Abstract
The cross sections of the 7 Be(n, α)4 He reaction for p -wave neutrons were experimentally determined at Ec.m. = 0.20−0.81 MeV close to the Big Bang nucleosynthesis (BBN) energy window for the first time on the basis of the detailed balance principle by measuring the time-reverse reaction. The obtained cross sections are much larger than the cross sections for s -wave neutrons inferred from the recent measurement at the n_TOF facility in CERN, but significantly smaller than the theoretical estimation widely used in the BBN calculations. The present results suggest the 7 Be(n, α)4 He reaction rate is not large enough to solve the cosmological lithium problem
Highlights
The primordial abundances of the light elements produced in the Big Bang nucleosynthesis (BBN) provide important insights into the early universe
It should be noted that the total 7Be(n,α)4He cross section for the s-wave neutrons was estimated to be as small as a few mb in the BBN energy window, and the s-wave reaction does not contribute to solve the cosmological Li problem
The present results suggest that the 7Be(n,α)4He reaction does not solve the cosmological lithium problem
Summary
The primordial abundances of the light elements produced in the Big Bang nucleosynthesis (BBN) provide important insights into the early universe. The primordial abundance of 7Li inferred from observations of metal-poor stars is 7Li/H = 1.6 ± 0.3 × 10−10 [2], whereas the calculated values by the BBN theory is as large as 7Li/H = 4.68 ± 0.67 × 10−10 [3] This discrepancy is known as the cosmological lithium problem, and has been of great interest in recent years. The cross section for this reaction was measured using a radioactive 7Be target irradiated by a low-energy neutron beam at 10 meV–10 keV [11], these reaction energies are still lower than the BBN energy. It should be noted that the total 7Be(n,α)4He cross section for the s-wave neutrons was estimated to be as small as a few mb in the BBN energy window, and the s-wave reaction does not contribute to solve the cosmological Li problem.
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