Evaluation of the fission track analysis for determination of trace-amounts of 239Pu

The fission track technique was studied and evaluated for critical circumstances in the overall process of the analysis. Fission Track Analysis (FTA) is used for determination of 239Pu at a femto gram level in biological and environmental samples. The technique is a type of neutron activation where 239Pu is determined by the registration of fission fragments in quartz after irradiation with thermal neutrons in a nuclear reactor. Plutonium is extracted from the sample by chemical separation processes in several steps in order to retrieve a pure plutonium sample. The method for the chemical separation was selected to suppress the influence from naturally occurring uranium in the sample since the cross-section of thermal neutron fission of 235U is in the same order of magnitude as the cross-section for 239Pu. The following parts of the fission track analysis were regarded as critical and subjects of further evaluation: the quenching of fission tracks, the efficiency of the solvent extraction used in the final step of the chemical separation, the efficiency of the uranium de-contamination, the background contribution from the yield determinant 236Pu, the interference from 232Th (which fissions with fast neutrons) and the calibration process. The study was performed in order to assure the accuracy of the 239Pu determination and showed that none of the mentioned critical parts prevent a successful performance of FTA with a μBq detection limit.