B. Bilwes

Energy-dependent renormalization coefficients of folding-model description of 32S+40Ca elastic scattering

Abstract Angular distributions for the elastic scattering of 32 S on 40 Ca have been measured at 32 S bombarding energies of 100, 120 and 151.5 MeV. Optical model analyses were performed using Woods-Saxon and double-folding potentials which yield a good description of the angular distributions. The energy dependence of the phenomenological optical potentials and renormal- ization coefficients of the double-folded potential were found. These results are anomalous in comparison with the elastic scattering of heavy projectiles (with A > 12) on the same target nucleus.

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.

Energy damping and intermediate-velocity fragment emission in peripheral Kr+Au collisions at 43 MeV/u☆

Abstract Triple and four-fold coincidences among fragments have been measured in the reaction 84 Kr+Au at 43 MeV/u. All events showing the projectile-like nucleus and fission fragments of the target-like nucleus, and all events with one additional intermediate-velocity fragment, were analysed in the frame of a dissipative collision and a participant-spectator model. The mechanism is basically that of a dissipative collision but the emission of the intermediate velocity fragment by the target differs from an equilibrated evaporation.

One-nucleon transfer reactions induced by 32S on 34S AT Einc = 90.0 and 97.09 MeV

Abstract The one-proton and one-neutron transfer reactions in the interaction of 34 S with incident 32 S were studied in the 0–3.5 MeV excitation energy range of the exit nuclei. The experimental set-up (kinematic identification technique, implanted targets, etc.) has allowed an angular resolution of 0.2°, a mass resolution of 1.5% and a Q -value resolution of 0.35 MeV. Comparison with DWBA computations using an analytical approximation for the radial integral, with insertion of suitable recoil corrections, demonstrates the absorptive direct mechanisms of the reactions, even when double excitation of residual nuclei is present. The extracted values for the products of spectroscopic factors are in very good agreement with the known values and they are confirmed by EFR-DWBA computations. The selectivities of the reaction receive about the same contributions from the nuclear structure as from the specific reaction mechanism.

The threshold anomaly in the 32S + 40Ca interaction☆

Abstract Angular distributions for elastic scattering of 32 S by 40 Ca, inelastic scattering leading to the first 2 + state in 32 S and the first 3 − state in 40 Ca and one-neutron stripping reactions have been measured at E lab =90 and 110 MeV. Angular distributions for alpha stripping reactions have been measured at E lab =90, 100 and 110 MeV. The data have been analyzed in terms of the double-folding model using the coupled-channel approximation for inelastic scattering. The energy dependence of the renormalization coefficient of the folding potential founded in a previous paper has been confirmed at these new energies and is consistent with the dispersion relation of Mahaux et al . The influence of the coupling of inelastic and transfer channels on the renormalization coefficient has been found to be important. The energy dependence of the potential diminishes with increasing coupling strength.

Folding-model analysis of the inelastic scattering of 32S on 40Ca at 100, 120 and 151.5 MeV

Abstract The angular distributions for the 2 + level of 32 S at 2.24 MeV and 3 − level of 40 Ca at 3.74 MeV have been measured at 100, 120 and 151.5 MeV 32 S bombarding energies and the 3 − level of 32 S at 5.01 MeV at 120 and 151.5 MeV. The levels have been analysed by the coupled-channel approximation using real potentials and form factors deduced from the folding model. It has been found that the energy dependence of the folding-model renormalization coefficient decreases in comparison with that found in the elastic-channel analysis and that an overall energy-independent renormalization coefficient seems to be needed.

Neck formation and decay in Pb + Au collisions at 29 MeV/u☆

Abstract Three fragment production has been studied in peripheral 208 Pb + 197 Au collisions at 29 MeV/u. The data suggests the formation and subsequent decay of necklike structures producing a small fragment emitted at mid-rapidity in between the two partners of a deep inelastic scattering. Data and trajectory calculations suggest also the existence of a competing process in which the neck is absorbed by one of the reactants, this latter decaying further by binary fission.

Complete energy damping in 29 MeV/nucleon Pb+Au two-body final-state reactions☆

Abstract Decay products emitted in highly dissipative Pb+Au reactions at 29 MeV/nucleon have been detected using a large area array. Multiplicities of fragments as large as 8 have been detected with a sizeable cross section. The m -fragment exit channels are fully compatible with the formation of a transient excited dinuclear system formed in damped collisions. The excitation energy function shows that fully damped collisions are achieved even for m = 2 indicating that heavy nuclei are able to sustain high excitation energy and end up as evaporation residues.

Evidence for fast and simultaneous multi-fragment emission in central Kr + Au collisions at 60 MeV/u

Abstract Decay products emitted in Kr+Au collisions at 60 MeV/u have been detected with a large experimental array. Events with fragment multiplicities up to 5 have been recorded. After a careful selection of central collisions leading to equilibrited events, the data are compared to two fragmentation models in which fragments are emitted either sequentially or simultaneously. It is shown that the data can be explained very well by the second model, thus suggesting a prompt multifragmentation mechanism. Possible expansion effects in the disassembly process are also investigated.

Nuclear disassembly time scales using space-time correlations

Abstract The lifetime, τ, with respect to multifragmentation of highly excited nuclei is deduced from the analysis of strongly damped Pb + Au collisions at 29 MeV/u. The method is based on the study of space-time correlations induced by “proximity” effects between fragments emitted by the two primary products of the reaction and gives the time between the reseparation of the two primary products and the subsequent multifragment decay of one partner. A lifetime significantly longer than the interaction time is found. It is compared with time scales corresponding to different types of dynamical instabilities which may be responsible for nuclear disassembly.