Formation and decay of super heavy systems

We investigate the formation and the decay of heavy systems which are above the fission barrier. By using a microscopic simulation of constraint molecular dynamics (CoMD) on Au+Au collision, we observe composite states stay for very long time before decaying by fission. The typical reaction mechanisms of heavy-ion collisions at lower incident energy are, depending on the energy and impact parameters, complete fusion, incomplete fusion, fusion-fission, molecular resonance, and deep inelastic collisions. Among the huge amount of studies in this field, collisions of very heavy nuclei have been investigated mainly for the creation of super heavy element (SHE). SHEs are produced in two ways: one is “cold fusion” which is complete fusion below the classical barrier, and the other is “hot fusion” which allows several neutrons to be emitted. Even though the name is “hot”, such reactions are still at very low energy near the barrier and the total mass number is very close to the aimed one. As far as the formation of SHE is concerned, the “fusion” of very heavy nuclei where the fission barrier no more exists is found to be ineffective [1,2]. Apart from the formation of SHE, the study of fission dynamics, including the spontaneous fission and the fusion-fission of heavy composite, has been one of the most important subjects. The competition of neutron emission between the Email addresses: [email protected] (Toshiki Maruyama), [email protected] (Aldo Bonasera), [email protected] (Massimo Papa), [email protected] (Satoshi Chiba). Preprint submitted to Elsevier Science 8 February 2008 fission and the fission delay have been discussed intensively. However almost all the discussion are done for mass regions where the classical fission barrier exists. Sometime ago many physicists paid attention to the low energy collision of very heavy nuclei in regard to the spontaneous positron emission from strong electric fields [3]. If a molecule state of, say, U and U is formed and stays sufficiently long time, the binding energy of a electron can exceed the electron mass and might create electron-positron pair by a static QED process. Unfortunately no clear evidence of static positron creation was observed below Coulomb energy region. They have pointed out [4] the importance of nuclear reaction which causes the time delay of separation of two nuclei. Although there increases the background component of positrons from nuclear excitation, which in this case is not interested in, the electron-positron from the static QED process is also expected to increase. However, the reaction mechanism of very heavy nuclei has not been discussed by fully dynamical models. In this paper we discuss the possibility of molecule-like states of heavy nuclei and the time scale of very heavy composite system formed by the fusionfission or deep inelastic processes. To investigate these problems theoretically we use a recently developed constraint molecular dynamics (CoMD) model [5]. This model has been proposed to include the Fermionic nature of constituent nucleons by a constraint that the phase space distribution should always satisfy the condition f ≤ 1. Among similar molecular dynamics models, there are quantum molecular dynamics (QMD) [6], Fermionic molecular dynamics (FMD) [7], and antisymmetrized molecular dynamics (AMD) [8]. QMD has been the most popular and feasible model. Unfortunately it cannot, in principle, deal with the Fermionic nature of nuclear system, although sometime the phenomenological Pauli potential is introduced for such a purpose. Therefore QMD model has been used mainly for higher energy phenomena except for some exceptions [9,10]. More sophisticated models, i.e. FMD and AMD, deal with antisymmetrization of the wave function and have succeeded in describing nuclear reactions at medium low energy and also in the study of nuclear structures. However, due to the four dimensional matrix element of two-body interaction, the CPU time necessary to work out calculations for systems with total mass larger than 200 is very large for practical studies. The constraint molecular dynamics, on the other hand, can deal to a certain extent with the Fermionic nature of the nuclear systems and it is still feasible for heavy systems. In this paper we apply CoMD to the investigation of Au+Au collisions at low energies where fusion-fission or deep-inelastic process may occur. In the following we give a brief review of the model [5]. The CoMD model mainly consists of two parts: classical equation of motion of