Speakers
Description
The ${}^{24}\mathrm{Mg}(p,\gamma){}^{25}\mathrm{Al}$ reaction plays a key role in the Mg–Al cycle, influencing the nucleosynthesis of intermediate-mass nuclei, the chemical evolution of asymptotic giant branch stars, and the production of radioactive ${}^{26}\mathrm{Al}$. Despite its astrophysical importance, the reaction rate at stellar energies remains poorly constrained due to the extremely low cross sections within the sub-resonant Gamow window.
This work presents an experimental study of the ${}^{24}\mathrm{Mg}(p,\gamma){}^{25}\mathrm{Al}$ reaction carried out at the INFN Laboratory of Gran Sasso within the LUNA collaboration. The underground environment provides a strong suppression of cosmic-ray induced background, enabling high-sensitivity measurements. However, beam-induced background from target contaminants remains a major source of systematic uncertainty, often exceeding the signal of interest by several orders of magnitude. For this reason, particular attention has been devoted to the production and characterization of high-purity magnesium targets.
Magnesium thin films were prepared using thermal evaporation and magnetron sputtering techniques, combined with different backing materials to optimize both purity and stability. The targets were characterized through Rutherford Backscattering Spectrometry and Nuclear Reaction Analysis, allowing precise determination of thickness, stoichiometry, and impurity depth profiles.
We focus our investigation on the experimentally accessible energy range between 200 and 400 keV, which includes two narrow resonances and a direct-capture component. Reaction $\gamma$-rays were acquired using two different setups: a high-efficiency $4\pi$ BGO detector and an HPGe detector.
A coincidence-based analysis technique was developed for the first setup, exploiting detector segmentation together with multiplicity and energy-gating conditions to enhance sensitivity to weak capture signals. This method was validated on the $E_p = 223$ keV resonance and subsequently applied to the off-resonance data, improving the determination of the cross section in the region where direct capture is dominant.
For the second, the excellent energy resolution of the detector and the stability of the LUNA - 400 accelerator, allowed an improved determination of the low energy resonance energy and provided an independent study of the off-resonance cross section.
In this talk, we present the results of both analysis, which demonstrate the effectiveness of combining high-purity targets, underground measurements, and advanced detection techniques for low-energy radiative-capture studies. We also present our stellar rate evaluations with uncertainties, performed with a Monte Carlo R-matrix analysis.