Observation of the Exotic Isotope ${}^{13}\mathrm{F}$ Located Four Neutrons beyond the Proton Drip Line

A ${}^{13}\mathrm{F}$ resonance was observed following a charge-exchange reaction between a fast ${}^{13}\mathrm{O}$ beam and a ${}^9\mathrm{Be}$ target. The resonance was found in the invariant-mass distribution of $3p+{}^{10}\mathrm{C}$ events and probably corresponds to a $5/2^{+}$ excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first $2^{+}$ state of ${}^{12}\mathrm{O}$, which subsequently undergo $2p$ decay. In addition, there may also be a significant proton decay branch to the second $2^{+}$ level in ${}^{12}\mathrm{O}$. The wave function associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the $2_2^{+}$ state of ${}^{12}\mathrm{O}$.

Figure 1

The histogram displays the spectrum of decay energy for ${}^{13}\mathrm{F}$ determined from the invariant mass of $3p+{}^{10}\mathrm{C}$ events. The transverse emission gate $|\cos {\theta }_H|<0.5$ was applied. The solid red curve shows a fit to this spectrum with a single ${}^{13}\mathrm{F}$ peak plus a smooth background (solid blue curve). The solid data points show the yields associated with sequential proton decay through the $0_1^{+}$ (red circle) and $2_1^{+}$ (blue square) intermediate states of ${}^{12}\mathrm{O}$ obtained for a gate on the fitted ${}^{13}\mathrm{F}$ peak. The width of this gate is indicated by the horizontal error bars. The open data points give the same components for bins on either side of the ${}^{13}\mathrm{F}$ peak in order to interpolate the background contributions in the peak gate. An interpolation was obtained from Gaussian fits to these data, which are shown by the dotted curves. The fitted yields per gate width have been scaled to put these data points on the same vertical scale (counts per 0.25 MeV) as the histogram.

Figure 2

Level diagram showing the location of the newly identified ${}^{13}\mathrm{F}$ state relative to states in ${}^{12}\mathrm{O}$, ${}^{11}\mathrm{N}$, ${}^{10}\mathrm{C}$, ${}^9\mathrm{B}$, and ${}^8\mathrm{Be}$. Energies for the ${}^{12}\mathrm{O}$ and ${}^{11}\mathrm{N}$ excited states are from Ref. [12].

Figure 3

Spectra of decay energy for ${}^{12}\mathrm{O}$ determined from the invariant mass of the three possible $2p+{}^{10}\mathrm{C}$ subevents from each detected $3p+{}^{10}\mathrm{C}$ events. (a) Events that lie in a 1.15-MeV-wide gate centered on the ${}^{13}\mathrm{F}$ peak, while (b) is for events in a background gate $8<E_T<9.3\mathrm{MeV}$. The red solid curve is a fit to this spectrum with contributions (dotted curves) from sequential proton decay through the $0_1^{+}$ (green), $2_1^{+}$ (blue), and $2_2^{+}$ (magenta) intermediate states of ${}^{12}\mathrm{O}$. The arrows show the centroid decay energies of the known ${}^{12}\mathrm{O}$ states.