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41CA measurement with low energy accelerator mass spectrometry (leams) at the Centro Nacional de Aceleradores

AuthorsVivo Vilches, Carlos CSIC ORCID
AdvisorLópez-Gutiérrez, J. M. CSIC ORCID
Issue Date28-Sep-2018
PublisherUniversidad de Sevilla
CSIC-JA-USE - Centro Nacional de Aceleradores (CNA)
AbstractThe 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.
DescriptionTesis llevada a cabo para conseguir el grado de Doctor por la Universidad de Sevilla--2018-09-28
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