Search Results (4 total)

Projectilelike fragments were detected and characterized in terms of A, Z, and energy for the reactions ³⁷Cl on ⁴⁰Ca and ²⁰⁹Bi at E=7.3 MeV/nucleon, and ³⁵Cl on ²⁰⁹Bi at E=15 MeV/nucleon, at angles close to the grazing angle. Mass and charge distributions were generated in the N-Z plane as a function of energy loss, and parametrized in terms of their centroids, variances, and coefficients of correlation. The results are compared to the predictions of two current models based on a stochastic nucleon exchange mechanism. The drifts of the charge and mass centroids for the system ³⁷Cl on ⁴⁰Ca are consistent with a process of mass and charge equilibration mediated by nucleon exchange between the two partners, followed by evaporation. The asymmetric systems show a strong drift toward larger asymmetry, with the production of neutron-rich nuclei. It is concluded that this is the result of a net transfer of protons from the light to the heavy partner, and a net flow of neutrons in the opposite direction. Model predictions fail to reproduce in detail the evolution of the centroids for asymmetric systems. The variances for all systems increase with energy loss, as would be expected from a nucleon exchange mechanism. However, the variances for the reaction ³⁷Cl on ⁴⁰Ca are higher than those expected from that mechanism, and the variances for the reaction ³⁵Cl on ²⁰⁹Bi start decreasing after about 100 MeV of energy loss. The coefficients of correlation indicate that the transfer of nucleons between projectile and target is correlated, as expected from Q-value constraints to the valley of β stability.

Mass, energy, and angular distribution data for fragments with A≥6 produced in intermediate-energy proton- and alpha-particle-induced reactions on ¹²C and ¹⁶O nuclei are compared with the results of an intranuclear cascade calculation followed by deexcitation of the residual nuclei via a Fermi breakup mechanism. In this latter step all possible particle-stable exit-channel states are included in the phase space available for decay. The shapes of the angular distributions are reproduced successfully by the calculation, even at relatively low bombarding energies (e.g., E_α=60 MeV). The energy spectra are also reproduced qualitatively. The isobaric cross sections of fragments heavier than the target nucleus are underpredicted for alpha-particle-induced reactions, whereas the yields of fragments with A=6-8 are overpredicted. The former discrepancy is attributed to the importance of alpha-particle breakup during the cascade while the latter may serve as a measure of the relative importance of statistical multibody breakup mechanisms.

[NUCLEAR REACTIONS Intranuclear cascade, Fermi breakup model, evaporation model; calculated σ(E,θ,A) for ¹²C(α,HI), ¹⁶O(p,HI), ¹²C(p,HI); HI: A=6-19.]

Inclusive mass, energy, and angular distributions of all fragments with A≥6 have been measured for alpha-particle-induced reactions on ¹²C at five energies between 49 and 159 MeV. From these data the evolution of the dominant reaction mechanisms with energy is characterized. Based on analysis of the mean value of the mass distribution, it appears that linear momentum transfer from projectile to target initially increases with beam energy and reaches a maximum at about 30-40 MeV/nucleon. The low-energy data are consistent with compound nucleus formation and simple transfer processes. At higher energies the growth of forward-peaked angular distributions, continuum energy spectra, and the disappearance of two-body states demonstrate the increasing importance of nucleon-nucleon collisions and multibody breakup mechanisms as the velocity of the projectile exceeds the Fermi velocity. The observed total reaction cross section decreases with increasing bombarding energy. Inclusion of these new data into the excitation functions for 6≤A≤11, which are required for nucleosynthesis calculations relevant to the origin of the elements Li, Be, and B, does not significantly alter conclusions based on previously available data.

A global study of the mass, energy, and angular distributions of all products formed in collisions of 180-MeV protons with ²⁷Al is reported. These data are compared with calculations based on intranuclear-cascade-plus-evaporation, preequilibrium hybrid, and semiempirical models. It is found that there is evidence for enhanced energy deposition in nucleon-nucleus collisions relative to predictions of intranuclear cascade calculations. In contrast, preequilibrium calculations produce stronger energy damping, more consistent with observed data.