18 Décembre – Thesis defense - Gwenaelle Gilardy
10 h Seminar room - CENBG (Gradignan)
Measuring the 7Li(a,g)11B reaction rate at temperatures relevant for the nu-process.
Part of the universal abundance of 11B is believed to come from the nu-process which takes place in core collapse supernovae at temperature between 1 GK and 5 GK. The main reaction chain responsible for the production of 11B is ending with the reaction 7Li(a,g)11B.
At the energies corresponding to the temperature range above, this reaction has only been measured by Paul et al. Phys. Rev.,164:1332–1342 (1967). Their measurement was suffering from background preventing the detection of any transition other than the transition corresponding to the direct capture of alpha, via resonance or not, to the ground state of 11B.
Our goal was to provide an additional measurement with the possibility to observe a gamma cascade and to update the stellar reaction rate accordingly. We measured gamma-rays coming from the 7Li(a,g)11B reaction between Ecm = 1.0 MeV and Ecm = 1.8 MeV using the Sta. Ana accelerator at the University of Notre Dame to produce the helium beam that was sent on a LiOH target. The gamma-rays were detected with a germanium detector at 45°. An R-matrix analysis of the differential cross section obtained was performed using the software Azure.
We found inconsistencies between the different measurements of the 7Li + a reaction. Two reaction rates were calculated using the measurement at 45° and the measurement of Paul et al. These two reaction rates are increased compared to the previous reaction rate recommended by NACRE.