First Observation of ${}^{54}\mathrm{Zn}$ and its Decay by Two-Proton Emission

The nucleus ${}^{54}\mathrm{Zn}$ has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasifragmentation of a ${}^{58}\mathrm{Ni}$ beam at $74.5\mathrm{MeV}/\mathrm{\text{nucleon}}$ in a ${}^{\mathrm{nat}}\mathrm{Ni}$ target. The fragments were analyzed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight ${}^{54}\mathrm{Zn}$ implantation events were observed. From the correlated decay events, the half-life of ${}^{54}\mathrm{Zn}$ is determined to be ${3.2}_{-0.8}^{+1.8}\mathrm{ms}$. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.

Figure 1

Two-dimensional identification plot for the setting optimized on ${}^{54}\mathrm{Zn}$. Plotted is the energy loss in the first silicon detector as a function of the time of flight between one micro-channel plate detector and the first silicon detector. Eight ${}^{54}\mathrm{Zn}$ events are identified. This figure presents only runs where a ${}^{54}\mathrm{Zn}$ nucleus was observed.

Figure 2

Decay energy spectrum for the first decay events in the same pixel after an implantation of ${}^{54}\mathrm{Zn}$. A decay energy of 1.48(2) MeV is determined from the seven events in the peak. The eighth decay event has a decay energy of 4.19 MeV. The inset shows the decay energy spectrum of ${}^{52}\mathrm{Ni}$ with the high-energy tails of the protons groups due to $\beta$ pileup.

Figure 3

The half-life of ${}^{54}\mathrm{Zn}$ is determined by means of the decay-time distribution of the first decay events after ${}^{54}\mathrm{Zn}$ implantation. The half-life determined is given in the figure.

Figure 4

Relation between half-life/decay width and decay energy as determined by the three-body model of Grigorenko et al. [13]. The picture compares the experimental datum to the three-body model (solid lines) with two different assumptions for the proton emitting orbital and with the di-proton model (dashed lines) for two different channel radii.