The 139 La(n ,γ) cross section: Key for the onset of the s -process

Abstract

The nuclear resonance parameters and the capture cross section of the neutron magic isotope La139 have been measured relative to Au197 in the energy range of 0.6 eV to 9 keV at the neutron time-of-flight (n_TOF) facility at CERN. We describe the experimental apparatus and the data analysis procedures, which concerned mainly the efficiency correction by means of the pulse height weighting function technique and the determination of different background components. We extracted the resonance parameters and the main nuclear quantities such as the resonance integral RI=10.8±1.0b, the average widths for s and p waves a 1=0=50.7 5.4 meV and l=1=33. 6±6.9 meV, the nuclear level densities l=0=252A 22 eV and l=1<250 eV, and the neutron strength functions S0=(0.82±0.05) Ö10-4 and S1=(0.55±0.04)Ö10-4. These results represent a significant improvement over previous data. The deduced Maxwellian-averaged capture cross sections are important for the interpretation of the most recent spectroscopic observations in low-metallicity stars.