First identification of ${}^{58}\mathrm{Zn}$ $\beta$-delayed proton emission

The $\beta$ decay of ${}^{57}\mathrm{Zn}$ and ${}^{58}\mathrm{Zn}$ was investigated in an experiment at the National Superconducting Cyclotron Laboratory of Michigan State University. For the first time $\beta$-delayed proton emission from ${}^{58}\mathrm{Zn}$ was observed with a branching ratio of 0.7(1)%. The proton-energy spectrum allowed for probing the Gamow-Teller strength distribution above the proton-separation energy in the daughter nucleus. Moreover, the absolute branching ration for delayed-proton emission from ${}^{57}\mathrm{Zn}$ was found to be compatible with 100%.

Figure 1

Decay scheme of ${}^{58}\mathrm{Zn}$ [9, 10], see text for details. Energy values are given in MeV.

Figure 2

ID-plot of (a) all fragments identified during the experiment and (b) triggering ions. Both plots refer to the ion-optics setting optimised for ${}^{60}\mathrm{Ge}$. See text for details.

Figure 3

Implantation profile of identified ${}^{58}\mathrm{Zn}$ ions (events in which only a single ion entered the chamber, $\approx 3\%$ of the total statistics). The abscissa shows the depth into the chamber along the beam direction. Only ions implanted within 10 and 90% of the chamber length were considered further in the analysis.

Figure 4

(a) and (b) Example CCD pictures (left) and respective fragments of the PMT signal corresponding to the proton track (right) collected during the experiment. The gray histogram shows the raw PMT signal, while the black curve is the same signal processed through a median filter. The decision on whether a proton is stopped inside the chamber or not is based on the observation of the Bragg peak in the CCD image rather than in the PMT signal because of the noise levels in the latter. See text for details.

Figure 5

Energy spectrum of the 33 reconstructed ${}^{58}\mathrm{Zn}\beta p$ events. The proton-stopping efficiency as a function of the energy, under the assumption that the ions are drifting, is shown with a black dashed line. See text for details.

Figure 6

(Top) B(GT) values for decay of ${}^{58}\mathrm{Zn}$ from experiment for $E_x>{\mathrm{S}}_p$ (full circles with error bars, this work) and from QRPA calculations (see text for details) up to 6 MeV excitation energy. The gray bars show the B(GT) calculations, while the dash-dotted blue line represents the calculated B(GT) folded with 0.2 MeV-wide Breit-Wigner distributions to account for the energy resolution. The vertical dashed line shows the ${\mathrm{S}}_p$ value in ${}^{58}\mathrm{Cu}$. (Bottom) Energy spectrum of $\beta p$ from ${}^{58}\mathrm{Zn}$ decay reported on the excitation-energy scale for comparison with the calculations. The spectrum has been corrected for the efficiency (see Fig. 5).