Astrophysical N15(n,γ)N16 reaction rate from precision measurement of the N15(d,p)N16 angular distributions

Abstract

Background: Asymptotic giant branch stars are one of the possible sites of fluorine production as confirmed by astronomical observation. The $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ reaction competes with $^{15}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{F}$ on the reaction chain and thus influences the production of the only stable fluorine isotope $^{19}\mathrm{F}$. The $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ reaction rate at low temperatures of astrophysical interest depends on the neutron spectroscopic factors of the four low-lying states in $^{16}\mathrm{N}$.Purpose: The $^{16}\mathrm{N}$ neutron spectroscopic factors from two previous measurements of $(d,p)$ reaction differ by a factor of $\ensuremath{\approx}2$. This work was intended to investigate these spectroscopic factors via a precision measurement of the $^{15}\mathrm{N}(d,p)^{16}\mathrm{N}$ angular distributions using a Q3D magnetic spectrograph.Methods: The high resolution of the Q3D magnetic spectrograph led to clear separation of these four closely spaced states. The neutron spectroscopic factors were extracted from the present angular distributions with the distorted wave Born approximation and adiabatic distorted wave approximation methods.Results: The neutron spectroscopic factors of the $^{16}\mathrm{N}$ ground, 0.120, 0.298, and 0.397 MeV states were extracted to be $1.32\ifmmode\pm\else\textpm\fi{}0.12, 1.33\ifmmode\pm\else\textpm\fi{}0.18, 1.10\ifmmode\pm\else\textpm\fi{}0.10$, and $1.30\ifmmode\pm\else\textpm\fi{}0.14$, respectively. The new results support the conclusion that these four states in $^{16}\mathrm{N}$ are all good single-particle levels. The cross section and reaction rate of $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ were calculated based on the present spectroscopic factors.Conclusions: The present work provides a precision measurement of the spectroscopic factors of the $^{16}\mathrm{N}$ states and the resulting $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ reaction rate. The present higher reaction rate of the $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ reaction suggests higher flow through $^{15}\mathrm{N}(n,\ensuremath{\gamma})^{16}\mathrm{N}$ compared with previous expectation.