Activation cross section measurement of the ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ astrophysical key reaction

Background: ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ is one of the key reactions of nuclear astrophysics, playing a role in various stellar processes and influencing energy generation of stars, stellar evolution, and nucleosynthesis. For a reliable reaction rate calculation, the low-energy cross section of ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ must be known with high accuracy. Owing to the unmeasurable low cross sections, theoretical calculations are unavoidable.

Purpose: High-precision experimental cross section data are needed in a wide energy range in order to provide the necessary basis for low-energy extrapolations. In the present work, the total ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ cross section was measured with a method complementary to the available data sets.

Method: The cross section was measured with activation, based on the detection of the annihilation radiation following the ${\beta }^{+}$ decay of the reaction product ${}^{15}\mathrm{O}$. This method, which provides directly the astrophysically important total cross section, was never used for the ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ cross section measurement in the studied energy range.

Results: The nonresonant cross section was measured between 550 and 1400 keV center-of-mass energies with total uncertainty of about 10%. The results were compared with literature data using an $R$-matrix analysis. It is found that the cross sections measured in this work are in acceptable agreement with the two recent measurements only if the weak transitions—not measured in those works—are included.

Conclusions: The present data set, being largely independent from the other available data, can be used to constrain the extrapolated cross sections to astrophysical energies and helps to make the astrophysical model calculations more reliable.

Figure 1

Upper panel: number of detected 511-keV $\gamma$ rays in the 5-s intervals as a function of time in the case of an activation of 21 activation cycles (the upper axis shows the cycle number). Lower panel: The cumulative number of detected events summing up the 21 cycles. The fit of the decay curves (shown by smooth red lines) were used to obtain the cross section.

Figure 2

Measured cross section of the ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$ reaction in the form of astrophysical $S$ factor. Besides the present results, the experimental data of Ref. [12] are shown as well as the sum of the cross sections of the two dominant transitions obtained from the $R$-matrix fits of Refs. [16, 17].

Figure 3

Same as Fig. 2, but the $R$-matrix calculations contain also the transitions not measured in Refs. [16, 17]. The partial $S$ factors for these transitions are taken from the work of Imbriani et al. [14].