Using fission track analysis techniques to measure plutonium isotopic ratios.

摘要

The alpha decay energies for the plutonium isotopes 240 and 239 are too similar for the resolution of standard alpha spectrometry detectors to differentiate the isotopes which are therefore routinely reported as a combined activity. Knowledge of the 240/239 ratio is important to understand the source and possible previous use of the plutonium. A process was investigated to measure the 240/239 ratio using a combination of alpha spectrometry and fission track analysis methods. Samples were chemically processed to isolate plutonium which was then electro-deposited on stainless steel planchettes. The activities of the samples were determined using an alpha spectrometer. The planchettes were then placed against polycarbonate detectors and the assembly was irradiated with a neutron flux. Neutrons caused the 239Pu to fission. Fission fragments entered the polycarbonate detector and caused a trail of damage. After etching the detectors, tracks were visible under a microscope from the fission fragment damage.; The mean chemical yield value for filter and soil samples was 21.7% and 9.2% respectively. The mean 240/239 ratio for four Waste Isolation Pilot Plant filters was 0.08 +/- 0.1 and 0.7 +/- 0.9 for five Rocky Flats soil samples. Due to low sample activities and low track densities the relative uncertainties for the measured samples were 0.63 and 1.07 for the Waste Isolation Pilot Plant air filter and Rocky Flats soil reference samples respectively. Uncertainty calculations and an empirical relationship of the process supported the high relative uncertainties for those samples. The primary source of uncertainty was determined to be the probabilistic nature of the fission process. Further calculations determined that the minimum relative uncertainty can be reduced to 0.042 if the track density was increased to approximately one thousand and the sample activity was 10 Bq or greater.