Systematic discrepancy in photoneutron cross sections for medium and heavy nuclei

The shape and magnitude of the photoneutron cross sections for Cu, Pr, Pb, and Bi are compared to find out a systematic discrepancy between the cross sections obtained using the NBS P2 chamber as a bremsstrahlung beam monitor and those obtained by other experimental techniques such as the positron annihilation-in-flight. The former cross sections in general are observed to be systematically larger than the latter. The discrepancy of about 10 percent is demonstrated in the peak cross section values for medium nuclei. This may be explained in the good direction by a systematic discrepancy of 6 percent in the calibration constants of the NBS P2 chamber given at several laboratories, because the lowest values in these calibrations have been widely used to determine the photoneutron cross sections from measured neutron yield curves. The highest values can almost eliminate the 10 percent discrepancy. The definite discrepancy can not be demonstrated for heavy nuclei because of the large discrepancies of 20 percent or more found in the cross sections obtained with γ-rays from positron annihilation-in-flight. 3 tables, 3 figures