Speaker: Guy Terwagne, University of Namur (FUNDP)

Abstract Summary:

Stellar nucleosynthesis occurs in stars during the process of stellar evolution. Fusion process is responsible of the formation of light elements between C and Ca. The main fusion reactions in stars cooler that Sun are includes in the p-p chain while CNO cycle is dominant for stars hotter than the Sun. Those nuclear reactions occurring in the core of stars are produced in the Gamow energy window, which is located below the Coulomb barrier where the cross sections are very low. It is necessary that nuclear physicists measure the reaction cross section at very low energies and thus be sensitive to very low reaction rates. In particular for radiative capture reactions, we must determine the optimal experimental conditions to detect the gamma rays. This means realizing very low background measurements. The present project gives the usual constraints and the solutions we have chosen for the particular 13C(p,?)14N resonant reaction that is involved in the CNO cycle and which plays a key role in the nucleosynthesis of heavy elements in AGB stars. The reverse kinematics reaction 1H(13C,?)14N have been studied by bombarding an hydrogenated silicon sample with 13C ions in the energy range which corresponds to energies around the 511 keV resonance in the CM system. The experimental set-up consists of an HPGe detector (138% efficiency) installed in a passive shielding. The germanium detector is placed near the target in lead castle of ultra low background material, which is cover with a plastic scintillator to detect the muons produced in the high atmosphere of Earth. The efficiency of the muons detector has been measured and reduced the background in the energy region of interest of the gamma rays detector by a factor of 3. The HPGe detector placed in the lead castle detects 8 MeV ?-rays emitted from the capture reaction 1H(13C,?)14N, which is the reverse kinematics of the proton capture by 13C occurring in the CNO cycle of stars. In order to reduce the background the gamma ray detector is in anti-coincidence with the muons detector placed above lead castle.

First results using this very low background facility will be shown and the efficiency of the anticoincidence technique will be explained in detail.