Speaker
Description
Summary
The synthesis of elements in stars are due to nuclear reactions, which
start from the most abundant and lightest element, the hydrogen, towards to the synthesis of the heaviest elements. Therefore nuclear fusion reactions determine the energy production, the neutrino luminosity and the evolution of stars. Most of the reactions between nuclei at stellar energies have very low cross sections, so that under typical laboratory conditions the reaction yield can be as low as few events/month or even below, i.e. prohibitively low with respect to the detector background induced by the cosmic rays. A drastic reduction of this background can be achieved by carrying out the measurement deep underground. The "Laboratori Nazionali del Gran Sasso" (LNGS) hosts the facility LUNA (Laboratory for Underground Nuclear Astrophysics), that is still the worldwide unique facility operating deep underground, in which cross sections at energies of astrophysical interest are measured. The very first accelerator used at LUNA was a $50~kV$ machine. The achievement of measuring the the $^3He(^3He,2p)^4He$ cross section inside the solar Gamow peak can be considered a milestone in the long way to the discovery of neutrino oscillation. In fact the existence of the Fowler resonance to explain the "Solar neutrino problem" was definitively rejected, pushing towards a new generation of experiments addressed to measure the neutrino oscillation parameters and the solar interior (Borexino, Kamland, SNO). The present LUNA400 accelerator (Terminal Voltage =400 kV) is mainly employed to study reactions of hydrogen burning in the Sun and in other celestial bodies, e.g. massive stars, AGB stars, Novae. Also important is the study of several leading processes of Big Bang Nucleosynthesis (BBN). As an example, the ongoing study of the $d(p,\gamma)^3He$ reaction is particularly important to derive the cosmological baryon density $\Omega_b$ and to constrain the possible existence of "dark radiation" at BBN epoch, i.e. the existence of light particles such as sterile neutrinos or axions, not foreseen in the standard model. Although the LUNA-400 accelerator is presently full operative, the future of LUNA is addressed to study the successive phases of stars, towards the synthesis of heavier and heavier elements. In this concern the LUNA-MV project has been approved and funded, and will be operative in the 2019. This machine has a terminal Voltage of 3.5 MV and can deliver beams of $^1H^+$, $^4He^+$, $^{12}C^+$ and $^{12}C^{2+}$. The starting program will last about 4 years and foresees the study of the carbon burning through the cross section measurement of $^{12}C+^{12}C$ processes, such as $^{12}C(^{12}C,p)^{23}Na$, $^{12}C(^{12}C,\alpha)^{20}Ne$ and $^{12}C(^{12}C,n)^{23}Mg$. Carbon Burning is of crucial importance to understand the evolution and fate of stars. the initial program with the LUNA-MV accelerator includes the study of the $^{13}C(\alpha,n)^{16}O$ and $^{22}Ne(\alpha,n)^{25}Mg$ reactions. These reactions release neutrons and, in turn, activate the s-process which allows the production of heavy elements. Finally, the measurement of the $^{12}C(\alpha,\gamma)^{16}O$ with LUNA-MV has been considered. This is a key reaction during the stellar phase of helium burning. However the possible existence of a beam induced background due to the $^{13}C(\alpha,n)^{16}O$ reaction suggests an inverse kinematic approach, with a $^{12}C$ ion beam impinging a $^4He$ gas target.
