Calculated Gamma-Ray Spectra for keV Neutron Capture in ²⁴⁰Pu, ²⁴²Pu, and ²³⁸U

Capture gamma-ray spectra of ²⁴⁰Pu, ²⁴²Pu, and ²³⁸U were calculated in the framework of the spherical optical model and the statistical model. A consistent set of input parameters was determined from available experimental information or from model-guided systematics. The complete gamma-ray cascades were calculated considering all possible transitions up to multiplicity seven. All experimental information on level schemes and gamma-ray transition probabilities of the compound nuclei was explicitly included as input. The capture gamm-ray spectra were used to correct experimental data for the capture cross sections of ²⁴⁰Pu and ²⁴²Pu from a relative measurement using a Moxon-Rae detector with a graphite converter and with ¹⁹⁷Au and ²³⁸U as standards. This correction is required to take into account that the detector efficiency is not exactly proportional to the gamma-ray energy. The resulting correction factors proved to be negligible for measurements relative to ²³⁸U; whereas, they are ∼3% if gold is used as a standard.