Improved Direct Measurement of the 64.5 keV Resonance Strength in the ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ Reaction at LUNA

The ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction plays a key role in various astrophysical scenarios, from asymptotic giant branch stars to classical novae. It affects the synthesis of rare isotopes such as ${}^{17}\mathrm{O}$ and ${}^{18}\mathrm{F}$, which can provide constraints on astrophysical models. A new direct determination of the $E_R=64.5\mathrm{keV}$ resonance strength performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) accelerator has led to the most accurate value to date $\omega \gamma =10.0\pm 1.4_{\mathrm{stat}}\pm 0.7_{\mathrm{syst}}\mathrm{neV}$, thanks to a significant background reduction underground and generally improved experimental conditions. The (bare) proton partial width of the corresponding state at $E_x=5672\mathrm{keV}$ in ${}^{18}\mathrm{F}$ is ${\mathrm{\Gamma }}_p=35\pm 5_{\mathrm{stat}}\pm 3_{\mathrm{syst}}\mathrm{neV}$. This width is about a factor of 2 higher than previously estimated, thus leading to a factor of 2 increase in the ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction rate at astrophysical temperatures relevant to shell hydrogen burning in red giant and asymptotic giant branch stars. The new rate implies lower ${}^{17}\mathrm{O}/{}^{16}\mathrm{O}$ ratios, with important implications on the interpretation of astrophysical observables from these stars.

Figure 1

Typical $\alpha$-particle spectrum from the $E_R=183\mathrm{keV}$ resonance at $E_p=193\mathrm{keV}$. The position and width of the alpha-particle peak (shaded area) is used to help define the ROI for the $E_R=64.5\mathrm{keV}$ resonance alpha particles. Error bars correspond to statistical uncertainties.

Figure 2

Overlay of background spectra taken overground (grey line) and underground (dashed line) showing a factor of 15 background reduction in the region of interest (vertical lines) of expected alpha particles from the $E_R=64.5\mathrm{keV}$ resonance in ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$. Also shown is the full energy spectrum (black line) taken on resonance at $E_p=71.5\mathrm{keV}$. Peaks arise from beam-induced reactions on target contaminants as labeled. The peak around 5 MeV is ascribed to intrinsic alpha activity in the silicon detectors [29].

Figure 3

Overlay of time-normalized on-resonance ($E_p=71.5\mathrm{keV}$), off-resonance ($E_p=65\mathrm{keV}$), and natural background spectra in counts/h (lines are to guides to the eye). Also shown is the histogram (in counts/C) obtained after a bin-by-bin subtraction of the time-normalized natural background spectrum from the on-resonance one. The shaded peak corresponds to the region of interest of the alpha particles from the 64.5 keV resonance. Note the different $y$ axes.

Figure 4

Ratio of the ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction rate of the present work (solid line) and of Buckner et al. [21] (dotted line) to the ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction rate of Iliadis [22]. Dashed and dash-dotted lines correspond to the upper and lower limits as given here and by Buckner et al., respectively.