Experimental Study of the Influence of the Structure of Partners in the Fusion of Nearly Symmetric Systems

Defended in 1998, December 4^th

    The cross-sections for the formation of evaporation residues in ⁷⁰Zn and ⁸⁶Kr induced reactions with ¹⁵⁰Nd and ^130,136Xe isotopes were measured for excitation energies of the compound nuclei ^216,220,222Th varied from 7 MeV, at the linear accelerator UNILAC of the nuclear facility GSI, Darmstadt (Germany).

    After de-excitation by evaporation (xn, pxn and αxn), the residual nuclei are separated from the primary beam and from parasitic reaction products by the velocity filter SHIP and are implanted into a silicon localisation detector. Their subsequently decay via alpha particles with characterised energies allows us to identify them and to deduce their production rate.

    Experimental fusion-evaporation excitation functions are compared with those leading to the same compound nuclei obtained with other projectile and target combinations and with those calculated with a code developed at GSI. This code allows us to evaluate the evolution of the fusion probability as a function of the incident energy for each system. The variation of cross-sections and of the fusion probability is studied as a function of the macroscopic and microscopic variables of the partners.  

    For the synthesis of super-heavy elements, theses results demonstrate quantitatively the interest in using partners of fusion with closed shell structures and rich in neutrons (the fusion cross-section increases by a factor of 9 for a complementary pair of neutrons). On the other hand, closed shell compound nuclei do not influence the fusion cross-section. It will be worth synthesising nuclei with Z greater then 110 with neutron rich projectiles coming from secondary beams, isotopes approaching the predicted stability valley.