Production of evaporation residues in the vicinity of the N =126 closure

Abstract

The shell structure and fission-barrier heights of nuclei involved in the compound- nucleus (CN) de-excitation chains leading to observable evaporation residues (ER) determine essentially fusion-evaporation cross sections for heavy fissile nuclei produced in the vicinity of the N=126 neutron shell in heavy ion (HI) reactions. The entrance-channel effect of quasifission (QF) lowering the fusion cross section in reactions with massive nuclei and the effect of the collective enhancement in the nuclear level density (CENLD), corresponding to the exit channel, should strongly reduce production cross sections for ER and increase the observed fission cross section in the vicinity of N=126. These effects were not evidently manifested in the analysis of cross sections obtained in different CN reactions leading to Po isotopes produced in a wide region of N. This analysis has been performed in the framework of standard statistical model (SSM) approximations. As a whole, production cross sections for neutron deficient Po nuclei produced in HI reactions can be reproduced in the framework of SSM with the reduced liquid-drop fission barriers [1]. We present a similar analysis of ER and fission cross section data obtained in the reactions induced by 3,4He on Pb and Bi targets, which lead to Po and At compound nuclei [2], which proved to be a good test of the preceding results [1]. Such reactions are characterized by much lower angular momenta transferred to a CN that allows to suggest more pronounced manifestation of CENLD effects in the CN decay. These reactions also do not reveal any indications of QF. This simplifies the data analysis and makes it more unambiguous. The results of the analysis of the 3,4He reactions performed in the present work demonstrate the same changes in the fission barrier heights for Po and At nuclei as obtained earlier with HI reactions data [1].