Abstract
Californium-252 is commonly used as a calibration source for neutron coincidence and neutron multiplicity counting in nuclear safeguards. Accurate knowledge of the neutron emission rate, age, and isotopics of the source (i.e., “source term”) are important for ensuring the best possible accuracies in item assay results. Californium-252 sources are not usually measured using gamma spectrometry, because of the assumption that their gamma-ray spectra appear to provide little information. However, gamma-ray signatures produced from 252Cf spontaneous fission products and odd-numbered Cf isotopes can be useful in determining source age and Cf isotopic composition. We demonstrate the utility of high-resolution gamma spectrometry in determining the age and isotopics of Cf neutron sources. In this work, five 252Cf sources were measured using a high-purity germanium detector. The neutron flux was small enough that radiation damage from prolonged neutron exposure was not substantial. Spectra were collected in 2-hour increments for 42–48 hours for quality control. The 2-hour spectra were added together to create a single spectrum for each source, and peak analyses were performed. Source ages were determined using a method that involved experimentally measuring the ratio of the gamma-ray emission rate of the 661.657 keV from 137Cs relative to a short-lived fission product and exploiting the dependence of this ratio on the source age. Source age was solved for using emission rates from four different gamma lines from four short-lived spontaneous fission products. Analysis results from all four fission products returned statistically similar source ages, but two had uncertainties greater than 25% because of high spontaneous fission product yield uncertainties as nominally high as 63%. Choosing short-lived fission products with small yield uncertainties is most important for accurately calculating source age. Additionally, the calculated ages did not always match the time since 248Cm separation well, which is often assumed to be the source age on the technical data sheet provided by the manufacturer. Activity ratios of 249Cf to 251Cf, the only Cf gamma emitters present in a 252Cf source, also did not match between experimental data and data on the technical data sheet provided by the vendor. It is thus recommended that newly purchased Cf sources be examined using gamma spectrometry to determine the source age and confirm isotopic composition of a 252Cf calibration source.
