Recoil study of Ar induced multinucleon transfer reactions: Capture of 1 to 9 charges by rare-earth targets

Abstract

Multinucleon transfer reactions induced with 282 MeV Ar ions and involving the capture by the target of 1 to 9 charges, have been studied. The targets were separated isotopes of rare earths from La to Tb. The experimental technique involved the measurement of the cross sections, angular distributions, and recoil ranges at each angle of the radioactive heavy residual nuclei 151Dy, 150Dy and 149gTb. For a given number y of protons captured, the variation of the cross section versus ΔA, the net gain of nucleons by the target, goes through a maximum for a value ΔA m, which depends on y: an increase of ΔA m, by two units is observed when y increases by one unit. The cross sections corresponding to these maxima decrease by a factor of about 1.5 to 2 when y increases by one unit. The total cross section for the capture of > 4 charges by the target is estimated to be about 60 mb, relative to a total cross section of about 2 b. The c.m. angular distributions dσ/ dθ are peaked backwards, at an angle which varies from about 165° to about 175° as the number of transferred charges is increased from 3 to 7. The energy distributions in the c.m. system correspond to large losses of kinetic energy by the colliding partners. Analysis of these data, under the assumption of a two-body two-step process, gives results in accord with the following description: A “composite system” of projectile and target, with very short lifetime, is formed during the initial collision. Charge equilibrium is reached in this composite system before its scission into two intermediate fragments. Therefore, the intermediate nuclei have a neutron/proton ratio equal to that of the composite system. In a second much slower step, these excited fragments deexcite by nuclear evaporation. In this analysis, the basic assumption is made that most of the excitation energy is taken up by the heavy intermediate nucleus. In support of this hypothesis, it is shown that the inverse assumption, namely that the light nucleus carries a significant part of the excitation energy, leads to conclusions that are inconsistent with other experimental and theoretical studies.