$\beta$-delayed two-proton decay of ${}^{27}\mathrm{S}$ at the proton-drip line

The $\beta$-delayed two-proton ($\beta 2p$) decay of ${}^{27}\mathrm{S}$ was studied using a state-of-the-art silicon array and Clover-type HPGe detectors. An energy peak at 6372(15) keV with a branching ratio of 2.4(5)% in the decay-energy spectrum was identified as a two-proton transition via the isobaric-analog state in ${}^{27}\mathrm{P}$ to the ground state of ${}^{25}\mathrm{Al}$ in the $\beta$ decay of ${}^{27}\mathrm{S}$. Two-proton angular correlations were measured by the silicon array to study the mechanism of two-proton emission. Based on experimental results and Monte Carlo simulations, it was found that the main mechanism for the emission of $\beta 2p$ by ${}^{27}\mathrm{S}$ is of sequential nature.

Figure 1

Schematic layout of the detection system locating at Terminal 2 of the RIBLL1. The zoom-in figures for silicon array marked with (a), (b). and (c) demonstrate the methods for the proton-proton coincidence. The ${}^{27}\mathrm{S}$ ions were stopped by DSSD2 and the escaping two protons after $\beta$ decay, $p_1$ and $p_2$, were (a) separately measured by DSSD3 and DSSD1, both measured by (b) DSSD3, and (c) DSSD1. ${\theta }_{pp}$ is the opening angle between $p_1$ and $p_2$ in which three pixels were fired simultaneously.

Figure 2

(a) Energy spectrum of $\beta$-delayed proton from the decay of ${}^{27}\mathrm{S}$ measured by DSSD3 above 5000 keV. (b) The decay time spectrum gated on the 6372-keV peak.

Figure 3

(a) Summed energy spectrum of the events in which three pixels were fired using proton-proton coincident method. (b) Decay time spectrum gated on the summed energy at 6320 keV ranging from 6200 to 6500 keV.

Figure 4

Energy-energy correlations of individual protons of $\beta 2p$ of ${}^{27}\mathrm{S}$. The red circles and the black triangles represent the 6.320-MeV $2p$ events and other $2p$ events, respectively.

Figure 5

Distribution of the opening angle of two protons with total energies ranging from 6200 to 6500 keV in Fig. 3. The colored lines are Monte Carlo simulation results and the black dots are the experimental results in present work.