The neutron total cross section of ¹²⁹l below 1 keV

The attention of the scientific community to the disposal and reprocessing of the spent reactor fuel has become the stimulus of more sophisticated R&D activities from which the stringent demand of safety has to benefit via an increased accuracy of the nuclear data. The CEA-IRMM collaboration voices since years the general interest of large nuclear power contractor, like France, in the transmutation of long lived fission products and minor actinides. ¹²⁹l is a very noxious isotope having a very long β half-life (1.57-10⁷ y), high volatility in the elemental state, high solubility in water, high geo-chemical mobility and potential long-term radiotoxicity: about 400 kg of ¹²⁹l are produced yearly in the reactors of the EU countries. An official request of France has been presented demanding neutron thermal and fast spectra measurements with emphasis on capture cross section data (NEA/NSC/DOC-97-4). By the capture of a neutron ¹²⁹l transmutes in ¹³⁰l, which decays β^- with 12.36 h half-life to the stable ¹³⁰Xe.