An Investigation of the Astrophysically Important ¹⁴N(p, γ)¹⁵O Reaction

¹⁵O nuclei, an important component of the solar neutrino flux, are produced via the ¹⁴N(p, γ)¹⁵O reaction in the CNO cycle in stellar cores. Recent measurements of CNO neutrinos from the decay of ¹⁵O at the Borexino detector provided the first direct measurement of CNO neutrinos, indicating a higher rate than previously expected. These findings provided new, independent information about the metalicity of the solar core.

There are still uncertainties in the rate of the ¹⁴N(p, γ)¹⁵O reaction at solar temperature conditions coming from nuclear sources which could provide independent verification of the Borexino results. Namely, the low-energy cross-section data for the transitions to the ground state and 6.79 MeV excited state and the lifetime of the subthreshold excited state in ¹⁵O at 6.79 MeV. In this work, we address these main sources of uncertainty in the nuclear physics inputs.

The result of a low energy ¹⁴N(p, γ)¹⁵O cross-section measurement taken at the CASPAR underground accelerator facility is presented. The measurement detailed here bridges gaps in existing data sets, aiming to find a resolution to existing discrepancies in the data sets and moving towards a better understanding of the low-energy behavior of the ¹⁴N(p, γ)¹⁵O cross-section.

This work also addresses the uncertainty coming from the lifetime of the excited state at E_x = 6.79 Mev, as previous measurements of this state's lifetime are significantly discrepant. This work details a measurement of this state's lifetime, as well as the lifetimes of the excited states in ¹⁵O at E_x = 5.18 MeV and E_x = 6.17 MeV.

The lifetimes have been determined by the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets of Mo, Ta, and W backing. The lifetimes were obtained from the weighted average of the three individual measurements, allowing us to account for systematic differences between the backing materials. For the 6.79 MeV state, we obtained a t = 0.6 +/- 0.4 fs. To provide cross-validation of our method, the lifetimes of the states at 5.18 MeV and 6.17 MeV to be t = 7.5 +/- 3.0 and t = 0.7 +/- 0.5 fs, respectively, in good agreement with previous measurements.

Ultimately, a multichannel R-matrix analysis was performed on all of the present data and was used to extrapolate the astrophysical S factors of the ground state and the 6.79 MeV transitions to low energies. The present extrapolated zero-energy S factors are S-6.79 (0) = 1.24 +/- 0.09 keV b, S-6.17 (0) = 0.12 +/- 0.05 keV b, and S-g.s. (0) = 0.33(+0.16/-0.08) keV b. The value for the S-6.79 (0) overlaps well with recently reported measurements. Overall, this indicates that this transition's extrapolation is quite robust to the addition of new data. For the ground state zero-energy extrapolation, S-g.s. (0), is slightly elevated but in overall agreement with previous literature.

The approach taken here for the study of the ¹⁴N(p, γ)¹⁵O reaction has succeeded in combining both the efforts of an improved lifetime measurement and new cross-section data in underground accelerator experiments. The combination of these two complementary measurements allows us to suggest an enhancement to the zero-energy S-factor of the ¹⁴N(p, γ)¹⁵O reaction, ultimately supporting the higher rate observed in the recent Borexino neutrino measurements. However, following the R-matrix analysis, consistent discrepancies between measured data and fits, both past and present, are identified and recommendations for future study are made.