Carbon burning in stars: an experimental study of the 12C(12C, p)23Na reaction towards astrophysical energies
Morales Gallegos2018.pdf (28.45Mb)
Morales Gallegos, Elia Lizeth
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Fusion reactions between 12C nuclei are among the most important in stellar evolution since they determine the destiny of massive stars (> 8 M). At thermonuclear energies (Ecm=1.5 ± 0.3 MeV), the 12C+12C reactions mainly proceed through 20Ne+α and 23Na+p channels. Since these energies are much lower than the height of the Coulomb barrier (Ecm= 6.1 MeV), the direct measurements of the 12C+12C reactions are very challenging because of the extremely small cross sections involved and the high beam-induced background originating from impurities in the targets. In addition, the 12C+12C reaction forms 24Mg at relatively high excitation energies (above the 12C 13.93 MeV thresholds) where molecular configurations are possible. Theoretical models fail to reproduce such structures and as a result, the extrapolation of high-energy cross section data towards the energy of astrophysical interest remain uncertain by 2-3 orders of magnitude. Further experimental efforts to measure at the lowest accessible energies are therefore in need. However, additionally to the extremely low cross sections and the resonant structure, the measurements at stellar energies of the 12C+12C reactions are troublesome due to natural hydrogen and deuterium contamination in the carbon targets. These target contaminants hamper the measurement of the 12C+12C process in all exit channels given that the 12C+1,2H reactions cross sections are considerably higher than that of the reaction of interest. In consequence, the use of ultra-low H content graphite targets and a study of the target behaviour under beam bombardment are necessary. This work focused on the experimental measurements of the 12C(12C,p)23Na and 12C(12C,α)20Ne reactions using charge particle detection. Although both channels were measured, only the proton channel was analysed and discussed in this thesis due to time constrains. The experiment was performed at the 3 MV pelletron tandem accelerator of the CIRCE (Centre for Isotopic Research on the Cultural and Environmental heritage) laboratory in Caserta, Italy. The experimental approach involved the development of optical calculations for optimal beam transportation (using the software COSY), the use of a four ΔE-Erest detectors system (a variable pressure CF4 ionization chamber used as the ΔE detector and a 300 mm2 Si detector used as the Erest) called GASTLY (GAs Silicon Two-Layer sYstem) and a study of the deuterium (hydrogen does not contribute to the beam-induced background at the detection angles and beam energies used here) contamination in graphite targets. The GASTLY detectors were placed at backward angles (121, 143 and 156o respect to the beam axis) and the 12C+12C reactions were investigated using carbon beams of Ecm=4.30 - 2.52 MeV with intensities of the order of μA. Highly Ordered Pyrolytic Graphite (HOPG) and highly pure (99.8%) natural graphite targets were used for the deuterium contamination study. A thermocamera was used to constantly monitor the target temperature during beam bombardment, allowing the investigation of target's deuterium content as a function of target temperature. Results showed a decrease in target's deuterium content of 53 - 80% in the target's temperature range of 200 - 1200 oC, depending on the type of target and detection angle. Furthermore, it was found that surrounding the scattering chamber with a nitrogen atmosphere while measuring low counting rate reactions (such as 12C+12C at low energies), the HOPG target's deuterium content decreases to about half its original value for a target temperature in the range between 800 - 1100 oC. For the 12C+12C reactions measurements, the HOPG target was used, maintaining high target temperatures. The p0-6 proton groups of the 12C(12C,p)23Na reaction were analysed and their yields, cross sections and astrophysical S-factors were obtained and are presented in this thesis. A comparison with previous data available in the literature is also presented, together with an indication for possible improvements in future investigations.