A method for investigation of the D(4He, γ)6Li reaction in the Ultralow energy region under a high background

The cosmological lithium problem, that is, a noticeable discrepancy between the predicted and observed abundances of lithium, is in conflict with the Standard Big Bang Nucleosynthesis model. For example, the abundance of ⁷Li is 2-4 times smaller than predicted by the Standard Big Bang Nucleosynthesis. As to the abundance of ⁶Li, recent more accurate optical investigations have yielded only the upper limit on the ⁶Li/⁷Li ratio, which makes the problem of ⁶Li abundance and accordingly of disagreement with the Standard Big Bang Nucleosynthesis predictions less acute. However, experimental study of the D(⁴He, γ)⁶Li reaction cross section is still of current importance because there is a theoretical approach predicting its anomalously large value in the region of energies below the Standard Big Bang Nucleosynthesis energy. The work is dedicated to the measurement of the cross section for the D(⁴He, γ)⁶Li reaction proceeding in zirconium deuteride at the incident ⁴He⁺ion energy of 36 keV. The experiment is performed at a pulsed Hall plasma accelerator with an energy spread of 20% FWHM. A method for direct measurement of the background from the reaction chain D(⁴He, ⁴He)D→D(D, n)³He→(n, γ) and/or (n, n‧γ) ending with activation of the surrounding material by neutrons is proposed and implemented in the work. An upper limit on the D(⁴He, γ)⁶Li reaction cross section σ≤7·10^-36 cm² at the 90% confidence level is obtained.