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 with 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 vegetation sample, onto an ion exchange resin. An inactive carrier of I-127 is added to the sample before separation, to monitor losses throughout 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,gamma) I-130 reaction with thermal neutrons. The I-127 carrier undergoes a (n,2n) reaction with fast neutrons to produce I-126. Iodine is extracted from the ion exchange resin after irradiation with an elution scheme which removes contamination from the radionuclide Br-82, the main interference in the analysis. Finally iodine is precipitated as AgI for gamma ray analysis. The sample is counted for 3 h on a Ge semiconductor detector to measure the radionuclide I-130, which has a half life of 12.4 h and I-126, which has a half life of 13.0 days. The measured 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 losses during processing using the measured activity of I-126. Th, detection 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 milk and grass samples the method has been used to measure I-129 deposition 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 velocity and retention coefficients of I-129 to grass.