Search Results (51 total)

The elastic scattering for the ^6,7Li+⁵⁹Co systems was investigated in the bombarding energy range 12  MeV≤E_lab≤30  MeV by means of an analysis using the São Paulo potential, through which the behavior of the real and imaginary parts as function of the bombarding energy was established. The experimental results suggest that overall there is an evidence of the usual threshold anomaly for both systems, although for the ⁶Li+⁵⁹Co system, an evidence of the breakup threshold anomaly could also be questioned.

The inelastic, two-neutron and α transfer and quasielastic cross sections for the ¹⁸O+¹¹⁰Pd system have been measured in the near barrier region (40 MeV ≤E_lab≤58 MeV). The experiments were performed in the São Paulo Pelletron laboratory. Coupled-channel analysis of the experimental data was performed using the São Paulo potential as a microscopic bare interaction. In the calculations, low-lying inelastic excitations, one- and two-neutron and α transfers to the target were considered as the main couplings, with no extra surface absorption. The agreement between the theoretical results and the experimental data is good. The role played by the coupled channels is very different in comparison with similar data analyses for the closed-shell region around ⁵⁸Ni.

Comprehensive high precision complete and incomplete fusion cross sections have been measured for the ⁶Li+²⁰⁹Bi, ⁷Li+²⁰⁹Bi, and ⁹Be+²⁰⁸Pb reactions, at energies near and below the Coulomb barrier. The experimental details and the analyses procedures for the characteristic α-decay and fission measurements are described. Three different methods are used to conclusively show the large suppression of complete fusion at energies around and above the average barrier, which is associated with the projectile nuclei having a low energy threshold against breakup. First, theoretical predictions of fusion cross sections above the average barrier are compared with the data, and second the area under the measured barrier distribution is compared with expectations. The sensitivity of the suppression factors to variables which can affect the calculated cross sections is thoroughly investigated. The third method, essentially model independent, compares the complete fusion cross sections for the ⁷Li+²⁰⁹Bi and ⁹Be+²⁰⁸Pb reactions with those for the fusion of nuclei with a high threshold against breakup, which produce the same compound nucleus. All methods give consistent results, showing that the complete fusion cross sections at energies around and above the barrier are suppressed by ∼30% compared with reactions of nuclei having a high energy threshold against breakup. The cross sections for incomplete fusion are found to be similar to the missing complete fusion cross sections. The experimental controversies regarding the effect of breakup on fusion is discussed, and the importance of unambiguously separating complete fusion from incomplete fusion is emphasized. This distinction is also important to achieve theoretically for realistic modelling of fusion of nuclei which break up readily.

We investigate the influence of the neutron halo and the breakup channel on the total ⁶He+²³⁸U fusion cross section at near-barrier energies. To include static effects of the 2n-halo in ⁶He nuclei, we use a single-folding potential obtained from an appropriate nucleon-²³⁸U interaction and a realistic ⁶He density. Dynamical effects arising from the breakup process are then included through coupled-channel calculations. These calculations suggest that static effects dominate the cross section at energies above the Coulomb barrier, while the coupling to the breakup channel is more important at sub-barrier energies. The comparison of our calculations with recent data suggests that the coupling to other channels may be influencing the cross section at very low energies.

Excitation functions for sub- and near-barrier total (complete + incomplete) fusion cross sections are presented for the ^6,7Li+⁵⁹Co reactions. Evaporation residues were identified by their characteristic γ rays and the corresponding yields measured with both the IReS Garel + array at the Vivitron facility and with the São Paulo Ge array at the 8UD Pelletron tandem facility using standard γ-ray techniques. The data extend to medium-mass systems previous works exploring the coupling effects (hindrance versus enhancement) in fusion reactions of both lighter and heavier systems. The results indicate a small enhancement of total fusion for the more weakly bound ⁶Li at sub-barrier energies, with similar cross sections for both reactions at and above the barrier.

An attempt was made to measure the excitation function of the cross section for the ⁸Li(n,γ)⁹Li reaction by performing the inverse reaction ⁹Li(γ,n)⁸Li, with the equivalent photons in the electric field of nuclei in a Pb target providing the γ rays for the reaction. The energy spectrum of lithium nuclei in coincidence with neutrons had no discernible peak where any beam-velocity ⁸Li’s would be located. Statistically, a Gaussian-shaped ⁸Li peak could have been present with 30±29 counts, which we interpreted as consistent with zero, with a two-standard-deviation upper limit of 87 counts. Using the fact that neutron capture on ⁸Li must be dominantly s-wave capture, and applying detailed balance, we obtained, with E in eV, σ_n,γ<930E^-1/2 μb. The corresponding limit on the astrophysical reaction rate is <790 cm³ mol^-1 s^-1. Theoretical predictions of the reaction rate have exceeded our upper limit by factors of 3–50.

Excitation functions have been measured for the fusion of the weakly bound nuclei ⁶Li and ⁷Li with ²⁰⁹Bi. The complete-fusion cross sections are lower than those predicted by fusion models, being only 65% and 75% for ⁶Li and ⁷Li, respectively. Within the uncertainties, this suppression is independent of beam energy. Distinguishing complete fusion from incomplete fusion, both experimentally and in theoretical models, is essential to understand the fusion process of weakly bound nuclei. A simple classical trajectory model which makes this distinction is presented. Further developments of the concepts of this model could be used for realistic predictions for the fusion of unstable weakly bound nuclei.

Binary decay excitation functions have been measured for the ¹⁶O+¹⁰B system at 22 MeV <E_c.m.<24.5 MeV (δE_c.m.∼20 keV) and 17 MeV<E_c.m.<25 MeV (δE_c.m.∼190 keV) and for the ¹⁹F+¹²C system at 22 MeV<E_c.m.<24.5 MeV (δE_c.m.∼20 keV). The binary fragments were identified by the kinematic coincidence technique. The excitation functions for these systems show strong fluctuations and average decay widths were obtained through the analysis of energy correlation functions. The small widths observed for the ¹⁶O+¹⁰B (about 20 keV) and ¹⁹F+¹²C (about 40 keV) systems show a constant behavior with the fragments’ excitation energy and scattering angle. These widths correspond to an intermediate system time scale of about 2×10^-20 s, which is larger than its revolution time. These features suggest the presence of the fusion-fission mechanism, where the compound nucleus takes a long time to acquire a relaxed shape and thermal equilibrium. For the ¹⁶O+¹⁰B system, larger widths (about 350 keV) were also observed, which are related to a faster process. These widths show a clear dependence with the scattering angle in the elastic scattering channel. These aspects are expected for a process where the time scale is comparable to the intermediate system revolution time, like an orbiting mechanism. Theoretical predictions for fusion-fission and orbiting mechanisms were compared to the experimental results and a good agreement was observed.

The binary decay of very light nuclear systems into the ground state exit channel has been investigated for light heavy-ion collisions at energies ranging from the Coulomb barrier up to ∼4 MeV per nucleon. A systematic analysis of 14 reactions and up to 50 angular distributions allowed the extraction of the energy thresholds (E_ef) for the fission channel forming the final products at the ground state (“elastic fission”). A significant suppression of the contribution of statistical processes is observed when the size of the system is reduced. This effect may be related to a relative increase of the fast and peripheral yields.

Multifragment emission following ¹²⁹Xe+¹⁹⁷Au collisions at 30A, 40A, 50A, and 60A MeV has been studied with multidetector systems covering nearly 4π in solid angle. The correlations of both the intermediate mass fragment and light charged particle multiplicities with the transverse energy are explored. A comparison is made with results from a similar system ¹³⁶Xe+²⁰⁹Bi at 28A MeV. The experimental trends are compared to statistical model predictions.

Energy-integrated reaction cross sections have been measured at energies ranging from 38 to 80 MeV/nucleon for various exotic neutron-rich isotopes of Al, Si, P, S, Cl, Ar, K, Ca, Sc, and Ti stopping in Si. An experimental technique is employed where Si detectors are used for both particle identification and to serve as the target material. The reduced strong absorption radii r₀² are deduced and compared with other experimental results. The radius dependence on the neutron number was studied and a trend of increasing reduced radius with neutron excess was found. This behavior is similar to that seen in lighter systems, although less pronounced than found there. The implications of this result on the conjectured existence of neutron halo or skin nuclei is discussed.

Complete fusion excitation functions for ⁹Be+²⁰⁸Pb have been measured to high precision at near barrier energies. The experimental fusion barrier distribution extracted from these data allows reliable prediction of the expected complete fusion cross sections. However, the measured cross sections are only 68% of those predicted. The large cross sections observed for incomplete fusion products support the interpretation that this suppression of fusion is caused by ⁹Be breaking up into charged fragments before reaching the fusion barrier. Implications for the fusion of radioactive nuclei are discussed.

We reply to the preceding Comment.

Elastic scattering angular distributions for the symmetric ²⁷Al+²⁷Al system were measured at bombarding energies from 50 to 70 MeV. The integrated inelastic cross section for the first two excited states was determined by the in-beam γ-ray spectroscopy method. The data were analyzed by the optical model and by coupled channel calculations. The spin-orbit couplings of the ground states of both nuclei are analyzed.

Two-particle azimuthal correlations have been used to extract reaction plane dispersion free triple-differential cross sections for d, t, and α particles for the midcentral collisions of ⁸⁴Kr+¹⁹⁷Au at E/A=35, 55, and 70 MeV. Both experimental measurements and extrapolations from lower incident energies suggest that rotational flow disappears at E/A≈100 MeV for light charged particles and that reaction plane dispersions introduce large uncertainties in extracting the disappearance of rotational flow.

Reaction cross sections have been measured at energies ranging from 50 to 70 MeV/nucleon for a variety of exotic neutron rich isotopes of Ar, K, Ca, and Sc. A method where Si detectors are used also as targets has been utilized for the measurements. The strong absorption radii r₀² have been determined and compared to the results of Glauber-type calculations which used density distributions extracted from the relativistic mean field theory. The isospin dependence of the radii was investigated and a trend similar to the one for lighter nuclei has been observed. The possible existence of halo or skin nuclei has also been addressed.

Charge and transverse energy distributions for intermediate mass fragments have been extracted for central ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35-400 MeV. The slopes of the measured fragment charge distributions decrease monotonically with incident energy, consistent with the expectations for highly charged systems, but not with recent critical exponent analyses. Statistical model calculations, which reproduce the experimental trends, suggest that post-breakup fragment secondary decays alter significantly the observed charge distributions. Radial expansion velocities extracted from these calculations follow the systematics of Au+Au collisions.

The dependence of intermediate-mass-fragment (IMF) element distributions on the multiplicity, N_IMF, of detected fragments has been measured for ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35, 55, 70, 100, 200, and 400 MeV. The observed dependence can be parametrized as P(N_IMF|Z)∝P(Z)exp(-c⋅N_IMF⋅Z), where c is a beam-energy and excitation-energy dependent parameter. Previous work indicated this parameter is zero in the liquid-gas coexistence region and positive in the gaseous phase. In contrast, we observe both negative and positive values for c, revealing the meaning of this parameter to be less straightforward than previously assumed. The magnitude of c appears nonetheless to provide a nontrivial test of multifragmentation models.

Complete fusion of light radioactive nuclei is predicted to be hindered at near-barrier energies. This feature is investigated in the case of the least bound stable nuclei. Evaporation residues resulting from the ^6,7Li+⁹Be and ^6,7Li+¹²C fusion reactions have been measured in order to study common features in reactions involving light weakly bound nuclei. The experimental excitation functions revealed that the fusion cross section is significantly smaller than the total reaction cross section and also smaller than the fusion cross section expected from the available systematics. A clear correlation between the fusion probability and nucleon (cluster) separation energy has been established. The results suggest that the breakup process has a strong influence on the hindrance of the fusion cross section.

Sidewards directed fragment flow has been extracted for ⁸⁴Kr+¹⁹⁷Au collisions at E/A  =  200 MeV, using techniques that are free of reaction plane dispersion. The fragment flow per nucleon increases with mass, following a thermal or coalescencelike behavior, and attains roughly constant limiting values at 4≤A≤12. Comparisons of the impact parameter dependences of the measured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field.

A new, sensitive method allows one to search for the enhancement of events with nearly equal-sized fragments as predicted by theoretical calculations based on volume or surface instabilities. Simulations have been performed to investigate the sensitivity of the procedure. Experimentally, charge correlations of intermediate mass fragments emitted from heavy ion reactions at intermediate energies have been studied. No evidence for a preferred breakup into equal-sized fragments has been found.

The azimuthal angular correlations of light charged particles and light intermediate mass fragments emitted from the reaction ³⁶Ar+¹⁹⁷Au at E/A  =  50  MeV are found to be reducible to the angular distributions of individual fragments. Thermal scaling is also observed in the coefficients of the angular correlations. Furthermore, the observed scaling with fragment mass seems to imply secondary emission from relatively small ( A≈15–30) primary fragments.

The charge ( Z) distributions from intermediate energy heavy-ion reactions depend upon the multiplicity n of intermediate mass fragments through a factor of the form e^-cnZ. Experimentally c starts from zero at low values of the transverse energy E_t and reaches a saturation value at high E_t. In a liquid-gas phase diagram c  =  0 for the saturated vapor, while c>0 for the unsaturated vapor. It is suggested that in the c≈0 regime the source evaporates down to a sizable remnant, while for c>0 the source vaporizes completely. Percolation of finite systems and nuclear evaporation portray a behavior similar to that observed experimentally.

A strong thermal signature is found in the charge distributions associated with multifragmentation from the reaction ³⁶Ar+¹⁹⁷Au at E/A  =  110 MeV. The n-fold charge distributions are reducible to the onefold charge distributions through a simple scaling that is dictated by fold number and charge conservation.