THE DETERMINATION OF I-129 IN MILK AND VEGETATION USING NEUTRON-ACTIVATION ANALYSIS

A new method has been developed to measure I-129 in the environment wi th detection limits below 10 mBq/kg of vegetation and 10 mBq/l of cows ' milk. The method is based on extraction of I-129 from the milk or ve getation sample, onto an ion exchange resin. An inactive carrier of I- 127 is added to the sample before separation, to monitor losses throug hout the entire procedure. The ion exchange resin is irradiated for 7. 5 h in a neutron flux of 10(16) n m(-2) s(-1) to induce the I-129 (n,g amma) I-130 reaction with thermal neutrons. The I-127 carrier undergoe s a (n,2n) reaction with fast neutrons to produce I-126. Iodine is ext racted from the ion exchange resin after irradiation with an elution s cheme which removes contamination from the radionuclide Br-82, the mai n interference in the analysis. Finally iodine is precipitated as AgI for gamma ray analysis. The sample is counted for 3 h on a Ge semicond uctor detector to measure the radionuclide I-130, which has a half lif e of 12.4 h and I-126, which has a half life of 13.0 days. The measure d I-130 activity is compared to a known standard to deduce the amount of I-129 in the sample, and the concentrations are corrected for losse s during processing using the measured activity of I-126. Th, detectio n limits for I-129 by this method are below 10 mBq/l for milk samples and 10 mBq/kg for vegetation. In addition to routine monitoring of mil k and grass samples the method has been used to measure I-129 depositi on on grass and soils in a field near the Sellafield plant. Results of these analyses, along with measurements of I-129 in air and rainfall using the same methodology, have been used to determine deposition vel ocity and retention coefficients of I-129 to grass.