The Study of Key Reactions Shaping the Post-Main Sequence Evolution of Massive Stars in Underground Facilities

Abstract

The chemical evolution of the Universe and several phases of stellar life are regulated by minute nuclear reactions. The key point for each of these reactions is the value of cross-sections at the energies at which they take place in stellar environments. Direct cross-section measurements are mainly hampered by the very low counting rate and by cosmic background; nevertheless, they have become possible by combining the best experimental techniques with the cosmic silence of an underground laboratory. In the nineties, the LUNA (Laboratory for Underground Nuclear Astrophysics) collaboration opened the era of underground nuclear astrophysics, installing first a homemade 50 kV and, later on, a second 400 kV accelerator under the Gran Sasso mountain in Italy: in 25 years of experimental activity, important reactions responsible for hydrogen burning could have been studied down to the relevant energies thanks to the high current proton and helium beams provided by the machines. The interest in the next and warmer stages of star evolution (i.e., post-main sequence and helium and carbon burning) drove a new project based on an ion accelerator in the MV range called LUNA-MV, able to deliver proton, helium, and carbon beams. The present contribution is aimed to discuss the state of the art for some selected key processes of post-main sequence stellar phases: 12C(α,γ)16O and 12C+12C are fundamental for helium and carbon burning phases, and 13C(α,n)16O and 22Ne(α,n)25Mg are relevant to the synthesis of heavy elements in AGB stars. The perspectives opened by an underground MV facility will be highlighted.

Final results on the 13C(α,n)16O cross section at low energies at LUNA

“…Thanks to excellent quality of the data, the reaction rate At T= 0.1 GK is evaluated with an overall uncertainty of about 15%. In particular we took into consideration the reaction rate at -2σ uncertainty, where more 13 C survives and it is it burned at a higher temperature (∼ 200 MK) into a convective shell powered by a subsequent thermal pulse. This generates a second neutron burst characterized by higher neutron density and lower exposure.…”

“…With the installation of the new LUNA MV facility, which will provide a maximum terminal voltage of 3.5 MV, the LUNA collaboration is planning to extend the measurement of the 13 C(α,n) 16 O at higher energies covering the energy range up to 1 MeV. This will give the unique possibility to provide a complete dataset over a wide energy range with well known uncertainties and avoiding normalizations [13].…”

“…MeV) (empty diamond). 13 C targets used during the measurement at LUNA have been produced evaporating 99% 13 C isotopically enriched powder on tantalum backings. To check target stochiometry, depth profile and uniformity immediately after the evaporation, an extensive target characterization was performed by means of Nuclear Resonant Reaction Analysis (NRRA) of the 13 C(p,γ) 14 N reaction at 1.75 MeV at the Tandetron accelerator installed at ATOMKI [10].…”

“…The 13 C(α, n) 16 O reaction is the main neutron source for the s-process in low mass AGB stars [1]. The reaction takes place subsequently complex convective motions in the so-called 13C pocket in a stellar environment of about 1−2•10 8 K, corresponding to a Gamow window between 140 1 and 250 keV, well below the Coulomb barrier of the reaction.…”

Probing the early Universe from deep underground

“…A large high-purity germanium detector (HPGe) faced to the reaction chamber at 90 • to the beam axis was used to detect the photons emitted by the D(p,γ) 3 He reaction. Possible gas impurities by in-leaking air were periodically checked by the strong E p = 278 keV 14 N(p,γ) 15 O resonance and were always below 0.5% [14]. The D(p,γ) 3 He (Q = 5.493 MeV) reaction proceeds through direct capture to the 3 He ground state with the emission of a single gamma with an energy in the range 5.5 -5.8 MeV.…”