Fast determination of I-129 extracted from milk by direct gamma-spectrometry for the surveillance of nuclear waste repositories

The German Guidelines for Monitoring the Emission and Immission from Nuclear Facilities stipulates the monitoring of I-129 in milk produced in the vicinity of nuclear waste repositories when the release of this nuclide is possible. The required limit of detection is 2 Bq I-129/litre milk. This study uses an improved, recently developed method for rapidly separating I-129 from liquid milk in a form suitable for direct gamma-spectrometry of the characteristic 39.6 keV emission. The improvement to the separation method is described, and the use of I-125 as yield tracer investigated. As the gamma and X-ray emissions of I-125 and I-129 are of low energies that are not detectable with conventional coaxial gamma detectors a new type, so-called "broad energy" detector was used for counting. The complex emission spectra of preparations containing equal activities of both nuclides were well resolved and accurate count rates obtained for all peaks. Analyses of six 500 mi milk samples contaminated with 2.0 Bq/l each of I-129 and I-125 resulted in yields in the range 79-85% and a mean I-129 activity concentration of 1.92 (standard deviation 0.07) Bq/l determined after 24 h counting of the gamma line at 39.6 keV. Similar results were obtained with the 8-fold stronger X-ray at 29.6 keV. Counting at this energy attains the statutory detection limit within 0.5 h. Counting for 24 h decreases the limit to a few mBq.