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.