The ${}^{20}\mathrm{Ne}{(p,\gamma )}^{21}\mathrm{Na}$ reaction is the slowest in the NeNa cycle and directly affects the abundances of the Ne and Na isotopes in a variety of astrophysical sites. Here we report the measurement of its direct capture contribution, for the first time below $E_{\mathrm{c}.\mathrm{m}.}=352$ keV, and of the contribution from the $E_{\mathrm{c}.\mathrm{m}.}=368$ keV resonance, which dominates the reaction rate at $T=0.03$–1.00 GK. The experiment was performed deep underground at the Laboratory for Underground Nuclear Astrophysics, using a high-intensity proton beam and a windowless neon gas target. Prompt $\gamma$ rays from the reaction were detected with two high-purity germanium detectors. We obtain a resonance strength $\omega \gamma =(0.112\pm 0.{002}_{\mathrm{stat}}\pm 0.{005}_{\mathrm{sys}})\mathrm{meV}$, with an uncertainty a factor of 3 smaller than previous values. Our revised reaction rate is 20% lower than previously adopted at $T<0.1$ GK and agrees with previous estimates at temperatures $T\ge 0.1$ GK. Initial astrophysical implications are presented.

The ¹² C+ ¹² C fusion reaction plays a crucial role in stellar evolution, including the occurrence of supernova explosions, and in the synthesis of the chemical elements. However, our understanding of its cross section remains severely deficient, particularly below E _cm = 2.5 MeV, the energy range of interest for astrophysics. To address these unresolved issues, the LUNA collaboration will conduct a dedicated study of the ¹² C+ ¹² C reaction at the Bellotti Ion Beam Facility (Bellotti IBF) located deep underground within the Gran Sasso National Laboratory (LNGS) in Italy. Based on the combination of passive and active shields, this campaign aims to achieve unprecedented sensitivity in measuring the cross sections of the two key reaction channels, ¹² C( ¹² C, α ) ²⁰ Ne and ¹² C( ¹² C, p ) ²³ Na in the low-energy regime via γ -ray detection. Here, we report on a sensitivity study for the upcoming campaign with a focus on the characterization of two detectors, namely a HPGe detector and a NaI(Tl) array. Furthermore, their intrinsic contamination is thoroughly investigated since this could potentially influence the overall sensitivity. Assuming typical beam intensities of the Bellotti IBF, we will be able to investigate reaction rates significantly below 100 counts per day. In case of the ¹² C+ ¹² C reaction we therefore expect to acquire experimental data well below the current limit of E _cm = 2.1 MeV. The results are supported by simulations to highlight the advantageous low-background environment, essential for high-precision nuclear astrophysics studies.

Nuclear reactions are responsible for the chemical evolution of stars, galaxies and the Universe. Unfortunately, at temperatures of interest for nuclear astrophysics, the cross-sections of the thermonuclear reactions are in the pico- femto-barn range and thus measuring them in the laboratory is extremely challenging. In this framework, major steps forward were made with the advent of underground nuclear astrophysics, pioneered by the Laboratory for Underground Nuclear Astrophysics (LUNA). The cosmic background reduction by several orders of magnitude obtained at LUNA, however, needs to be combined with high-performance detectors and dedicated shieldings to obtain the required sensitivity. In the present paper, we report on the recent and future detector-shielding designs at LUNA.

In the nova temperature range, 0.1 GK < T < 0.4 GK, several low-energy resonances dominate the Ne 21 ( p , γ ) Na 22 reaction rate, which is currently affected by large uncertainties. We present a high-precision study of the resonances at E r lab = 127.3 , 271.4, 272.3, 291.5, and 352.6 keV, measured directly at the Laboratory for Underground Nuclear Astrophysics in Italy. The strengths of the 127.3-, 271.4-, and 291.5-keV resonances are consistent with previous measurements within 1 σ . However, for the 272.3- and 352.6-keV resonances, we report strength values of (129.9 ± 5.8) meV and (14.9 ± 0.8) meV, respectively, more than a factor of 1.5 higher than literature values. In addition, we report on new branching ratios for the 127.3-, 272.3-, and 352.6-keV resonances, leading to updated decay schemes. Finally, we present a revised thermonuclear reaction rate and investigate its impact on the NeNa nucleosynthesis.

The ¹³C(α,n)¹⁶O reaction is the main neutron source of the s-process taking place in thermally pulsing AGB stars and it is one of the main candidate sources of neutrons for the i-process in the astrophysical sites proposed so far. Therefore, its rate is crucial to understand the production of the nuclei heavier than iron in the Universe. For the first time, the LUNA collaboration was able to measure the ¹³C(α,n)¹⁶O cross section at E _c.m. = 0.23−0.3 MeV drastically reducing the uncertainty of the S(E)-factor in the astrophysically relevant energy range. In this paper, we provide details and critical thoughts about the LUNA measurement and compare them with the current understanding of the ¹³C(α,n)¹⁶O reaction in view of future prospect for higher energy measurements. The two very recent results (from the University of Notre Dame and the JUNA collaboration) published after the LUNA data represent an important step forward. There is, however, still room for a lot of improvement in the experimental study of the ¹³C(α,n)¹⁶O reaction, as emphasized in the present manuscript. We conclude that to provide significantly better constraints on the low-energy extrapolation, experimental data need to be provided over a wide energy range, which overlaps with the energy range of current measurements. Furthermore, future experiments need to focus on the proper target characterisation, the determination of neutron detection efficiency having more nuclear physics input, such as angular distribution of the ¹³C(α,n)¹⁶O reaction below E _α < 0.8 MeV and study of nuclear properties of monoenergetic neutron sources and/or via the study of sharp resonances of ¹³C(α,n)¹⁶O. Moreover, comprehensive, multichannel R-matrix analysis with a proper estimate of uncertainty budget of experimental data are still required.