Direct Observation of Two Protons in the Decay of ${}^{54}\mathrm{Zn}$

The two protons emitted in the decay of ${}^{54}\mathrm{Zn}$ have been individually observed for the first time in a time projection chamber. The total decay energy and the half-life measured in this work agree with the results obtained in a previous experiment. Angular and energy correlations between the two protons are determined and compared to theoretical distributions of a three-body model. Within the shell model framework, the relative decay probabilities show a strong contribution of the $p^2$ configuration for the two-proton emission. After ${}^{45}\mathrm{Fe}$, the present result on ${}^{54}\mathrm{Zn}$ constitutes only the second case of a direct observation of the ground state two-proton decay of a long-lived isotope.

Figure 1

Left: Decay-time distribution obtained in the decay of ${}^{54}\mathrm{Zn}$, giving rise to a half-life of ${1.59}_{-0.35}^{+0.60}\mathrm{ms}$. Right: Energy spectrum of the ${}^{54}\mathrm{Zn}$ decay events determined with the charge signals of one GEM. The total decay energy $E_{2p}=1.28\pm 0.21\mathrm{MeV}$ is estimated from 11 events in the peak (see text for more details).

Figure 2

Example of energy-loss spectra obtained with the $XY$ strip detector and associated with the implantation of a $2p$ emitter in the TPC. Top: Signals obtained for the implantation of ${}^{54}\mathrm{Zn}$: the ion enters in the chamber and stops at a given ($X_0,Y_0$) depth. The arrows indicate the direction of the incoming beam. The solid line is the fit of the implantation profile whereas the vertical line indicates the determined implantation position. Middle: The decay of ${}^{54}\mathrm{Zn}$ takes place at the stopping point of the implantation event described above. The two protons are clearly identified. Their tracks are determined by fitting the decay signal with a sum of two foldings of a straight line and a Gaussian. The vertical full line corresponds to the starting point of the trajectories determined from the implantation profile of ${}^{54}\mathrm{Zn}$ whereas the dashed lines are the two stopping points of the protons trajectories. Bottom: Corresponding time signals. The spectrum on the $Y$ dimension is fitted by a straight line for each proton track, giving the directions of each individual proton.

Figure 3

(a) Two-dimensional view of a ${}^{54}\mathrm{Zn}$ decay event as reconstructed from the $XY$ strip detector. The color corresponds to the energy loss detected by the strips. (b) Same decay event reconstructed in the three-dimensional space.

Figure 4

Energy sharing between the two protons emitted in the decay of ${}^{54}\mathrm{Zn}$. The dashed line is the energy distribution of the three-body model [10] folded with the response of the detector, which fits well the experimental distribution.

Figure 5

Top: Experimental angles between the two protons in the three-dimensional space. Each event is convoluted with a Gaussian representing the angular resolution. Middle: Experimental angular distribution between the two protons. Seven events are represented in the histogram. The dashed and full lines represent two possible angles for three not fully reconstructed events. Bottom: The three lines are the theoretical predictions of the three-body model, each line corresponding to different weights of the $p^2$ configuration. These model distributions are folded with a Gaussian function representing the detector angular resolution.