Lifetime of Heavy Composite Systems Formed by Fusion between Heavy Nuclei

Abstract

Collisions between heavy nuclei at relatively low energy region have attracted strong interests of heavy-ion physicists for 3 different reasons, namely the creation of super heavy elements (SHEs), fission dynamics of very heavy systems, and creation of electron-positron pair due to the strong Coulomb field of the composite heavy nuclei as a verification of the QED process. In these processes, the lifetime of the composite system, created by the fusion of the projectile and the target, which decays eventually by fission is the key issue to understand the underlying reaction mechanisms and to estimate the probability of occurrence of these processes. 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. In the study of fission dynamics of heavy systems including the spontaneous fission and the fusion-fission of heavy composite, the competition of neutron emission between the 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. If a molecule state of, say, U and U is formed and stays sufficiently long time, the binding energy of an 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 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 fusion-fission or deep inelastic processes. To investigate these problems theoretically we use a recently developed constrained molecular dynamics (CoMD) model. 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. ∗Corresponding author. E-mail: maru@hadron02.tokai.jaeri.go.jp. 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.