Low-Energy Accelerator Mass Spectrometry of actinides (236U, 237Np, 239, 240Pu) at the Centro Nacional de Aceleradores and its applications in oceanography
Bibliographic record
Abstract
The development of high sensitivity Accelerator Mass Spectrometry (AMS) systems has allowed the study of a series of long-lived actinides at ultra-trace levels, opening new research topics. 236U (T1/2 = 2.34•107 y), 237Np (T1/2 = 2.14•106 y), 239Pu (T1/2 = 2.41•104 y) and 240Pu (T1/2 = 6.35•103 y) are present in the general environment mainly as a consequence of different uses of nuclear energy. Because of their specific inputs to the oceans and their long half-lives, they are very useful tools in oceanography to study different processes according to their characteristic behaviour in the marine environment. However, because of their extremely small concentrations in seawater, their analysis entails important challenges and the level of development and establishment of their corresponding techniques have been different. Although Pu isotopes have been commonly study in seawater, these studies have mainly focussed on the combined analysis of 239+240Pu by radiometric techniques. More recently, the study of 240Pu/239Pu isotopic ratios has been possible thanks to high sensitivity mass spectrometry techniques. During the last years, many efforts have been developed to study 236U in seawater because of its great potential as tracer of ocean currents but much work is still necessary to complete the currently scarce data set worldwide. In contrast, 237Np studies in seawater are almost non-existent mainly due to the challenges associated to its chemical isolation and the lack of a long-live Np isotope to be used as yield tracer and as normalization isotope for AMS analysis. In this context, this PhD thesis is targeted at the implementation of the 236U, 237Np and 239,240Pu AMS techniques for seawater studies at the CNA. In collaboration with the IAEA Environment Laboratories (IAEA-NAEL), in Monaco, a new method for the sequential extraction of 236U, 237Np and 239,240Pu has been successfully stablished based on the use of 242Pu to control not only the 239,240Pu radiochemical recovery but also the 237Np one. For the optimization of the AMS analysis of these radionuclides at the CNA, we have developed a comprehensive study of the use of He as stripper. The new figures of merit of the technique for 236U, 239Pu and 240Pu has been established and, furthermore, we have developed and implemented the 237Np AMS measurement technique at this facility by using 242Pu as normalization isotope (non-isotopic normalization). Finally, the new techniques developed in this thesis have been applied for the study of 236U, 237Np and 239,240Pu in different ocean environments (Namibian coast, in the south-eastern Atlantic Ocean, Canada Basin, in the Arctic Ocean and the north-western Mediterranean Sea), validating the corresponding procedures and contributing to increase the current database of these radionuclides in seawater.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".