| Environmental Pu239 arises from the development, production and testing of nuclear weapons. Fallout of this weapon debris has produced an average worldwide Pu239 level of about 1 ppb. Traditional methods are available to measure Pu239 at fCi levels, but these methods are unable to detect Pu239 at environmental levels (100 aCi). Such an ultra sensitive method chemically separates Pu239 from naturally occurring uranium and other fissile isotopes. Then the technique is applied to the Pu239 burden effluent.;Fissile samples are exposed to distinct neutron spectra to achieve quantitative analysis of each fissile isotope using fission track analysis without chemical separation. The University of Utah's 100 kW TRIGA reactor was used to deliver thermal and epithermal neutron fluences. Each isotope has a unique fission cross-section for different neutron energy ranges. It was possible to differentiate between the isotopes based on the number of fission tracks produced. The proposed technique can measure the amount of Th232, U235, and Pu239 in the sample simultaneously. Samples containing known amounts of Pu239, natural uranium (U235 and U238) were deposited on the polycarbonate Lexan detectors and irradiated in different positions in the reactor core. They were chemically etched and fission tracks were counted. The concentrations were calculated using the computer code COG and the mathematical correlations for the two irradiators. NU=-2.754 0+/-0.25x1011 T1+1.9654+/-0.24 x1012T2 NPu=3.3 861+/-0.27x107 T1-1.4973+/-0.25 x108T2 aU,1= 6.2479+/-0.17x10-13 ,aU,2= 1.1304+/-0.23x10-13 aPu,1= 8.1976+/-0.16x10-11 ,aPu,2= 1.1492+/-0.23x10-11;T1 and T2 are the number of tracks counted from samples when exposed to a known neutron spectra. Without chemical processing, the detection limit of ∼120 aCi/g can easily be achieved. Calculated amounts ranged within a factor of two to three from a known quantity deposited on the Lexan detectors. The uncertainties associated with this method stem from the differences in theoretical and actual neutron flux in the irradiators, the inefficiencies with etching process, and errors with the track counting activity. |
