Charged-particle branching ratios above the neutron threshold in ${}^{19}\mathrm{F}$: Constraining ${}^{15}\mathrm{N}$ production in core-collapse supernovae

Background: Spatially correlated overabundances of ${}^{15}\mathrm{N}$ and ${}^{18}\mathrm{O}$ observed in some low-density graphite meteoritic grains have been connected to nucleosynthesis taking place in the helium-burning shell during core-collapse supernovae. Two of the reactions which have been identified as important to the final abundances of ${}^{15}\mathrm{N}$ and ${}^{18}\mathrm{O}$ are ${}^{18}\mathrm{F}(n,\alpha ){}^{15}\mathrm{N}$ and ${}^{18}\mathrm{F}(n,p){}^{18}\mathrm{O}$. The relative strengths of the ${}^{18}\mathrm{F}(n,\alpha ){}^{15}\mathrm{N}$ and ${}^{18}\mathrm{F}(n,p){}^{18}\mathrm{O}$ reactions depend sensitively on the relative ${\alpha }_0$ and $p_0$ decay branches from states above the neutron threshold in ${}^{19}\mathrm{F}$ in addition to other properties such as the spins, parities, and neutron widths. However, experimental data on the charged-particle decays from these highly excited states are lacking or inconsistent.

Purpose: We measure the charged-particle decay branches from states around the neutron threshold in ${}^{19}\mathrm{F}$.

Method: Two experiments were performed using proton inelastic scattering from LiF targets and magnetic spectrographs. The first experiment used the high-resolution Q3D spectrograph at Munich to constrain the properties of levels in ${}^{19}\mathrm{F}$. A second experiment using the Orsay split-pole spectrograph and an array of silicon detectors was performed in order To measure the charged-particle decay branches from states around the neutron threshold in ${}^{19}\mathrm{F}$.

Results: A number of levels in ${}^{19}\mathrm{F}$ have been identified along with their corresponding charged-particle decays. The first state above the neutron threshold which has an observed proton-decay branch to the ground state of ${}^{18}\mathrm{O}$ lies 68 keV ($E_x=10.5$ MeV) above the neutron threshold. The $\alpha$-particle decays from the neutron-unbound levels are generally observed to be much stronger than the proton decays.

Conclusion:Neutron-unbound levels in ${}^{19}\mathrm{F}$ are observed to decay predominantly by $\alpha$-particle emission, supporting the role of ${}^{18}\mathrm{F}(n,\alpha ){}^{15}\mathrm{N}$ in the production of ${}^{15}\mathrm{N}$ in the helium-burning shell of supernovae. Improved resonant-scattering reaction data are required in order to be able to determine the reaction rates accurately.

Figure 1

Excitation-energy spectra resulting from the ${}^{19}\mathrm{F}(p,p^{\prime }){}^{19}\mathrm{F}$ reaction at ${\theta }_{\mathrm{Q}3\mathrm{D}}={25}^{\circ }$. The total fit is shown as a solid red line and individual state contributions are shown as dashed red lines. The 10.345-MeV state in ${}^{16}\mathrm{O}$ is shown as a solid purple curve. Vertical solid black lines show the locations of states with the associated indices corresponding to those listed in Table 2. The two different fields correspond to different strengths of the magnetic fields of the Q3D to center two different excitation energies on the focal plane of the spectrograph. Note that the $y$ axis for this spectrum does not start at 0 and so the extent of the uniform background on the focal plane is somewhat hidden.

Figure 2

As in Fig. 1 but for data at ${\theta }_{\mathrm{Q}3\mathrm{D}}={35}^{\circ }$.

Figure 3

As in Fig. 1 but for data at ${\theta }_{\mathrm{Q}3\mathrm{D}}={40}^{\circ }$.

Figure 4

As in Fig. 1 but for data at ${\theta }_{\mathrm{Q}3\mathrm{D}}={50}^{\circ }$.

Figure 5

Matrices of experimental and simulated coincidence events. The excitation energy (abscissa) is plotted against the missing energy (ordinate) for double-sided silicon strip detector (DSSSD) detectors 1 and 2 covering the same angular range. (Top left) The experimental coincidence matrix assuming ${}^{19}\mathrm{F}(p,p^{\prime }){}^{19}\mathrm{F}\left(\alpha \right){}^{15}\mathrm{N}$ reaction kinematics. (Top right) The simulated coincidence matrix assuming ${}^{19}\mathrm{F}(p,p^{\prime }){}^{19}\mathrm{F}\left(\alpha \right){}^{15}\mathrm{N}$ reaction kinematics for various different reactions, including (black) ${}^{19}\mathrm{F}$ with $\alpha$ decays to the ground state of ${}^{15}\mathrm{N}$, (red) ${}^{19}\mathrm{F}$ with $\alpha$ decays to the first excited $E_x=5.27$-MeV state in ${}^{15}\mathrm{N}$, (green) ${}^{12}\mathrm{C}$ decays through the ground state of ${}^8\mathrm{Be}$, and (blue) ${}^7\mathrm{Li}$ decay into $\alpha +t$. (Bottom left) The experimental coincidence matrix assuming the ${}^{19}\mathrm{F}(p,p^{\prime }){}^{19}\mathrm{F}\left(p\right){}^{18}\mathrm{O}$ reaction kinematics. (Bottom right) As top right but assuming ${}^{19}\mathrm{F}(p,p^{\prime }){}^{19}\mathrm{F}\left(p\right){}^{18}\mathrm{O}$ reaction kinematics. The locus at high values of missing energy is due to the detection of ${}^{15}\mathrm{N}$ ions and low-energy $\alpha$ particles from the decay of ${}^8\mathrm{Be}$.

Figure 6

(Top) Energy of the particle detected in the silicon detector (abscissa) against the time difference between the focal-plane event and the particle detected in the silicon detector (ordinate). The additional timing gate on the proton events is also shown. This spectrum was generated using events from detectors 1 and 2 with a gate on $E_{\mathrm{x}}$ between 10.2 and 10.7 MeV and on values of the missing energy (assuming proton kinematics) between 7.95 and 8.1 MeV. (Bottom) Missing energy spectra with a gate on time differences from $-20$ to 20 (blue) and with the more constrictive timing gate (red). The suppression of the events from the contaminant channels is clear. The dotted vertical lines in the bottom panel show the gates on the missing energy which are used to generate the time-difference spectrum in the upper panel.

Figure 7

Focal-plane spectra from the Orsay experiment. Top: Singles (inclusive) spectrum. Middle: ${\alpha }_0$-gated focal-plane spectrum. Bottom: $p_0$-gated focal-plane spectrum. The solid lines show the overall fits of the spectra. The dotted and dashed lines show the contributions from individual resonances. The green and blue additional lines in the ${\alpha }_0$ and $p_0$ show the linear background component and the additional background component from the contaminant channels in the $p_0$ locus in detectors 5 and 6. Resonances which are not shown in the $p_0$ spectrum are consistent with no signal and instead the maximum yield is computed as an upper limits; see the text for additional details.

Figure 8

The (black) experimental and (red) fitted yields per region for the state at $E_x=10.256$ MeV.