Big Bang 6 Li nucleosynthesis studied deep underground (LUNA collaboration)

Abstract

a b s t r a c tThe correct prediction of the abundances of the light nuclides produced during the epoch of Big Bang Nucleosynthesis (BBN) is one of the main topics of modern cosmology. For many of the nuclear reactions that are relevant for this epoch, direct experimental cross section data are available, ushering the so-called "age of precision". The present work addresses an exception to this current status: the 2 H( α, γ ) 6 Li reaction that controls 6 Li production in the Big Bang. Recent controversial observations of 6 Li in metal-poor stars have heightened the interest in understanding primordial 6 Li production. If confirmed, these observations would lead to a second cosmological lithium problem, in addition to the well-known 7 Li problem. In the present work, the direct experimental cross section data on 2 H( α, γ ) 6 Li in the BBN energy range are reported. The measurement has been performed deep underground at the LUNA (Laboratory for Underground Nuclear Astrophysics) 400 kV accelerator in the Laboratori Nazionali del Gran Sasso, Italy. The cross section has been directly measured at the energies of interest for Big Bang Nucleosynthesis for the first time, at E cm = 80, 93, 120, and 133 keV. Based on the new data, the 2 H( α, γ ) 6 Li thermonuclear reaction rate has been derived. Our rate is even lower than previously reported, thus increasing the discrepancy between predicted Big Bang 6 Li abundance and the amount of primordial 6 Li inferred from observations.

Influence of orthogonalization procedure on astrophysical S-factor for the direct α + d →6Li + γ capture process in a three-body model

“…The reaction rate evaluations are based on the straightforward calculations starting from the astrophysical S-factor. The data set of the LUNA collaboration available at energies E=94 keV and E=134 keV [3] was recently renewed with additional data at E=80 keV and E=120 keV [4]. The results of the LUNA collaboration for the reaction rates turn out to be even lower than previously reported.…”

Nuclear Astrophysics in underground laboratories: the LUNA experiment

“…The accelerated deuterium particles, after Rutherford scattering with the α-beam, interact with the gas itself producing the two reactions: 2 H(d,n) 3 He and 2 H(d,p) 3 H. In particular, the neutrons emitted by the 2 H(d,n) 3 He were responsible for a high beam induced background in the γ-spectrum. A long and detailed study of this background was required in order to perform the analysis of the data [58][59][60].…”

Nuclear astrophysics at Gran Sasso Laboratory: the LUNA experiment

“…The detection efficiency of Ge1 can be evaluated by the ratio between the number of events that has triggered the acquisition and the number of events actually acquired by the detector itself. The effect of the coincidence acquisition is shown in figure 2, where the spectrum of the 14 N(p,γ) 15 O resonance acquired by Ge1 in inclusive configuration is shown on the left. Selecting the resonance emitted photon at 1.3 MeV with Ge2, the resulting coincidence spectrum of Ge1 (on the right in figure 2) shows the only peak of the corresponding cascade photon at 6.1 MeV.…”

“…The high efficiency (∼ 70%) of the BGO detector reduces the dependence of the reaction yield on the angular distribution of the emitted γ rays and thus allows to achieve a low systematic uncertainty. The detection efficiency has been determined by Monte Carlo simulations, as well as by measurements with radioactive sources ( 60 Co, 137 Cs, 88 Y) and 14 N(p,γ) 15 O resonant reaction [12,13]. The study of systematic uncertainties included also the determination of the beam heating effect obtained by varying the target pressure and beam intensity.…”

“…In particular, the first cross section to be measured will be the 14 N(p,γ) 15 O in a much wider energy range compared to what was done at LUNA400. This way the tuning of LUNA MV will be done and we will more precisely extrapolate the reaction cross section within the Gamow peak of the Sun, i.e.…”