Fig.1: Comparison for the isotopic chains (Ca, Ni, Sn, <strong>and</strong> Pb) of the HFB pairing gaps n with the odd-even mass staggering deduced from experimental masses (diamond). Fig.2: Comparison for the isotonic chains (N=20, 28, 50, <strong>and</strong> 82) of the HFB pairing gaps p with the odd-even mass staggering deduced from experimental masses (diamond). 69
Evaporation-residue residue cross sections: role of the entrance channel <strong>IPN</strong>O Participation: N. Rowley Collaboration: Nabila Saffi dine-Grar (Department of Physics, University Ferhat Abbas of Sétif, 19000 Algérie) Sections efficaces de résidus d’évaporation : rôle de la voie d’entrée Pour une réaction entre deux ions lourds, la création des résidus d’évaporation est un processus très complexe, surtout pour les systèmes symétriques où la quasi-fission peut intervenir très fortement. L’étude d’une gamme de réactions menant au même noyau composé, permet une séparation approximative des effets de la voie d’entrée (choix du projectile et de la cible) et les effets parvenant des propriétés du noyau composé lui-même. Nous définissons deux distributions de barrière ; une pour la capture (étape initial da la réaction) et, en exploitant la section efficace des résidus d’évaporation, une deuxième pour la création du noyau composé. Une comparaison des deux permet de voir l’importance de la quasi-fission dans les différents systèmes. For heavy systems, the formation of high-Z evaporation residues via heavy-ion capture is a very complex process. In the early stages of the <strong>reaction</strong>, the colliding partners encounter an external Coulomb barrier which they must surmount or penetrate for the <strong>reaction</strong> to proceed. Once the barrier has been crossed, the composite system must evolve to form an equilibrated compound nucleus, though of course there may be pre-equilibrium fission before this state is reached; a systemdependent process referred to as quasifission. Furthermore, if <strong>and</strong> when a compound nucleus is formed, it may decay by true fission, as well as becoming a longer-lived evaporation residue through the emission of low-mass particles (neutrons, protons, alpha-particles...). At sufficiently high angular momentum all of the compound <strong>nuclei</strong> will fission <strong>and</strong> their contribution to the evaporation residue cross section will fall to zero. Even in its first stage, the <strong>reaction</strong> may be further complicated by the presence of a “distribution of Coulomb barriers” arising from the internal <strong>structure</strong>s of the <strong>reaction</strong> partners (principally their highly collective states). This phenomenon will modify the coefficents for crossing the external barriers <strong>and</strong> entering into the composite system. Well above the highest barrier, all of the low partial waves that are capable of surviving fission will be fully transmitted, <strong>and</strong>, in the absence of quasifission, the reduced cross section for evaporationresidue formation at a given compound-nucleus excitation energy, becomes independent of the <strong>reaction</strong> partners [1]. This significantly simplifies the <strong>reaction</strong> dynamics. However, when attempting to optimize compound-nucleus formation (for example in superheavy-element creation) one might choose a lower energy, where transmission is not complete but survival is much greater. We investigate the differences that arise from different choices of target <strong>and</strong> projectile in this lower-energy Fig. 1. A comparison of the experimental barrier distribution ( 96 Zr + 124 Sn <strong>reaction</strong>) for fusion (compound nucleus creation) with the theoretical capture distribution shows the dominant role of quasi-fission for all but the highest barriers. Both curves are normalized to 1 for comparison, but the true fusion probability is around 8%. regime. They essentially come from: 1) the compound nucleus excitation energy at the corresponding barrier, 2) the “inertial parameter” 2MR 2 which determines the energy range over which the evaporationresidue cross section saturates, <strong>and</strong> 3) the entrance-channel barrier distribution due to the intrinsic <strong>structure</strong>s of the reactants. A good underst<strong>and</strong>ing of these effects will facilitate the study of the role of quasi-fission which is also system dependent, being significantly more important for mass-symmetric systems. It may also possess a deformation dependence (the survival probability being greatest for the most “compact” configurations at contact), <strong>and</strong> may be further influenced by closed shells in the target <strong>and</strong>/or projectile (with the fragment-mass drift favouring the closed-shell <strong>structure</strong>s). We derive the correct structural form of the evaporation-residue cross section <strong>and</strong> demonstrate our 70
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