On the Mechanism of Multinucleon Transfer Reactions between Complex Nuclei

Abstract

997A model for multinucleon transfer reactions is suggested, according to which the main contribution to the reaction is due to collisions with orbital angular momenta lying between the critical angular momentum for formation of a compound nucleus and the angular momentum of the grazing trajectory. It is shown that the time of such collisions is comparable to the time of flight of the projectile in the field of the target but much longer than the time of nuclear orbiting. An expression is derived for the reaction amplitude on the assumption that different partial waves are allowed to interfere as in the case of a direct reaction and that the partial wave cross sections are proportional to the level density of the given reaction product. The energy spectra calculated with this model are found to agree with the experimental spectra of products of the interaction of 145 MeV "N with 282 Th in those cases in which many nucleons are transferred from the projectile to the target. § l. IntroductionThe assumption that multinucleon transfer reactions between two complex nuclei proceed through the stage of formation of a compound nucleus leads to a satisfactory description of the Gaussian distribution of the masses of the reaction products/> the exponential dependence of the cross sections of these reactions on the Q value of the ground-state-ground-state transition 2 > and the enhancement of the proton-stripping over proton-pickup cross sections. 8 > On the othe~ hand, the calculation of the energy spectra of heavy-ion reaction products by means of the statistical theory of the compound nucleus reactions yields curves which are much narrower than the corresponding experimental curves.'> Moreover, the angular distributions of the products of these reactions show the maxima at the grazing angles as well as forward peaks and are small at backward angles. 5 > This could be considered as an indication that multinucleon transfer reactions are direct and fast reactions. However, the standard distorted wave theory of direct reactions normally explains a simple dependence on the actual Q value of the reactions and not on the Q value of the ground-state-ground-state transitions. Siemens et al. 6 l suggested that such unusual dependence on the ground-state Q value is a, result of the kinematics of the reaction, and claimed that a modified version of the distorted-wave Born approximation in which the