Reactions in Stars Measured in Underground Laboratories

Abstract

The main goal of nuclear astrophysics is the study of nuclear reactions responsible for energy production in stars and for the synthesis of chemical elements in the Universe. In stellar interiors, thermonuclear reactions occur under extreme temperature and density conditions, driving the processes that power stars and determine their evolution. Understanding these reactions requires precise knowledge of nuclear reaction cross sections at stellar energies. At such energies, reaction probabilities are extremely small because the interacting nuclei must overcome the electrostatic repulsion of the Coulomb barrier. As a result, direct experimental measurements are often challenging and may be limited by very low counting rates and background signals.

To address these difficulties, experiments in nuclear astrophysics are frequently conducted in underground laboratories. The rock overburden above these facilities strongly reduces the flux of cosmic rays reaching the detectors, leading to a significant suppression of background radiation. This environment enables the measurement of nuclear reactions with extremely low cross sections that would otherwise be obscured by cosmic-ray-induced signals at Earth’s surface. Underground accelerators and low-background detection systems are therefore essential tools for studying key reactions involved in stellar hydrogen burning and other nucleosynthesis processes.

Several underground facilities have played an important role in advancing this field. The Laboratory for Underground Nuclear Astrophysics, located in the Gran Sasso National Laboratory in Italy, was the first to operate deep underground and has produced numerous measurements of reactions relevant to BBN and hydrogen burning in stars. Another important site is the Felsenkeller underground laboratory in Dresden, Germany, which provides a shallower underground environment but still offers a substantial reduction of cosmic-ray background. Both laboratories employ specialized experimental techniques designed to maximize the signal-to-noise ratio, including high-intensity ion beams, ultra-sensitive detectors, and shielding systems optimized for low-background measurements.

Research in underground nuclear astrophysics continues to expand toward the study of more advanced stages of stellar evolution such as helium burning and carbon burning. These stages are activated at higher energies in the mega–electron-volt range and play a central role in the later phases of stellar life cycles and in the production of heavier elements. The precise knowledge of the nuclear reactions during these phases provides a deeper understanding of stellar structure, nucleosynthesis pathways, and the chemical evolution of the cosmos.