DOI: 10.7274/26143123
title: Investigations of the <sup>15</sup>N(α, γ)<sup>19</sup>F Reaction Relevant to Fluorine Production in Asymptotic Giant Branch Stars

description: 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, <sup>19</sup>F, has only been observed in the Asymptotic Giant Branch (AGB) stars. The <sup>15</sup>N(α, γ)<sup>19</sup>F reaction leads to its production inside AGB stars.However, the observed <sup>19</sup>F abundance in AGB stars is 6 times more than the <sup>19</sup>F abundance in the solar system. Therefore, understanding the reaction rate of <sup>15</sup>N(α, γ)<sup>19</sup>F at AGB star conditions will shed light into the overabundance issue from the observations. The rate of <sup>15</sup>N(α, γ)<sup>19</sup>F at temperatures relevant to AGB stars is mostly dominated by the direct capture, the narrow resonance at <i>E</i><sub>c.m</sub>. = 364 keV, and the low energy tails of the <i>E</i><sub>c.m</sub>. = 1323 and 1487 keV resonances. Recent measurements have shown discrepancies in the resonance energy, strength, and alpha width of the <i>E</i><sub>c.m</sub>. = 1323 and 1487 keV resonances, casting additional uncertainties in the total reaction rate.This work investigates the resonance properties of the <i>E</i><sub>c.m</sub>. = 1323 and 1487 keV resonances using a gamma-spectroscopy measurement with a solid Ti<sup>15</sup>N 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 <sup>19</sup>F produced from the reaction, correcting for the Doppler shift effect. The resonance energies for the lower and higher resonance are measured to be <i>E</i><sub>c.m</sub>. = 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 <sup>15</sup>N(α, γ)<sup>19</sup>F at T &lt; 0.1 GK is increased by 15%, requiring additional astrophysical simulation studies to understand its impact to the <sup>19</sup>F 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.