Low level measurement of (60)Co by gamma ray spectrometry using γ-γ coincidence

A comprehensive assessment of two-decade radioactivity monitoring around the Channel Islands

The Channel Islands are located in the Normand-Breton Gulf (NBG), in the mid-part of the English Channel (France, Normandy). In the northern part, off Cap La Hague, controlled amounts of radioactive liquid waste are discharged by the ORANO La Hague nuclear fuel reprocessing plant (RP). Radionuclides were monitored in the NBG to assess the dispersion of radioactive discharges from the RP in the marine environment. The temporal and spatial distribution of the data are consistent with the history of the discharges, with most gamma emitter radionuclide environmental levels being close to or below the current limits of detection. A clear fingerprint of H-3, C-14 and I-129 radionuclides discharged from the RP is measured. The hydrodynamics in the NBG do not yield a simple gradient with linear distance from the outfall of the RP. Modelling tools were used to understand how radioactive discharges spread from the source of input. Dispersion patterns clearly illustrate the different behaviours of soluble and non-soluble radionuclides. The study indicated that the footprint of radioactive liquid discharges by French nuclear facilities was still measurable in species collected from the NBG for the mostly dissolved radionuclides. The less conservative ones, with a high affinity for suspended matter, are potentially influenced by old releases. These pathways could be investigated by dedicated hydrodynamic dispersion models. Overall, in the Channel Islands the levels are low and consistent with the general decrease in liquid radionuclide discharges by the RP since the 1990s.

Measurements of thorium and uranium daughters in radioenvironmental samples using γγ-coincidence spectrometry

We present the performance of a γγ-coincidence spectrometer for measuring the activities of thorium and uranium daughters in environmental samples. The spectrometer consists of two NaI(Tl) detectors facing each other inside a low-background passive shield. We present coincidence gating schemes for achieving the best signal-to-noise ratios, coincidence detection efficiencies, background levels, and minimum detectable activities. The spectrometer is simulated using Geant4 to correct sample efficiencies for self-absorption effects. The device is used to measure thorium and uranium daughter activities in Brazil nuts, potting mix, and magazine paper. Our results for Brazil nuts agree with some, but not all, previous measurements. Thorium or uranium daughter activities have previously not been reported for commercial potting mix. For magazine paper, our measured activities are lower than most previously determined values.

Leda: A gamma-gamma coincidence spectrometer for the measurement of environment samples

This paper presents a new gamma-gamma coincidence spectrometer to measure the radioactivity in environmental samples. This system, called Leda, is made of 2 HPGe and 1 NaI(Tl). The different analysis channels (single, in coincidence or in anti-coincidence) possible thanks to the digital electronics are described. Results obtained with environmental samples are shown. Despite its low efficiency, this method improves the detection limits for all emitters due to its very low background, and thus decreases the counting time.