Investigations of the ¹⁵N(α, γ)¹⁹F Reaction Relevant to Fluorine Production in Asymptotic Giant Branch Stars

Despite being one of the most common elements people see and use in everyday life, the origin of fluorine is still a widely debated issue in the field of nuclear astrophysics. The production of its only stable isotope, ¹⁹F, has only been observed in the Asymptotic Giant Branch (AGB) stars. The ¹⁵N(α, γ)¹⁹F reaction leads to its production inside AGB stars.

However, the observed ¹⁹F abundance in AGB stars is 6 times more than the ¹⁹F abundance in the solar system. Therefore, understanding the reaction rate of ¹⁵N(α, γ)¹⁹F at AGB star conditions will shed light into the overabundance issue from the observations. The rate of ¹⁵N(α, γ)¹⁹F at temperatures relevant to AGB stars is mostly dominated by the direct capture, the narrow resonance at E_c.m. = 364 keV, and the low energy tails of the E_c.m. = 1323 and 1487 keV resonances. Recent measurements have shown discrepancies in the resonance energy, strength, and alpha width of the E_c.m. = 1323 and 1487 keV resonances, casting additional uncertainties in the total reaction rate.

This work investigates the resonance properties of the E_c.m. = 1323 and 1487 keV resonances using a gamma-spectroscopy measurement with a solid Ti¹⁵N target at the Nuclear Science Laboratory of the University of Notre Dame. This work determines the resonance energy by measuring the gamma-ray energies from the de-excitation of ¹⁹F produced from the reaction, correcting for the Doppler shift effect. The resonance energies for the lower and higher resonance are measured to be E_c.m. = 1321.6 ± 0.6 keV and 1479.4 ± 0.6 keV, respectively. From the measured excitation functions of the two resonances, this work deduced the strength and alpha width of the lower energy resonance are omega-gamma= 1.65 ± 0.17 eV and Gamma-alpha = 2.1 ± 0.3 keV and the strength and alpha width of the higher energy resonance are omega-gamma = 4.20 ± 0.49 eV and Gamma-alpha = 6.5 ± 0.4 keV. The impact of these new resonance parameters on the total reaction rate is investigated using the program RatesMC. The reaction rate of ¹⁵N(α, γ)¹⁹F at T < 0.1 GK is increased by 15%, requiring additional astrophysical simulation studies to understand its impact to the ¹⁹F abundance inside AGB stars.

In addition, this work also performed the same excitation function measurement of the two resonances with the St.~George recoil mass separator to investigate the recoil charge state fraction. Knowing the charge state fraction of the recoil is critical for St.~George measurement to correct the measured experimental yield to the full yield. This work shows that the semi-empirical model that assumes recoils are at a charge state equilibrium and predicts the charge state fraction of the recoils is more energy dependent than anticipated. The measured charge state fraction of the recoils for the two resonances show contradicting results to the semi-empirical model predicted values. Moreover, the lack of understandings of the gas target thickness prevents further studies on this. It is recommended that further investigations of the charge state fraction of the recoils and a systematic study of the gas target thickness should be performed for future St.~George measurements.