Online spectral unmixing in gamma-ray spectrometry.

We introduced in a previous paper a time-dependent full-spectrum analysis algorithm speeding up the estimation of the activity of the radionuclides present in a sample. In this paper, we present a new version of the algorithm allowing online estimation. It uses only on a buffer of few segments while keeping the time information by using a time dependent regularization, thus reducing the size of the data matrices and the length of the processing of each iteration. The algorithm is optimized and tested on both simulated and measured spectra of aerosol samples.

Analysis of gamma-ray spectra with spectral unmixing - Part I: Determination of the characteristic limits (decision threshold and statistical uncertainty) for measurements of environmental aerosol filters.

Poisson-statistics based spectral unmixing has been shown to be an efficient analysis tool for the radionuclides activity estimation from gamma-ray spectrometry measurements. However, the calculation of the corresponding characteristic limits has not been investigated so far. In this paper, we present the quantification of the decision threshold and the limits of the coverage interval for the metrological use of such spectral unmixing algorithms. The proposed approach is evaluated and validated with simulated spectra of HPGe and NaI measurements by comparing the results to characteristic limits calculated from Monte-Carlo simulations. We focus particularly on the validation of the method for the metrological analysis of environmental measurements, for which the low-level activity quantification requires an accurate characteristic limits determination. Along with the instrument calibration, we establish a metrological analysis tool by using the spectral unmixing algorithm for environmental aerosol filters measured by gamma-ray spectrometry.

Analysis of gamma-ray spectra with spectral unmixing, Part II: Recalibration for the quantitative analysis of HPGe measurements.

In the context of radioactivity measurements, the quantitative analysis of a gamma-ray spectrum depends on the analysis algorithm. To that end, we recently introduced a Poisson statistics-based spectral unmixing approach. However, it also relies on a proper instrument recalibration as well as on an uncertainty estimation, for which no solution has been proposed so far. The goal of this article is twofold: i) we first present a novel method to correct for the instrument calibration of an HPGe detection system, which is tailored to spectral unmixing algorithms, and ii) we apply this new approach to the quantitative analysis of real data as well as on the evaluation of the uncertainty. Along with the characteristic limits determination investigated, this paper introduces the first full metrological analysis sequence of aerosol filter measurements based on spectral unmixing, which allows to quantify both the radionuclides' activities and their associated uncertainties.

Europe-Wide Atmospheric Radionuclide Dispersion by Unprecedented Wildfires in the Chernobyl Exclusion Zone, April 2020.

From early April 2020, wildfires raged in the highly contaminated areas around the Chernobyl nuclear power plant (CNPP), Ukraine. For about 4 weeks, the fires spread around and into the Chernobyl exclusion zone (CEZ) and came within a few kilometers of both the CNPP and radioactive waste storage facilities. Wildfires occurred on several occasions throughout the month of April. They were extinguished, but weather conditions and the spread of fires by airborne embers and smoldering fires led to new fires starting at different locations of the CEZ. The forest fires were only completely under control at the beginning of May, thanks to the tireless and incessant work of the firefighters and a period of sustained precipitation. In total, 0.7-1.2 TBq 137Cs were released into the atmosphere. Smoke plumes partly spread south and west and contributed to the detection of airborne 137Cs over the Ukrainian territory and as far away as Western Europe. The increase in airborne 137Cs ranged from several hundred µBq·m-3 in northern Ukraine to trace levels of a few µBq·m-3 or even within the usual background level in other European countries. Dispersion modeling determined the plume arrival time and was helpful in the assessment of the possible increase in airborne 137Cs concentrations in Europe. Detections of airborne 90Sr (emission estimate 345-612 GBq) and Pu (up to 75 GBq, mostly 241Pu) were reported from the CEZ. Americium-241 represented only 1.4% of the total source term corresponding to the studied anthropogenic radionuclides but would have contributed up to 80% of the inhalation dose.

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.

Coordinated underground measurements of gamma-ray emitting radionuclides for plasma physics research.

Forty-eight samples made of CaF2, LiF and YVO4 were placed inside the KSTAR Tokamak and irradiated by neutrons and charged particles from eight plasma pulses. The aim was to provide information for plasma diagnostics. Due to the short pulse durations, the activities induced in the samples were low and therefore measurements were performed in five low-background underground laboratories. Details of the underground measurements, together with data on the quality control amongst the radiometric laboratories, are presented.

Size distributions of airborne radionuclides from the fukushima nuclear accident at several places in europe.

Segregation and radioactive analysis of aerosols according to their aerodynamic size were performed in France, Austria, the Czech Republic, Poland, Germany, and Greece after the arrival of contaminated air masses following the nuclear accident at the Fukushima Dai-ichi nuclear power plant in March 2011. On the whole and regardless of the location, the highest activity levels correspond either to the finest particle fraction or to the upper size class. Regarding anthropogenic radionuclides, the activity median aerodynamic diameter (AMAD) ranged between 0.25 and 0.71 µm for (137)Cs, from 0.17 to 0.69 µm for (134)Cs, and from 0.30 to 0.53 µm for (131)I, thus in the "accumulation mode" of the ambient aerosol (0.1-1 µm). AMAD obtained for the naturally occurring radionuclides (7)Be and (210)Pb ranged from 0.20 to 0.53 µm and 0.29 to 0.52 µm, respectively. Regarding spatial variations, AMADs did not show large differences from place to place compared with what was observed concerning bulk airborne levels registered on the European scale. When air masses arrived in Europe, AMADs for (131)I were about half those for cesium isotopes. Higher AMAD for cesium probably results from higher AMAD observed at the early stage of the accident in Japan. Lower AMAD for (131)I can be explained by the adsorption of gaseous iodine on particles of all sizes met during transport, especially for small particles. Additionally, weathering conditions (rain) encountered during transport and in Europe in March and April contributed to the equilibrium of the gaseous to total (131)I ratio. AMAD slightly increased with time for (131)I whereas a clear decreasing trend was observed with the AMADs for (137)Cs and (134)Cs. On average, the associated geometric standard deviation (GSD) appeared to be higher for iodine than for cesium isotopes. These statements also bear out a gaseous (131)I transfer on ambient particles of a broad size range during transport. Highest weighted activity levels were found on the 0.49-0.95 µm and on the 0.18-0.36 µm size ranges in France and in Poland, respectively. The contribution from resuspension of old deposited (137)Cs was assessed for the coarse particle fractions only for the first sampling week.