Anthony MARCHIX - GANIL, France

1. Research background 

    Superheavy nuclei do not exist in the nature, but they are predicted by theory. These nuclei are in a region (proton number is between 114 and 126), where they are unstable with fission according to Liquid Drop Model. Halflifes of these nuclei decrease very quickly with the increase of their masses. Actually, the method used to synthesis these nuclei is the fusion of heavy ions, but synthesis cross-sections are very low, of the order of picobarn. So, the study of these nuclei is an experimental and theoritical challenge due to very low probabilities and due to the fact that physical variables are unknown in this region.
    At Ganil, there is an experimental group which try to search the island of stability by the measurement of the fission time of superheavy nuclei. The experiment is focalised on nuclei Z=114, Z=120 and Z=124, which permit to have an idea on the proton closure shell. It is an original work which need some theoretical tools to explain their observations. So, my PhD subject is to develop these tools, in collaboration with Professor Abe, from Osaka University, who already has been collaborating with my PhD advisor David Boilley.

2. Research methodology

    To synthesis superheavy nuclei, one generally uses heavy-ions reactions. The very rare compound nucleus which comes from the fusion of the two heavy nuclei, evaporates light particles and emits gamma-rays to cool down. In a theoretical point of view, we divide the reaction in three steps :

    1. The approaching phase with the passage of the Coulomb barrier to reach the contact point.
    2. The fusion phase to reach the compound nucleus.

    3. And the desexcitation of the compound nucleus by evaporation of light particles.

    We can separate the fusion and the desexcitation phase due to Bohr’s approximation which stipulates that there is no memory of the initial conditions when we have formed a compound nucleus. So, we can treated separately the fusion phase and the desexcitation one. In our case the fusion phase is divided in two steps, the approaching phase and the fusion phase. In lighter system, the fusion phase correspond to the approaching phase due to the fact that the Coulomb and fusion barrier are mixed up. But for the formation of superheavy nuclei, the contact point is located outside the fusion barrier, so we can treated in two differents ways these two phases. For the approaching phase, we use the “Surface Friction Model” [Gro78] developped by Gross and Kalinowski in the seventies, for the fusion phase we use a multidimensional Langevin code [She02] to calculate the diffusion above the fusion barrier. And finally, for the desexcitation phase, we use a code called “Kewpie2” [Bou04].
    About my work, I study the desexcitation phase and I improve Kewpie2 which takes into account the competition between the evaporation of light particles and fission. Due to very low probabilities, this code does not use classical Monte-Carlo methods, but propose a new scheme for having all the cascade of disintegration of the compound nucleus, which include all the energy spectrum of the daughter nuclei.
    But the main characteristic of Kewpie2 is its time evolution, which permits us to include the dynamics of the reaction, and get the fission time by solving Bateman’s equations. It is very useful to evaluate long fission times that cannot be calculated by Langevin and Fokker-Planck formalism.