Development of 135Cs/137Cs analysis adapted to low-level contaminated soils and sediments to identify the origin of radiocesium released into the environment ; Développement de l'analyse isotopique 135Cs/137Cs adaptée aux sols et aux sédiments faiblement contaminés pour identifier l'origine du radiocésium dans l'environnement

Several sources of radiocesium can contribute to the radioactive contamination of the environment. To discriminate them, the use of an isotopic ratio little explored until now is proposed: 135Cs/137Cs. To date, few data are available in the environment due to the existence of several analytical challenges.The current thesis has two main objectives. The first one is devoted to the development and the validation of the protocol allowing the 135Cs/137Cs ratio measurement of environmental samples. The second objective is to contribute to the knowledge of the radiocesium radiological background in France by analyzing samples of interest.The analytical performances of the developed protocol are very successful for soil or sediment samples of up to a hundred grams. The Cs recoveries exceed 80% and high purities of the measured fractions are obtained in only five days.The measurement is performed by mass spectrometry (ICP-MS/MS). The optimization of gas conditions in the collision/reaction cell allowed to obtain a sensitive method for cesium while eliminating the last traces of interferents.After the protocol development, its accuracy was verified by comparing the 135Cs/137Cs ratios measured in samples impacted by Chernobyl or Fukushima fallout with those reported in the literature.Finally, the validated protocol was applied, for the first time, to twenty-one soil or sediment samples collected in regions of France mainly impacted by fallout from nuclear weapons testing and/or Chernobyl accident. The acquired data allowed, first, to confirm and refine the isotopic signature of each 137Cs source and then to estimate the contributions of each of them in environmental samples. ; Plusieurs sources de radiocésium peuvent contribuer à la contamination radioactive de l'environnement. Pour les identifier, un rapport isotopique peu exploré jusqu'à présent est proposé : 135Cs/137Cs. À ce jour, peu de données sur ce rapport sont disponibles dans l'environnement en raison de l'existence de plusieurs défis analytiques.Cette thèse ...