnitudes of cross sections have been studied for (12C, 11B) and (16O, 15N). The 2p transfer reactions leading to 20Ne and 28Si have been analyzed using realistic microscopic wave functions. Effects of configuration mixings on both the relative and the absolute cross sections are presented. The validity of approximating the 2p wave function by a 0s cluster component is discussed and some nuclear structure information is derived. The 26Mg(12C, 9Be)29Si reaction populates preferentially the same levels as the 28Si(d, p)29Si reaction. A DWBA analysis using a 3He cluster form factor gives relative spectroscopic strengths close to those derived from (d, p) stripping.

Angular correlations and angular distributions of the fission fragments produced in the bombardment of a 232 Th target with protons, deuterons and α-particles in the energy range between 35 and 1000 MeV/nucleon have been measured. From these measurements, the distributions of linear momentum imparted to fissioning nuclei have been deduced in the various energy regimes; dominating reaction mechanisms are classified according to the fraction of the available incident momentum transferred to the target. The experimental results are compared to the predictions of intra-nuclear cascade calculations. An optimum excitation energy supported by the fissioning nuclei could be the dominant limitation to momentum transfer at high incident energies. The angular distributions of the fission fragments were used to extract fission cross sections and upper limits of the angular momentum imparted to the fissioning nuclei.

Angular distributions have been measured for the elastic and inelastic scattering of 64 MeV 16 O ions on 116 Sn targets and analysed through coupled-equation calculations. The one-proton transfer has also been studied and the DWBA analysis gives results consistent with those derived from ( 3 He, d). The two-proton transfer has been analysed in the framework of both DWBA and CCBA; theoretical wave functions for Te isotopes have been tested and the importance of multistep processes is discussed.

Formation and decay properties of composite-like nuclei produced by 58 Ni projectiles bombarding 232 Th target nuclei are investigated at different energies by means of fission fragment angular correlation measurements. Experimental results are compared to the excition model and excitation energies in the fissioning nuclei are deduced. By comparing the present data with the results on other systems, the influences of entrance channel conditions and composite-like nuclei temperatures are discussed to explain the evolution of the high linear momentum transfer region. The coexistence of both massive transfer mmechanism and head-on collisions followed by preequilibrium is suggested.

States in 20 Ne have been studied through the 12 C( 14 N, 6 Li) 20 Ne reaction. Excitation functions have been measured from 20 MeV to 60 MeV in steps of 5 MeV at different angles for 20 Ne states up to 10 MeV excitation energy. States of 24 Mg have been also populated using the 12 C( 14 N, d) 24 Mg reaction; excitation functions of 24 Mg states up to 9 MeV excitation energies as well as angular distributions at 35 MeV bombarding energy have been obtained. Comparisons of data with Hauser-Feshbach calculations show clearly that the compound nucleus mechanism is the main process for both 12 C( 14 N, 6 Li) 20 Ne and 12 C( 14 N, d) 24 Mg reactions. Strong evidence has been provided for inhibition of the 26 Al compound nucleus formation for angular momenta higher than critical values. The location of the yrast line in the 26 Al nucleus is discussed.

Angular distributions for the scattering cross section of 14.6 MeV neutrons on sulphur have been measured by the time-of-flight technique. The measurements included elastically scattered neutrons and two inelastically scattered neutron groups, corresponding respectively to the 2.24 MeV level in S32 and to several levels between 4 and 5 MeV that are not resolved in the present experiment.

Single proton hole states in odd indium isotopes are investigated using the (d, 3 He) reaction on even tin isotopes at 22 MeV bombarding energy. DWBA calculations using the JULIE Code have been performed to assign l p values and to extract spectroscopic factors. A “ J -dependence” in the l p = 1 proton transfers has been observed; distorted wave calculations predict this effect. The level schemes of residual nuclei are discussed.

The (t, α) reactions on even tellurium isotopes ( fA = 122 to 130) have been studied at an incident triton energy of 12 MeV. Alpha angular distributions of transitions leading to states of antimony nuclei have been compared to DWBA calculations. Angular momentum transfers and spectroscopic factors have been obtained. Present results show significant configuration mixing of the proton wave function in the tellurium targets. Proton hole states in Sb isotopes have been located and the two-particle-one-hole interaction is discussed.

Deuteron angular distributions from the (3He, d) reaction on the even Sn isotopes leading to states of the corresponding odd Sb isotopes (A = 113 to 125) have been studied at an incident 3He energy of 18 MeV. DWBA calculations using the JULIE code have been performed to assign lp values and extract spectroscopic factors. On the basis of this analysis, levels were found carrying large fractions of the 1g72, 2d52 and 1h112 proton single-particle strength. The splitting of the 2d32 and 3s12 strength in many levels is more pronounced. Spacings and spectroscopic factors for the low-lying states are in fair agreement with calculations of Kisslinger and Sorensen.

Expérience GANIL; Angular correlations between fission fragments induced by 1760 MeV 40Ar bombarding 197Au and 232Th targets have been measured. Energies, velocities and masses have been obtained for both fragments. The momentum transfer and energy deposited were deduced from an event-by-event analysis. The recoil velocity and total mass spectra show that mass transfer is still a dominating process. Momentum transfer of at least 6.5 GeV/c and 900 MeV excitation energy are deduced.