“Angular distribution studies for the 16O(p,gamma)17F reaction at the LUNA400 accelerator”

In Nuclear Astrophysics, the proton capture reaction 16O(p, γ)17F plays an important role since it takes place in the H-burning zones of stellar interiors, particularly in the pp-chain and the CNO cycle. This reaction is crucial for the NO portion of the CNO cycle as it determines the abundances of 16O and 17O. These isotopes are found in the presolar meteorite grains which are assumed to be formed in the red giant stars. As such, it is important to determine the abundances of the Oxygen isotopes in the red giant stars. The 16O(p, γ)17F reaction occurs at energies of astrophysical interest (Q = 600.27 keV) through direct capture either to the ground state or the first excited state of 17F. To evaluate the thermonuclear reaction rate at the Gamow window (20 - 120 keV), extrapolations have been used since experimentally it is still challenging to reach that energy range. With the use of the LUNA400 accelerator, cross-sections can be measured at proton energies below 400 keV. Thus the uncertainties in the extrapolation to the astrophysical energy range can be reduced. In this thesis, the cross-section is measured by the detection of prompt gamma radiation which allows us to measure the contributions from the transitions to the ground state and to the 495.33 keV excited state of 17F. In particular, the angular distribution of emitted gammas has been measured. The measurement of the 16O(p, γ)17F cross section is done by impinging a proton beam on a solid Ta2O5 target. The target was prepared by the anodization of 0.25mm-thick Ta disks. The experiment has been carried out in the underground laboratory in order to minimise the background radiation contribution. The gamma rays from the de-excitation of 17F were detected by three detectors: one HPGe at 55°, one CeBr3 at 0° and one CeBr3 at 90°. The detectors were enclosed in a 10-cm-thick lead shielding. Thus, the angular distribution of the gamma rays has been analysed. The thesis contains the efficiency analysis of the detectors using three different sources: 137Cs, 60Co and 133Ba. The areas of the gamma peaks have been measured, followed by the fitting of the data to obtain efficiency calibration. Then yield calculation for three different energies has been performed. The areas of the peaks corresponding to the energies give the yields. Thus the angular distribution calculation on the basis of the efficiency normalised area has been performed. The yield obtained allowed the determination of the 16O(p, γ)17F cross section.