First observation of 55 , 56 Zn

In an experiment at the SISSI/LISE3 facility of GANIL, the most proton-rich zinc isotopes 55,56 Zn have been observed for the first time. The experiment was performed using a high-intensity 58 Ni beam at 74.5 MeV/nucleon impinging on a nickel target. The identification of 55,56 Zn opens the way to 54 Zn, a good candidate for two-proton radioactivity according to theoretical predictions. Nuclear structure experiments near the proton drip line represent an important tool to investigate the properties of the atomic nucleus. The mapping of the proton drip line provides a first stringent test for mass models. One of the most exciting new phenomena at the proton drip line is probably the occurence of the two-proton ground-state (2p) decay which has been predicted about 40 years ago [1]. Although considerable experimental and theoretical efforts have been made in order to observe this new radioactivity, no evidence was found up to now. Instead, a three-body break-up regime has been observed in the ground-state decays of 6 Be [2] and 12 O [3]. In β-delayed processes where the emission occurs from excited states, a sequential-emission picture via an intermediate state is able to describe the experimental data (see e.g. [4,5]. For the 2p-decay candidates, the two-proton separation energy is negative. In the case of nuclei, for which one-proton emission is energetically forbidden, the emission of the two protons has to be simultanous. Therefore, there are two possibilities: i) an uncorrelated emission where the two protons occupy the whole phase space or ii) a correlated emission where an angular correlation of the two protons may be observed. According to mass predictions [6,7,8,9], the best candidates for this radioactivity are 45 Fe, 48 Ni, and 54 Zn with predicted half lives in the 1µs-1ms range. 45 Fe and 48 Ni have been observed in recent experiments at GSI [10] and at GANIL [11]. The production rate of 45 Fe at the LISE3 separator [12] of GANIL reaches now a level where spectroscopic studies become feasible to investigate the principal decay modes of the exotic proton-rich nuclei. However, 48 Ni is produced only with 1-2 counts per day, making spectroscopic studies difficult, if not impossible. Therefore, it seems to be very interesting to search for new candidates for this decay mode. In the lighter mass region, the proton drip line has been reached for all even-Z nuclei up to nickel. The search for new 2p …