Critical discussion of the concept of critical angular momentum in heavy ion reactions

Abstract

After a presentation of the concept of critical angular momentum, lcrh, and its relation to the compound nucleus cross section, the experimental methods used to determine Rcr are discussed. Criteria for testing the compound nucleus formation are presented. Then a number of experimental results is given, mainly on argon induced reactions which show how lcrℏ strongly depends on the entrance channel and particularly on the bombarding energy. A critical distance of approach, Rcr, is extracted as the relevant parameter limiting complete fusion of two heavy ions. It corresponds to an aproximate formulation for the compound nucleus cross section σCN = πRcr2 (1 − Vcr/E) where Vcr is the Coulomb + nuclear potential at the distance Rcr between the centers. The fission barrier for a rotating nucleus is discussed, as well as the separation between evaporation residues and fission. Finally, the possibility of existence of an inferior critical angular momentum for very heavy projectiles is presented.