Radiative Capture Cross Sections for Fast Neutrons

Neutron capture cross sections have been measured for 28 elements in the neutron energy range 175 to 1000 kev. The method used was detection of capture gamma radiation in a one-meter-diameter liquid scintillator, the capture samples being placed at the center for irradiation by a pulsed neutron beam. Absolute cross sections have been determined by comparison with the known capture-plus-fission cross section of ${\mathrm{U}}^{235}$; both capture and fission events are detected with nearly 100% efficiency in this arrangement. Corrections have been made for loss of capture-gamma-ray energy, by means of pulse height analyses and comparison between one-meter and 48-cm scintillator results; corrections have also been made for change in path length due to scattering. The results obtained appear to agree reasonably well with the predictions of resonance theory, with level densities given by the Fermi gas model together with the effective ground state of Hurwitz and Bethe. The low capture cross sections of magic-number target nuclides appear well correlated with the magic-number effects on the mass.