Spectroscopic factors for low-lying 16N levels and the astrophysical 15N(n,γ)16N reaction rate.

Background: Fluorine is a key element for nucleosynthetic studies since it is extremely sensitive to the physical conditions within stars. The astrophysical site to produce fluorine is suggested to be asymptotic giant branch stars. In these stars the 15N(n,γ)16N reaction could affect the abundance of fluorine by competing with 15N(α,γ)F19. Purpose: The 15N(n,γ)16N reaction rate depends directly on the neutron spectroscopic factors of the low-lying states in 16N. Shell model calculations and two previous measurements of the (d,p) reaction yielded the spectroscopic factors with a discrepancy by a factor of ~2. The present work aims to explore these neutron spectroscopic factors through an independent transfer reaction and to determine the stellar rate of the 15N(n,γ)16N reaction. Methods: The angular distributions of the 15N(Li7,Li6)16N reaction populating the ground state and the first three excited states in 16N are measured using a Q3D magnetic spectrograph and are used to derive the spectroscopic factors of these states based on distorted wave Born approximation analysis. Results: The spectroscopic factors of these four states are extracted to be 0.96 ± 0.09, 0.69 ± 0.09, 0.84 ± 0.08, and 0.65 ± 0.08, respectively. Based on the new spectroscopic factors we derive the 15N(n,γ)16N reaction rate. Conclusions: The accuracy and precision of the spectroscopic factors are enhanced due to the first application of high-precision magnetic spectrograph for resolving the closely spaced 16N levels which cannot be achieved in most recent measurements. The present result demonstrates that two levels corresponding to neutron transfers to the 2s1/2 orbit in 16N are not good single-particle levels although 15N is a closed neutron-shell nucleus. This finding is contrary to the shell model expectation. The present work also provides an independent examination to shed some light on the existing discrepancies in the spectroscopic factors and the 15N(n,γ)16N rate. [ABSTRACT FROM AUTHOR]