Year
2019
Abstract
Californium-252 is commonly used as 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 are important in assuring best possible accuracies in sample assay results. 252Cf sources are not usually measured using gamma spectrometry for metrological purposes, 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 indeed 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 californium neutron sources. In this work, five 252Cf sources were measured using a High Purity Germanium (HPGe) detector. The neutron flux was small enough that radiation damage from prolonged neutron exposure was not substantial. Spectra were collected in two-hour increments for 42-48 hours. The two-hour spectra were added together to create a single spectrum and peak analyses were performed. Sources ages were determined using a method that involved experimentally measuring the intensity of a gamma ray from a short-lived fission product relative to the intensity of the 661.657 keV from 137Cs,and exploiting the dependence of this ratio on the source age. Source age was solved for using intensities from five different gamma lines from five short-lived fission products. Analysis results from all five fission products returned statistically similar source ages, but two had uncertainties greater than 25% because of high fission product yield uncertainties as high as 63%. Choosing short-lived fission products with small yield uncertainties is most important for accurately calculating source age. Additionally, calculated ages did not match time since 248Cm separation, 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, also did not match between experimental data and data on the technical data sheet. It is thus recommended that newly purchased Cf sources be examined using gamma spectrometry to find source age and confirm isotopic composition of a 252Cf calibration source.