Evidence for ${}^{15}\mathrm{O}+\alpha$ resonance structures in ${}^{19}\mathrm{Ne}$ via direct measurement

States in the astrophysically important ${}^{19}\mathrm{Ne}$ nucleus have been populated directly using the ${}^{15}\mathrm{O}(\alpha ,\alpha ){}^{15}\mathrm{O}$ reaction with a radioactive ${}^{15}\mathrm{O}$ beam. Due to the challenges involved, this represents the first such ${}^{15}\mathrm{O}$ measurement with high resolution and the first dedicated to exploring the cluster structure. This measurement comprises three experiments: the first to develop the ${}^{15}\mathrm{O}$ radioactive beam, the second the resonant scattering study, and the third to measure the precise energy loss of O ions in ${}^4\mathrm{He}$ to reduce systematic uncertainties. The results show evidence for ${}^{15}\mathrm{O}+\alpha$ cluster structure which could explain earlier results pointing to the increased $\alpha$ branching ratio in the astrophysical reaction rate for high-spin near-threshold resonances. Absolute differential cross sections have been measured. Additionally, spins, parities, and partial decay widths, essential for nuclear reaction-rate calculations, have been extracted using an $R$-matrix analysis.

Figure 1

Schematic (not to scale) of the experimental apparatus. The detection system consisted of a DSSSD and two PPACs required for the time-of-flight measurement and beam tracking.

Figure 2

A plot of detected energy versus TOF. Alpha particles originating from elastic scattering (and selected for further analysis) correspond to the locus inside the black long-dashed contour. The corresponding simulated distribution is indicated by the red continuous line. The red short-dashed line corresponds to $\alpha$ particles originating from inelastic scattering, demonstrating that events from this process can be excluded from the analysis, but in this instance, have not been observed. The intensities are given by the heat-map color scale. Note, that as inelastic excitation reduces the available kinetic energy, these reactions take place in the gas closer to the window for the same detected $\alpha$ energy. Since the $\alpha$ particles travel faster than the ${}^{15}\mathrm{O}$ ions, this reduced ${}^{15}\mathrm{O}$ flight path leads to a shorter TOF, giving the position of the inelastic locus as shown here.

Figure 3

Excitation function for ${}^{19}\mathrm{Ne}$ at $0^{\circ }$. The data points from the current work are overlaid by the $R$-matrix fit (solid red line). The fit without the newly observed levels is shown for comparison (dashed blue line). See text and Table 1 for details.

Figure 4

Energy levels of ${}^{19}\mathrm{Ne}$ corresponding to the ${}^{15}\mathrm{O}+\alpha$ rotational bands. The levels predicted in Ref. [45] (in the middle) are compared with the levels experimentally measured in the present work and Ref. [33] (on the left) and to the corresponding ${}^{15}\mathrm{N}+\alpha$ negative parity band in ${}^{19}\mathrm{F}$ (on the right) [33, 46].