Medidas de 10Be y 26Al en espectrometría de masas con acelerador de baja energía en el Centro Nacional de Aceleradores

Résumé

Cosmogenic radionuclides with long half-life are located in the nature at trace levels, therefore, their measurement is really difficult. Being that the conventional Mass Spectrometry is not able to distinguish between the radionuclide and its isobar, the design and the setting up of a new system was needed. Accelerator Mass Spectrometry (AMS) is a technique adopted to measure radionuclides with a large half-life at trace levels, where a tandem electrostatic particles accelerator is coupled to a mass spectrometer, which has magnetics and electrics elements to carry and select the ions by mass and energy. Isotopes are identified and counted with nuclear detection techniques, which can be used because the ions' energy is around 1 MeV/nucleon. The concentration of the problem isotope can be determined comparing the counts which are reaching the detector with the stable ions current, because the measurement has a relative character in AMS. The main features of the technique are the high sensitivity, almost a total selectivity and a great range of measurement of the isotope concentration. The development of the AMS has increased the detection sensitivity of several orders of magnitude with respect to the conventional Mass Spectrometry. Furthermore, the isobar and molecular interference can be separated almost totally. The first tandem accelerator mass spectrometers began to be installed during the 70's with the main aim of 14C measurement (T1/2= 5730 y) in small samples, with applications in archaeological dating or climatic changes studies. It was quickly checked the great potential of such a system to the measurement of other radionuclides and other applications like Basic Nuclear Physics, Astrophysics, Material Sciences, Biomedicine, Geology, Environmental and Palaoenvironmental Sciences; therefore the use of the AMS was extended to both the light radionuclides such as 10Be (T1/2 = 1,51×106 y), 26Al (T1/2 = 7,17×105 y) and 36Cl (T1/2 = 3,01×105 a) and the heavier ones like 129I (T1/2 = 1,57×105 a), 239Pu (T1/2 = 24360 a) and other Pu’s isotopes. The high cost of High Energy AMS technique (3-14 MV) stopped its setting up in laboratories around the world. Over the years, the evolution of the AMS has been focused on the viability of radionuclides measurements in low energy systems, whose terminal voltage is lower than 1 MV. In 1999, the Particle Physics group of ETH Zürich designed and built up the first Low Energy AMS (TANDY), with a terminal voltage of 600 kV and an area of 4,5×6,5 m2 , being the beginning of this kind of systems. In 2005, the Accelerators National Center (CNA) in Seville purchased a Low Energy AMS system built by High Voltage Engineering Europe, with 1 MV terminal voltage and a 3.8×6.3 m2 area named SARA (Spanish Accelerator for Radionuclides Analysis). SARA installation has allowed the implantation of the AMS technique in Spain through the development of several Doctoral Thesis centered on the use of the system for the measurements 5 of distinct radionuclides in environmental samples. With this thesis, the studies which has been performed so far are completed, introducing the measurements of 26Al and 10Be radionuclides. Both 10Be and 26Al find application fields in several Sciences. 10Be is a powerful tool very used in Geology and Geochronology, because it’s a very important tracer on the Earth. It’s used to investigate the shaping and the transformations of landscapes, weather variations, changes in the cosmic rays flux, rocks dating (together with 26Al), the erosion and denudation rates in surfaces and sediments, its productions in the atmosphere and subsequent distribution, recycled sediments by volcanos and wastes of nuclear powers facilities. On the other hand, because of its production on the Earth, the 26Al is used on rocks, sediment and surfaces dating as well as 10Be and on meteorite and lunar rocks. During the last decades, the 26Al started being adopted in Biomedicine as a tracer in living organisms’ metabolism. Studies about its absorption in kidneys and brain led to use it in researches concerning sicknesses as the Alzheimer. Due to the importance of both radionuclides, specific studies had to be carried on at the National Accelerator Centre. First of all, the capacity of Low Energy AMS to detect 26Al was tried on the acceptation tests, which were carried out after the setting up. With this thesis, the work related to the optimization of 26Al measurements and to the development of the associated radiochemistry needed to isolate and take the two radionuclides off from different samples has been made. Those points are the main aims of this thesis. Thanks to the acquired abilities and having the possibility to make routine measurements to both radionuclides, new research lines will be opened for the Applied Nuclear Physics group, for the National Accelerators Centre and for other research groups at the National Accelerators Centre. In this thesis, the optimization of the system for 26Al measurements has been realized studying the transmission of the beam along the stripper, measuring the correlation curve for several standards and evaluating the background level against the stripper pressure in blanks for both charge states +1 and +3. A study about the variation of the current over the metal matrix and its concentration have been performed and compared with the same test at the TANDY. The radiochemical procedures have been adapted in our laboratory to meteoric and in situ 26Al and 10Be radionuclides in quartz and sediments. In addition, a new radiochemical procedure is described for measurement of both 26Al and 10Be in aerosol filters. To complete the radiochemical procedure, studies about the variation of the current against the calcination temperature of Al2O3 and the use of Fe, to coprecipitate the Al(OH)3 have been done. The ratios 26Al/27Al, 10Be/9 Be and the concentrations of 26Al and 10Be have been measured in quartz samples of rocks and sediments of terraces. Those samples were early measured at SUERC and the results were compared with our ones, being consistent between them for ratios and concentrations for 26Al and 10Be. 26Al in the atmosphere doesn’t have been so studied; thus, 26Al and 10Be were extracted from aerosols filters in Seville city, with the aim to provide new data about the concentrations of both radionuclides and the ratio 26Al/10Be. The results were one order of magnitude higher than what can be found in the literature, showing the presence of an extra contribution for 26Al from other places near Seville. 26Al and 10Be are currently measured in aerosol filters in Antarctic, high latitudes, high altitudes, in lower stratosphere or upper troposphere. The measured aerosol filters were collected at sea level, midlatitudes and down town. 10Be meteoric was measured on sediments samples from Guadiana River, with the aim to get the denudation rate of the basin of the river. The datum was compared with the one obtained by the sediment budget method which is a twofold higher than our result. Concerning the waste 6 of nuclear powers, it’s important to know the radionuclides which are in the wastes to provide a right store of them. At the CNA, 10Be has been measured in resins exchange ions, to know the amount of 10Be in the reactor and the store pool water. Dawsonites and Hidrotalcites are selective absorbers of cations and anions. They have been measured to try their capability of absorbing 10Be from waters of nuclear powers. Finally, it will be done a description of the study of recycled 10Be from sediments under the sea which are eruptioned from volcanos in the Andes, measuring 10Be in volcanos ashes samples in SARA system at National Accelerators Centre.