Measurement of ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ between the narrow resonances at $E_r^{\mathrm{lab}}=193$ and $519$ keV

The ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ reaction sensitively influences hydrogen burning nucleosynthesis in a number of stellar sites, including classical novae. These thermonuclear explosions, taking place in close binary star systems, produce peak temperatures in the range of $T=100–400$ MK. Recent results indicate that the thermonuclear rates for this reaction in this particular temperature range are dominated by the direct capture process. We report on the measurement of the ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ cross section between the narrow resonances at $E_r^{\mathrm{lab}}=193$ and $519$ keV, where the $S$ factor is expected to vary smoothly with energy. We extract the direct capture contribution from the total cross section and demonstrate that earlier data are inconsistent with our results.

Figure 1

Relevant parts of the energy level diagram for the ${}^{18}\mathrm{F}$ compound nucleus. Energies and $J^{\pi }$ values are from Refs. [5, 11, 12]. The value in parentheses represents the ($p,\gamma$) $Q$ value. Resonance energies are given in the laboratory system. Only those bound states are shown from which we observed secondary $\gamma$-ray transitions to the ground state. Most of the $\gamma$-ray cascades proceed through the first excited state at $E_x=937$ keV. The red vertical bar on the right-hand side shows the range of Gamow peak centroids for typical nova peak temperatures ($T=0.1–0.4$ GK, corresponding to $E_0=103-261$ keV), while the blue vertical bar on the left-hand side indicates the bombarding energy region covered in the present experiment.

Figure 2

Ratios of individual reaction rate contributions and the total recommended rate [5]. Resonant contributions are labeled by their energy in the laboratory, while contributions from direct capture and the low-energy tails of broad resonances are denoted by “DC” and “TBR”, respectively. Direct capture dominates the total reaction rates at nova peak temperatures, $T=0.1–0.4$ GK.

Figure 3

Estimated ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ direct capture $S$ factor from Rolfs [13] (dotted line) and Fox et al. [5] (horizontal dashed line). The solid line corresponds to the estimated incoherent sum of resonant capture ($E_r^{\mathrm{c}.\mathrm{m}.}=557$ and $677$ keV) and the direct capture contribution from Ref. [5]. The energy range between the two thin vertical lines shows the region of the classical nova Gamow peak centroids (for $T=0.1–0.4$ GK). Previous $S$-factor data from Rolfs [13] and Chafa et al. [7] are shown as full circles (green) and full square (blue), respectively. The former data points include an $18\%$ uncertainty of the absolute cross-section scale [13]. The two low-energy resonances at $E_r^{\mathrm{lab}}=194$ and $519$ keV are not displayed since they are too narrow to be relevant for the present discussion.

Figure 4

Schematic setup including beam tube, target (“T ”), passive shielding, and $\gamma$-ray detector. The detector is located at an angle of $\theta ={55}^{°}$ with respect to the beam direction. The lead shield is shown as the grey-shaded area. The detector crystal is surrounded on almost all sides by at least 5 cm of lead.

Figure 5

Sample spectrum measured in the ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ reaction at a laboratory energy of $E_{\mathrm{lab}}=400$ keV ($E_{\mathrm{eff}}^{\mathrm{c}.\mathrm{m}.}=375$ keV). At this energy the direct capture contribution dominates over the resonant component. $\gamma$-ray transitions in ${}^{18}\mathrm{F}$ are marked in red, where “R/DC” refers to the primary transitions. All other peaks arise from environmental and beam-induced background.

Figure 6

Same as Fig. 3, except that our measured $S$ factors are included as full red circles and the vertical scale is linear instead of logarithmic. All data points refer to measured total $S$ factors. Our lowest data point at $E_r^{\mathrm{c}.\mathrm{m}.}=257$ keV appears at the high-energy boundary of the classical nova Gamow peak region (for $T=0.1–0.4$ GK). Our extracted direct capture $S$ factor agrees within uncertainties with the prediction of Fox et al. [5] (horizontal dashed line), but disagrees with the result of Rolfs [13] (dotted line) by a factor of $\approx 2$. Note that the solid line is not a fit to our data, but represents the total $S$ factor predicted by Fox et al. [5].