Studies of charged-particle reactions for low-energy nuclear astrophysics require high sensitivity, which can be achieved by means of detection setups with high efficiency and low backgrounds, to obtain precise measurements in the energy region of interest for stellar scenarios. High-efficiency total absorption spectroscopy is an established and powerful tool for studying radiative capture reactions, particularly if combined with the cosmic background reduction by several orders of magnitude obtained at the Laboratory for Underground Nuclear Astrophysics (LUNA). We present recent improvements in the detection setup with the Bismuth Germanium Oxide (BGO) detector at LUNA, aiming to reduce high-energy backgrounds and increase the summing detection efficiency. The new design results in enhanced sensitivity of the BGO setup, as we demonstrate and discuss in the context of the first direct measurement of the 65 keV resonance (E _x = 5672 keV) of the ¹⁷O(p,γ)¹⁸F reaction. Moreover, we show two applications of the BGO detector, which exploit its segmentation. In the case of complex γ-ray cascades, e.g. the de-excitation of E _x = 5672 keV in ¹⁸F, the BGO segmentation allows to identify and suppress the beam-induced background signals that mimic the sum peak of interest. We demonstrate another new application for such a detector in form of in situ activation measurements of a reaction with β ⁺ unstable product nuclei, e.g. the ¹⁴N(p,γ)¹⁵O reaction.

The ${}^{12}\mathrm{C}/{}^{13}\mathrm{C}$ ratio is a significant indicator of nucleosynthesis and mixing processes during hydrogen burning in stars. Its value mainly depends on the relative rates of the ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ and ${}^{13}\mathrm{C}(p,\gamma ){}^{14}\mathrm{N}$ reactions. Both reactions have been studied at the Laboratory for Underground Nuclear Astrophysics (LUNA) in Italy down to the lowest energies to date ($E_{\mathrm{c}.\mathrm{m}.}=60\mathrm{keV}$) reaching for the first time the high energy tail of hydrogen burning in the shell of giant stars. Our cross sections, obtained with both prompt $\gamma$-ray detection and activation measurements, are the most precise to date with overall systematic uncertainties of 7%–8%. Compared with most of the literature, our results are systematically lower, by 25% for the ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ reaction and by 30% for ${}^{13}\mathrm{C}(p,\gamma ){}^{14}\mathrm{N}$. We provide the most precise value up to now of $3.6\pm 0.4$ in the 20–140 MK range for the lowest possible ${}^{12}\mathrm{C}/{}^{13}\mathrm{C}$ ratio that can be produced during H burning in giant stars.

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 ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ reaction is the onset process of both the CNO and hot CNO cycles that drive massive star, red and asymptotic giant branch star, and novae nucleosynthesis. The ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ rate affects the final abundances of the stable ${}^{12,13}\mathrm{C}$ nuclides with ramifications for meteoritic carbon isotopic abundances and the $s$-process neutron source strength. Here, an underground measurement of the ${}^{12}\mathrm{C}(p,\gamma ){}^{13}\mathrm{N}$ cross section is reported. The present data, obtained at the Felsenkeller shallow-underground laboratory in Dresden (Germany), encompass the 320–620 keV center of mass energy range to include the wide and poorly constrained $E=422$ keV resonance that dominates the rate at high temperatures. This work's $S$-factor results, lower than literature by 25%, are included in a comprehensive $R$-matrix fit, and the energy of the ${\frac{1}{2}}^{+}$ first excited state of ${}^{13}\mathrm{N}$ is found to be 2369.6(4) keV with a radiative and proton width of 0.49(3) eV and 34.9(2) keV, respectively. A reaction rate, based on the present $R$-matrix fit and extrapolation, is suggested.