N Cross Section—A Prominent Beam-Induced Background Reaction in Underground Nuclear Astrophysics Experiments*

The ¹³C(α,n)¹⁶O reaction is the primary source of neutrons for the main branch of the slow neutron capture process (s-process) of nucleosynthesis. Direct measurement of the cross section within the Gamow window is difficult due to low yields. Prior measurements have constrained the ¹³C(α,n)¹⁶O cross section down to 279 keV, but with large statistical uncertainties. These uncertainties, compounded by the unknown influence of a 1/2⁺ resonance in ¹⁷O near the α-capture threshold, make extrapolation into the Gamow window unreliable, necessitating additional measurements at low energies. Measurement is further complicated by beam-induced background from the ¹³C(d,n)¹⁴N reaction, resulting from deuterium contamination in the α-particle beams of most accelerators. At astrophysical energies, the ¹³C(d,n)¹⁴N cross section is many orders of magnitude greater than that of ¹³C(α,n)¹⁶O. Thus, a direct measurement of ¹³C(d,n)¹⁴N in the energy range of interest is needed. The ¹³C(d,n)¹⁴N cross section was accordingly measured at laboratory energies between 165 and 250 keV at Oak Ridge National Laboratory’s Multicharged Ion Research Facility. Preliminary results are discussed.