Low-energy cross-section measurement of the B-10(alpha, n)N-13 reaction and its impact on neutron production in first-generation stars

Nucleosynthesis in the first generation of massive stars offers a unique setting to explore the creation of the first heavier nuclei in an environment free of impurities from earlier stellar generations. In later generations of massive stars, hydrogen burning occurs predominantly through the CNO cycles, but without the carbon, nitrogen, and oxygen to catalyze the reaction sequence, first stars would have to rely on the inefficient pp chains for their energy production. Observations of second and third generation stars show pronounced abundances of carbon and oxygen isotopes, which suggests a rapid conversion of the primordial abundances to heavier elements. While the triple-alpha-process primarily facilitates this conversion, there are alternative reaction sequences, such as H-2(alpha, gamma)Li-6(alpha, gamma)B-10(alpha, n)N-13, that may play a significant role. To study such alternate reaction pathways for production of carbon and heavier nuclei, a number of new measurements are needed. In this work, new measurements are reported for the B-10(alpha, n)N-13 reaction, extending the cross section down to 575 keV incident alpha-particle energy. The measurements were made using a state-of-the-art deuterated liquid scintillator and a spectrum unfolding technique. An R-matrix analysis was performed in order to facilitate a comparison of the underlying nuclear structure with the reaction measurements. An unexpected upturn is observed in the low-energy S factor that indicates the presence of a new low-energy resonance. A revised reaction rate is determined that takes into account the present data as well as other previous measurements from the literature that were previously neglected.