Nuclear astrophysics in underground laboratories: The LUNA experiment

One of the main ingredients of nuclear astrophysics is the knowledge of the thermonuclear reactions responsible for powering the stellar engine and for the synthesis of the chemical elements. At astrophysical energies the cross section of nuclear processes is extremely reduced by the effect of the Coulomb barrier. The low value of cross sections prevents their measurement at stellar energies on Earth surface and often extrapolations are needed. The Laboratory for Underground Nuclear Astrophysics (LUNA) is placed under the Gran Sasso mountain and thanks to the cosmic-ray background reduction provided by its position can investigate cross sections at energies close to the Gamow peak in stellar scenarios. Many crucial reactions involved in hydrogen burning has been measured directly at astrophysical energies with both the LUNA-50kV and the LUNA-400kV accelerators, and this intense work will continue with the installation of a MV machine able to explore helium and carbon burnings. Based on this progress, currently there are efforts in several countries to construct new underground accelerators. In this talk, the typical techniques adopted in underground nuclear astrophysics will be described and the most relevant results achieved by LUNA will be reviewed. The exciting science that can be probed with the new facilities will be highlighted. Nuclear processes generate the energy that makes stars shine. Moreover they are responsible of the synthesis of the elements (and isotopes) in stars. As a matter of fact, hydrogen, helium and all isotopes until lithium and beryllium are synthesised during the Big Bang Nucleosynthesis (BBN). All other nuclei are produced during the different characteristic phases of the star evolution [1]. The understanding of these nuclear processes is the goal of nuclear astrophysics and, in particular, the knowledge of the nuclear cross-sections involved in that processes. The astrophysical relevant energies is given by the folding of two strongly energy dependent functions [1, 2]: the Maxwell-Boltzmann velocity distribution and the cross section of charged particle induced reactions. The folding results in a structure called the Gamow peak, which peaks at the energy EG (called Gamow energy): EG ≈ 0.1220(Z2 1Z 2μ) 1 3 T 2 3 9 (1) where Z1,2 are the charges of the two reaction partners, μ = m1m2/(m1 + m2) their reduced mass, and T9 = T/109 K is the temperature of the astrophysical scenario under study. The Gamow energy of nuclear reactions taking place in the Sun, with its core temperature of T9 ≈ 0.016, is typically 20 keV depending on the precise reaction, leading to cross sections in the range of pbarn and below. As matter of fact, in this energy range the cross section is highly reduced by the effect of the Coulomb repulsion and the nuclear reactions can occur only via tunnel effect. In particular the e-mail: caciolli@pd.infn.it cross section can be written as: