Exploring Stars in Underground Laboratories: Challenges and Solutions

TL;DR

The paper reviews how underground laboratories enable measurement of thermonuclear reaction cross sections at stellar energies, addressing the problem of extremely small cross sections and cosmic-ray backgrounds. It outlines the thermonuclear reaction framework, including the Gamow peak and the $S(E)$ factor, and discusses experimental requirements like target stability, energy calibration, and background mitigation. It then reviews LUNA's 30-year legacy across gamma, charged-particle, neutron channels, and activation methods, highlighting key results that refine solar, BBN, and stellar nucleosynthesis models, and surveys parallel underground facilities (CASPAR, JUNA, Felsenkeller) and the LUNA MV upgrade. The work demonstrates that deep underground nuclear astrophysics is essential for constraining reaction rates that power stars and synthesize elements, with broad implications for astrophysical observations and galactic chemical evolution.

Abstract

For millennia, mankind has been fascinated by the marvel of the starry night sky. Yet, a proper scientific understanding of how stars form, shine, and die is a relatively recent achievement, made possible by the interplay of different disciplines as well as by significant technological, theoretical, and observational progress. We now know that stars are sustained by nuclear fusion reactions and are the furnaces where all chemical elements continue to be forged out of primordial hydrogen and helium. Studying these reactions in terrestrial laboratories presents serious challenges and often requires developing ingenious instrumentation and detection techniques. Here, we reveal how some of the major breakthroughs in our quest to unveil the inner workings of stars have come from the most unexpected of places: deep underground. As we celebrate 30 years of activity at the first underground laboratory for nuclear astrophysics, LUNA, we review some of the key milestones and anticipate future opportunities for further advances both at LUNA and at other underground laboratories worldwide.

Figures (5)

• Figure 1: (Color online) The Gamow peak curve (green) arises from the product of the Maxwell-Boltzmann distribution at a temperature $T$ (blue) (here $T = 15\times10^6 \,\mathrm{K}$) and the tunnelling probability between the interacting nuclei (red). The integral of the peak (shaded area) is proportional to the reaction rate. Note how, at the same temperature, the Gamow peak curves shift to higher energies for reactions between heavier charges and become progressively smaller because of the correspondingly lower tunnelling probabilities $\exp(-2\pi\eta)$ (shown here only for the p+p case).
• Figure 2: (Color online) Picture of the LUNA-400kV accelerator (in the background, top right) and the two beam lines (in the foreground). Both solid-target (left) and gas-target (centre) setups are surrounded by massive lead shields for further background suppression.
• Figure 3: (Color online) Upper panel: Background spectra recorded with a HPGe detector on the Earth's surface (black) and underground at LUNA, with the copper and lead shielding described in Cavanna2014. Lower panel: Astrophysical S-factor of the $^2$H$(\mathrm{p},\gamma)$$^3$He reaction. The LUNA results are compared to the literature.
• Figure 4: (Color online) Background spectra obtained with passivated implanted silicon detectors overground (black lines) and underground (red lines) with and without lead shielding (solid and dashed, respectively) Bruno2015. The vertical lines indicate the region of interest (ROI) for the detection of low energy $\alpha$ particles from the $^{17}$O$(\mathrm{p},\gamma)$$^{14}$N reaction.
• Figure 5: Upper panel: Qualitative comparison between background spectra for thermalized neutrons (peak at $764\,\mathrm{keV}$) as obtained with $^3$He counters of different size and material, overground (black) and underground (red and blue histograms). Note how the low levels of intrinsic radioactivity of the stainless steel housing affords a further background reduction underground compared to traditional aluminium counters. Lower panel: Astrophysical $S$-factor of the $^{13}$C$(\alpha,\mathrm{n})$$^{16}$O reaction. The high precision of the LUNA data (solid symbols) Ciani2021, for the first time within the Gamow window (green shaded area), places stronger constraints on the theoretical fit (black line). Previous literature data are taken from Refs. Drotleff1993Heil2008.