First measurement of the Nb-94(n,gamma) cross section at the CERN n_TOF facility

Abstract

One of the crucial ingredients for the improvement of stellar models is the accurate knowledge of neutron capture cross-sections for the different isotopes involved in the s-,r- and i- processes. These measurements can shed light on existing discrepancies between observed and predicted isotopic abundances and help to constrain the physical conditions where these reactions take place along different stages of stellar evolution.

In the particular case of the radioactive Nb-94, the Nb-94(n,gamma) cross-section could play a role in the determination of the s-process production of Mo-94 in AGB stars, which presently cannot be reproduced by state-of-the-art stellar models. There are no previous Nb-94(n,gamma) experimental data for the resolved and unresolved resonance regions mainly due to the difficulties in producing high-quality samples and also due to limitations in conventional detection systems commonly used in time-of-flight experiments.

Motivated by this situation, a first measurement of the Nb-94(n,gamma) reaction was carried out at CERN n_TOF, thereby exploiting the high luminosity of the EAR2 area in combination with a new detection system of small-volume C6D6-detectors and a high quality Nb-94-sample. The latter was based on hyper-pure Nb-93 material activated at the high-flux reactor of ILL-Grenoble. An innovative ring-configuration detection system in close geometry around the capture sample allowed us to significantly enhance the signal-to-background ratio. This set-up was supplemented with two conventional C6D6 detectors and a high-resolution LaCl3(Ce)-detector, which will be employed for addressing reliably systematic effects and uncertainties.

At the current status of the data analysis, 18 resonance in Nb-94+n have been observed for the first time in the neutron energy range from thermal up to 10 keV.