Investigation of the di-nuclear system U+U

In heavy ion collisions at Coulomb barrier energies the two collision partners can be brought into nuclear contact. During their closest approach the two nuclei form a dinuclear system (nuclear molecule) which is seen by the electrons as a common charge centre. The lifetime of the di-nuclear system (nuclear contact time) is correlated with the energy dissipation during the collision. While the contact times in quasi-elastic collisions are 10 to 10 s, for collisions with high energy dissipation one should expect lifetimes which are 10 – 100 times longer. Correlated with these long contact times is a large mass flow between the two nuclei which can lead to the exchange of up to 20 protons and as many neutrons as was shown in previous experiments for different collision systems up to U+U [1,2]. In April 2006 we started new investigations of the system U+U at the VAMOS spectrometer of GANIL, which have mainly two objectives: a) Due to the high mass flow expected during nuclear contact, two reaction products can emerge. One has a proton number below uranium and the other one the complementary proton number beyond uranium. In this case we could obtain neutron rich nuclei in the transuranium region, which are otherwise experimentally not accessible. b) If we can observe nuclear contact times of at least 10 s in very heavy collision systems like U+U, the respective reactions could be used as a "trigger" for a fundamental, experimentally hitherto unobserved phenomenon: The decay of an overcritical vacuum in a static electric field by the emission of e+epairs.