Measurements of the Total-Neutron Cross-Section Minima in Natural Iron

Transmission measurements on natural iron samples were carried out at the Nevis Synchrocyclotron to determine the minima in the total cross section occurring between 10 and 500 keV. We used the NVS 200-m time-of-flight system and obtained an effective fwhm resolution of ≈0.6 nsec/m for these measurements. Values of the total cross section at the minima points were determined by the transmission-of-the-difference method for four thick samples ranging in inverse thickness (1/n) = 1.53 to 0.38 b/atom. Our value of the cross section at the important minimum at 24.4 keV is (0.51 ± 0.02) b. This value is close to the one appearing in the KEDAK data file (0.557 b), but considerably higher than the values in ENDF/B-I (0.15 b) and ENDF/B-II (0.285 b) files. Our value is in agreement with recent integral spectral measurements at RPI which show a preference for a value close to the KEDAK evaluation.