Underground study at LUNA of proton-induced reactions on 6Li at astrophysical energies

Abstract

Big Bang Nucleosynthesis (BBN) theory coupled with Cosmic Microwave Background measurements provide predictions on the modern day abundances of low mass isotopes, which are then compared to observations from stellar environments. Specifically 6,7Li isotopic abundances are determined from spectral analysis of radiation emitted from the outer atmospheres of low mass pre-main sequence (PMS) stars. Current literature reports measured abundance of 6Li is 1000 times higher than predicted. Two reactions which destroy 6Li: 6Li(p,a)3He and 6Li(p,g)7Be, have an impact on the predicted 6Li abundances. A previous study measured the 6Li(p,g)7Be reaction at relevant energies and proposed a new resonance which may affect the 6Li abundances predicted from BBN. Until now the existence of this resonance has yet to be confirmed by an independent measurement.

This thesis work reports an experimental campaign aimed to measure both proton-induced destructive reactions on 6Li across astrophysically relevant energies. These reactions were measured concurrently at the Laboratory for Underground Nuclear Astrophysics (LUNA) located under the Gran Sasso mountain in Italy, which benefits from a reduced natural background (10^4 - 10^5 reduced gamma-ray flux) compared to surface laboratories. A proton beam was accelerated at energies Ep = 80 - 395 keV onto 6Li-enriched solid targets nominally composed of Li2O or Li2WO4 and evaporated on a tantalum backing. The charged particles and gamma rays were simultaneously detected using a Silicon and a High Purity Germanium detector, respectively.

The experimental yields were calculated from the data and deconvolved using a median energy approach to determine astrophysical S-factors for both 6Li destruction reactions. Due to incomplete knowledge of the target stoichiometries during beam bombardment, the present S-factors were normalised to previously reported 6Li(p,a)3He reaction S-factors. Results from this work do not confirm the existence of the proposed resonance in 6Li(p,g)7Be. Present thermonuclear reaction rates are calculated to the highest precision to date and the astrophysical implications discussed. Comparisons to previous literature are presented.