Direct Measurements of the 23Na(p,γ)24Mg Cross Section at Stellar Energies

Asymptotic Giant Branch (AGB) stars play a fundamental role in the determination of the observed abundances of light mass elements in the universe. In spite of great efforts, model predictions for the abundance of nuclides in AGB stars are rather uncertain due to the complex physical processes in the star. The lack of precise cross-section data, for the relevant proton capture reactions in particular, worsens the reliability of the model predictions for light element yields. The reaction 23Na(p, γ) 24Mg links the NeNa and the MgAl cycles of stellar burning, and is active in stars that burn at the highest temperatures (such as AGB stars), or in explosive scenarios. This link between the two cycles, in competition with the reaction 23Na(p, α) 20Ne, makes the reaction rate of 23Na(p, γ) 24Mg a relevant input parameter, whose precise knowledge is important for the determination of the abundances of – for example – neon, sodium, magnesium and aluminum. The subject of this work is an experiment for direct cross section measurements of 23Na(p, γ) 24Mg at LUNA, the Laboratory Underground for Nuclear Astrophysics. The location of this experiment, underground at the Gran Sasso National Laboratory (LNGS) allows gamma ray measurements with greatly enhanced sensitivity. In the first chapter of this thesis, a brief introduction to nuclear astrophysics is given, with a focus on the experimental aspects of direct cross section measurement. The current knowledge about 23Na(p, γ) in view of astrophysics is summarized in this chapter. Chapter two describes the experimental setup for the experiment, from the accelerator to the used detector setups. The third chapter presents the detector characterization in more detail, including Monte Carlo simulations and detector calibration. Chapter four exposes the measurements and their analyses, starting with the development and experimental study of sodium targets, and coming to the measurements with the two detector setups. The final chapter of this work discusses the possible implications on the 23Na(p, γ) 24Mg reaction rate from the current results, and gives an outlook about the next steps and future experimental work to further improve the knowledge about this reaction.