Neutron Capture Cross Section Measurement of ^129 I and ^127 I Using the NaI(Tl) Spectrometer of the ANNRI Beamline at J-PARC

The neutron capture cross sections of ^129 I and ^127 I were measured from the thermal to the keV energy regions with the NaI(Tl) spectrometer of the Accurate Neutron-Nucleus Reaction Measurement Instrument beamline in the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex. The neutron capture yield was determined by means of the total energy detection principle with the pulse-height weighting technique. The present cross section results for ^127 I were normalized using the saturated resonance method with a thick-enough ^197 Au sample and provide good overall agreement with JENDL-5 from thermal to about 500 keV. A resonance analysis with the REFIT code was performed and the resonance parameters for both ^129 I and ^127 I below 100 eV are presented in this work. In the case of ^129 I, four large resonances of ^127 I were employed for the cross section normalization since it was included as an impurity. The present results for ^129 I are the first experimental data to cover a wide neutron region from 10 meV to about 30 keV. In the neutron region between thermal and 20 eV, the present results display a different energy dependence than that in the JENDL-5 and JEFF-3.3 and much similar to that in ENDF/B-VIII.0. Notwithstanding, good agreement was found at the thermal region between the present measurement of 31.0 ± 1.4 b and both evaluated and most experimental data.