Measurement of Pu-isotopes in lake sediments by alpha spectrometry

In this work, a technique for the measurement of Pu-isotopes (''239+240 Pu and ''238Pu) activity concentration in lake sediments, by alpha spectrometry, will be shown. For the measurement was used an alpha spectrometry system with four independent ion-implanted Si detectors. This system is devoted exclusively to the Pu-isotopes measurement and, consequently, a low background level can be kept for a long time and very accurate ''238Pu/''239+240 Pu activity ratios can be determined. In our working conditions (15000 s of counting time and 25% of efficiency) the Minimum Detectable Activity (MDA) was calculated to be 0.038 mBq. Sediment samples were treated in order to isolate the plutonium present in the sample and to adequate it to the measurement conditions. Some 5-10 g of sediment sample previously dried and homogenized, were traced with ''242 Pu, also alpha emitter, and reduced to ashes. Pu-isotopes were leached with 8M HNO3 during 8 hours. Liquid phase is separated by filtration and plutonium was co-precipitated with Fe''3+ as hydroxides. Since other actinides were also co-precipitated, it was necessary to isolate the plutonium. for it, the AG1x8 anion exchange resin was used. Finally, the sample was electroplated onto a stainless steel planchet and measurement in the semiconductor detector. The radiochemical yield range was 50-90% The goodness and accuracy of the radiochemical method of measurement was corroborated in an intercomparison exercise. (Author) 11 refs