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Most of the anthropogenic radionuclide 129I released to the marine environment from the nuclear fuel reprocessing plants (NFRP) at Sellafield (England) and La Hague (France) is transported to the Arctic Ocean via the North Atlantic Current and the Norwegian Coastal Current. 129I concentrations in seawater provides a powerful and well-established radiotracer technique to provide information about the mechanisms which govern water mass transport in the Nordic Seas and the Arctic Ocean and is gaining importance when coupled with other tracers (e.g. CFC, 236U). In this work, 129I concentrations in surface and depth profiles from the Nordic Seas and the North Atlantic (NA) Ocean collected from four different cruises between 2011 and 2012 are presented. This work allowed us to i) update information on 129I concentrations in these areas, required for the accurate use of 129I as a tracer of water masses; and ii) investigate the formation of deep water currents in the eastern part of the Nordic Seas, by the analysis of 129I concentrations and temperature-salinity (T-S) diagrams from locations within the Greenland Sea Gyre. In the Nordic Seas, 129I concentrations in seawater are of the order of 109 at·kg− 1, one or two orders of magnitude higher than those measured at the NA Ocean, not so importantly affected by the releases from the NFRP. 129I concentrations of the order of 108 atoms·kg− 1 at the Ellet Line and the PAP suggest a direct contribution from the NFRP in the NA Ocean. An increase in the concentrations in the Nordic Seas between 2002 and 2012 has been detected, which agrees with the temporal evolution of the 129I liquid discharges from the NFRPs in years prior to this. Finally, 129I profile concentrations, 129I inventories and T-S diagrams suggest that deep water formation occurred in the easternmost area of the Nordic Seas during 2012., This work has been partially financed by the project FIS2015-69673-P by the Spanish Ministerio de Economía y Competitividad.

Trabajo presentado en la 14th International conference on Accelerator Mass Spectrometry, celebrada en Ottawa (Canadá), del 14 al 18 de agosto de 2017

Tesis llevada a cabo para conseguir el grado de Doctor por la Universidad de Sevilla--2018-09-28, The accelerator mass spectrometry (AMS) technique, developed 40 years ago for 14C dating, was soon used to measure other long-lived radionuclides. One of the radionuclides measured by AMS since these early years is 41Ca and for a variety of applications. Some of these applications are: the calculation of the terrestrial age of meteorites, the study of nuclear reactions of astrophysical interest, the understanding of the calcium metabolism, and the characterization of low-level nuclear waste. The main challenge in 41Ca AMS is dealing with the interference caused by its stable isobar, 41K. This interference is reduced by using calcium 2uoride (CaF2) samples and the extraction of the (CaF3)- ion. At large AMS facilities, besides, 41K ions can be discriminated from 41Ca ions using different detection techniques based on the energy loss dependence on the atomic number. The 41Ca measurement with low energy AMS systems, like the 1 MV system at the Centro Nacional de Aceleradores (CNA), is quite challenging, since this discrimination is not possible. Nevertheless, the 41K contribution can be estimated and, therefore, corrected, thanks to the sequential detection of the other stable isotope of potassium, 39K (K-correction). Although the sensitivity achieved in 41Ca AMS at low energies is 3-4 orders of magnitude lower than those achieved at larger facilities, it allows the competitive measurements for biomedical applications, and the characterization of concrete samples from nuclear reactor bioshields. Since 41Ca AMS at low energies is limited to the estimation of 41K interference, it is advisable to study the different ways toward the production of this interference. Some factors related to it have been studied in different experiments performed with the 1 MV AMS system at CNA (SARA) and the 600 kV AMS system at the ETH Laboratory of Ion Beam Physics in Zurich (TANDY). For instance, we could demonstrate that 41K can be injected also as the (41K57Fe)- molecular ion. As a consequence, 41K interference is dependent on the materials used during the sample pressing. We also proved that, even when both 41K/40Ca and 39K/40Ca ratios change over time, the relation between both, 41K/39K remains constant. Therefore, the Kcorrection is a robust method to estimate the 41K interference. The information provided from these experiments has contributed to the setting up and optimization of the 41Ca measurements with the SARA system at CNA, which was the main goal of this thesis. The results are not only useful for measurements at our system, but also for other very similar HVE 1 MV AMS systems. Several tests have been performed during these years to study and optimize all the performance parameters of 41Ca measurements with this system: ionization eWciency, transmission and destruction of the molecular background in the stripper, ion optical transport, detection eWciency, and 1nal sensitivity. Mixing the CaF2 samples with silver powder, our ion source produces stable (40CaF3)- currents between 50 and 150 nA. In comparison with the TANDY system at ETH, the slightly lower transmission for the 2+ state through the helium stripper (40% at SARA, 50% at TANDY) is compensated by the better optical transmission in the high energy sector (90-100% at SARA, 80-85% at TANDY). This is due to the quadrupole triplet which refocus the beam at the exit of our accelerator. The capabilities of both systems for 41Ca AMS are equivalent. 41Ca/40Ca backgrounds found in the system, in the 10-12 range, allow, among other applications, the characterization of the 41Ca content in the bioshield from nuclear reactors. Within the colaboration between the Spanish radioactive waste management agency (ENRESA) and the AMS Research group at CNA, a detailed study of the 41Ca spatial distribution in the bioshield of the Jos´e Cabrera nuclear power plant has been performed. A radiochemical method for concrete samples have been developed in order to deal with the relatively large number of samples involved in this study. The measured 41Ca/40Ca ratios in the areas of maximum neutron 2uence were on the 10-6 range, while they get down to the 10-10 range in regions far from the reactor cavity. While the 41Ca/40Ca attenuation pro1le follows an ideal behavior in some areas, it does not in other parts where the in2uence of diffused thermal neutrons is higher.

We present a detailed study of the performance parameters for 41Ca measurements at the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA) in Seville. Mixing CaF2 with Ag, we get stable (40CaF3)− currents between 50 and 100 nA. Transmission of the 2+ state in the He stripper is 41%, while optical transmission in the HE sector is typically higher than 95%. At low energies we cannot separate 41Ca from the isobar 41K at the detector, but the interference is reduced by using (41CaF3)− ions. The remaining contribution is corrected by measurement of the other stable isotope of potassium, 39K, to estimate this interference (K-correction). After this correction, we reach a 41Ca/40Ca background level between 5 × 10−12 and 8 × 10−12, making possible measurements of 41Ca/40Ca ratios higher than 4 × 10−11. Intercomparisons with the 0.6 MV AMS system at ETH Zurich show a quite good correlation. Ministerio de Economía y Competitividad FIS2015-69673-P

Trabajo presentado en las IX Jornadas CPAN (Centro Nacional de Física de Partículas, Astropartículas y Nuclear), celebradas en Santander (España), del 23 al 25 de octubre de 2017

Trabajo presentado en la 14th International conference on Accelerator Mass Spectrometry, celebrada en Ottawa (Canadá), del 14 al 18 de agosto de 2017

Trabajo presentado en la 43ª Reunión Anual de la Sociedad Nuclear Española (SNE), celebrada en Malága (España), del 4 al 6 de octubre de 2017

The Centro Nacional de Aceleradores (CNA) hosts a 1 MV accelerator mass spectrometry (AMS) apparatus since September 2005. In order to improve its overall performance, several updates have been made on the existing facility during the last 10 years of operation. In this paper, two modifications conducted in 2015 will be described. To increase the transmission of the ions through the accelerator, the stripping gas on the 1 MV CNA machine was changed from Ar to He. The measured maximum transmission for almost every isotope results to be higher, especially for heavy masses: for instance, in the case of uranium in the 3+ charge state, the transmission increased from 11% with Ar gas to about 38% with He gas. The second advance consisted of the substitution of the existing gas ionization chamber with a new one provided by ETH Zurich. The ETH detector features with its miniaturized design and is optimized for low energy AMS (i.e. very low electronic noise and efficient charge collection). As the electronic noise is the most important contribution to the resolution for light ions, the total energy resolution has been reduced by 15% in the case of 10Be, allowing a better discrimination against its isobar, 10B. For the heaviest radionuclides where the quality of the spectra is determined by the charge carrier production in the gas, the resolution for 2.7 MeV uranium ions was improved by 30%, probably due to a more efficient charge collection.

Trabajo presentado al VIII CPAN Days: Encuentros en Física Nuclear, celebrado en Zaragoza del 28 al 30 de noviembre de 2016., 1.Introduction. 2.41Ca measurements at low energy. 3.Performance of the 1 MV AMS system for 41Ca. 4.Measurements from biomedical samples; intercomparison with ETH Zürich 0.6 MV system. 5.Conclusions and prospects.