Characterization of californium sources by gamma spectrometry: relevance for nuclear forensics

High-resolution gamma spectrometry was used to study characteristics of californium neutron sources—age, isotopic composition and impurity radionuclides, as well as the effect of the isotopic composition on uncertainties involved in age determination. These characteristics are relevant during criminal investigations related to californium sources found outside of regulatory control. Age determination relies on measuring the activity ratios of long-lived and short-lived fission products of ²⁵⁰Cf and ²⁵²Cf. It is based on a known method but also introduces new pairs of isotopes that can be used for this purpose. The isotopic composition was estimated from the direct measurement of the activities of ²⁴⁹Cf and ²⁵¹Cf isotopes. The methods were tested using 5 californium reference sources at JRC-Karlsruhe. Additional raw data (gamma spectra) were provided by the German Radiation Protection Authority, and by the University of Novi Sad, Serbia. It has been shown that gamma spectrometry can be successfully used to characterize californium sources for nuclear forensic purposes.