342 ANALYSIS OF YIELDS OF FUSION-FISSION AND QUASIFISSION FRAGMENTS IN HEAVY ION COLLISIONS A. K. Nasirov 1,2 , G. Giardina 3 , F. Hanappe 4 , S. Heinz 5 , S. Hofmann 5 , G. Mandaglio 3 , M. Manganaro 3 , A. I. Muminov 2 , W. Scheid 6 1 Joint Institute for Nuclear Research, Dubna, Moscow region, Russia 2 Institute of Nuclear Physics, Tashkent, Uzbekistan 3 INFN, Sezione di Catania, and Dipartimento di Fisica dell’Università di Messina, Italy 4 Université Libre de Bruxelles, Bruxelles, Belgium 5 Gesellschaft für Schwerionenforschung, Darmstadt, Germany 6 Institut für Theoretische Physik der Justus-Liebig-Universität, Giessen, Germany The decrease of the evaporation residues yield in reactions with massive nuclei is explained by increase of the competition between quasifission and complete fusion processes and by the decrease of the survival probability of the heated and rotating compound nucleus against fission. The experimental data on the yields of evaporation residues, fusion-fission and quasifission fragments in the 48 Ca + 154 Sm reaction are analyzed simultaneously in the framework of the theoretical method based on the DNS concept and advanced statistical model. The measured yields of evaporation residues and fission fragments for the 48 Ca + 154 Sm reaction have been well reproduced by using the partial fusion and quasifission cross sections obtained in the DNS model. Such way of calculation is used to find optimal conditions for the synthesis of the new element Z = 120 (A = 302).We compare the excitation functions of evaporation residues of the three reactions 54 Cr + 248 Cm, 58 Fe + 244 Pu, and 64 Ni + 238 U. Our estimations show that the 54 Cr+ 248 Cm reaction is preferable in comparison with the two others because the excitation function of the evaporation residues is some orders of magnitude higher and the optimal energy for the synthesis is lower than that for the 58 Fe + 244 Pu and 64 Ni + 238 U reactions. 1. Introduction The correct estimation of the fusion probability is not an easy task for the reactions with massive nuclei. Different theoretical models use different assumptions about the fusion process they can give different cross sections. The experimental methods of estimating the fusion probability depend on the unambiguity of identification of the complete fusion reaction products. The problem is that quasifission fragments can be considered as fusion-fission fragments when there is overlap of their mass (charge) and angular distributions. As a result the complete fusion cross sections may be overestimated. We know that quasifission fragments show anisotropic angular distributions [1-2] and this is a way to separate them from the fusion-fission fragments which should have isotropic angular distribution. But fission fragments in reactions with heavy ions also show anisotropic angular distributions which is explained by the assumption that an equilibrium K-distribution is not reached (K is the projection of the total spin of the compound nucleus on its axial symmetry axis). Our recent calculations have shown that, at some values of the orbital angular momentum, the angular distribution of quasifission fragments may be isotropic. This is discussed in Section 2. As example, we consider the 48 Ca + 154 Sm reaction which was experimentally studied in detail in Ref. [3]. In Section 3, we present results of estimation of the evaporation residue yields to find the preferable reactions for synthesizing the superheavy element Z = 120. 2. About overlaps of the fusion-fission, quasifission and fast-fission fragments distributions All reaction channels with the full momentum transfer take place through the stage of the dinuclear system (DNS) formation and can be called capture reactions. The formation of the compound nucleus (CN) in reactions with massive nuclei has a hindrance: not all of the dinuclear systems formed at capture of the projectile by the target-nucleus can be transformed into CN. The decay of the DNS into two fragments without passing the stage of the CN formation we call quasifission. In the fast-fission reactions a mononucleus is formed with very large angular momentum L CN and its fission barrier disappears. Therefore, the fast rotating mononucleus goes immediately to fission forming two fragments without reaching of the equilibrium stage of a CN. Fig. 1 taken from Ref. [3] shows the overlap of the measured mass distributions of the 48 Ca+ 154 Sm reaction products in the mass range 55 < A < 145. Upper panel presents the data for the beam energy corresponding to excitation energy of CN of * CN E = 63 MeV and lower panel corresponds to the excitation energy 49 MeV. In Ref.[3], the measured data approximated by the solid Gaussian lines of Fig.1 are interpreted as fusion-fission products. The variances of the mass distribution of fission fragments were calculated simply by using the saddle point temperature T by formula (1) 2 M σ = (98.1 ± 15.1)T + (0,05 ± 0.01) <I 2 > (1) The large open circles correspond to the symmetric part of the mass-angular distributions, i.e. to the isotropic angular distribution. The calculations performed in the framework of the DNS model [see Refs. 5 - 6] showed that at lower energies the contribution of fusion-fission (dashed line in the lower panel of Fig. 2) to the yield of binary fragments is small in comparison with quasifission. At low energies the projectile-like fragments (A < 55) give a large