Deep underground measurement of ¹¹B(α,n)¹⁴N

The primordial elemental abundance composition of the first stars leads to questions about their modes of energy production and nucleosynthesis. The formation of 12C has been thought to occur primarily through the 3α process, however, alternative reaction chains may contribute significantly, such as 7Li(α,γ)11B(α,n)14N. This reaction sequence cannot only bypass the mass A=8 stability gap, but could also be a source of neutrons in the first star environment. However, the efficiency of this reaction chain depends on the possible enhancement of its low energy cross section by α-cluster resonances near the reaction threshold. A new study of the reaction 11B(α,n) 14N has been undertaken at the CASPAR underground facility at beam energies from 300–700keV. A 4π neutron detector in combination with pulse shape discrimination at low background conditions resulted in the ability to probe energies lower than previously measured. Resonance strengths were determined for both the resonance at a laboratory energy of 411keV, which was measured for the second time, and for a new resonance at 337keV that has been measured for the first time. This resonance, found to be significantly weaker than previous estimates, dominates the reaction rate at lower temperatures (T<0.2KG) and reduces the reaction rate in first star environments.