Measurement of the total cross section of the H-2(a,g)Li-6 radiative capture reaction

Big-bang nucleosynthesis (BBN) calculations of the abundances of the light isotopes H-2, He-3, He-4 and Li-7 in the universe have been successfully confirmed via observation. For primordial Li-6 a clear detection in low-metallicity objects, which could show a primordial component, is not yet available. Only a theoretical determination of the abundance of Li-6 exists. However these calculations require the experimental measurement of the H-2(a,g)Li-6 radiative capture reaction cross section.
At present the value of the reaction cross section at the energies appropriate to the Big-bang temperatures (approx. energy: 50-200 keV) has not been measured directly. The direct measurement is a challenging task since the estimated cross section is in the nano or even sub-nanobarn domain. Additionally the deuterons of the reaction are a strong source of background when they breakup and produce neutrons. Past efforts have avoided this issue by the use of thin gas-targets of deuterium allowing the detection of the recoil Li-6 and hence reconstructing the full reaction channel. The drawback is a loss of luminosity, which is prohibitive at lower energies, where the cross section drops down exponentially.
At the 4MV Cockroft-Walton accelerator in Basel we tried to improve this situation by measuring the total cross section of capture reaction down to a c.m. energy of around 400 keV and demonstrating the feasibility of a thick target experiment.