Effect of Calcium Oxide Microstructure on the Diffusion of Isotopes

Calcium oxide (CaO) powder targets have been successfully used at CERN-ISOLDE to produce neutron deficient exotic argon and carbon isotopes under proton irradiation at high temperatures (>1000°C). These targets outperform the other related targets for the production of the same beams. However, they presented either slow release rates (yields) from the beginning or a rapid decrease over time. This problem was believed to come from the target microstructure degradation, justifying the material investigation. In order to do so, the synthesis, reactivity in ambient air and sintering kinetics of CaO were studied, through surface area determination by N2 adsorption, X-ray diffraction for crystalline phase identification and crystallite size determination, and scanning and transmission electron microscopy to investigate the microstructure. The synthesis studies revealed that a nanometric material is obtained from the decarbonation of CaCO3 in vacuum at temperatures higher than 550°C, which is very reactive in air. This reactivity was studied, and it was observed that the CaO powder microstructure is changed through the reaction with air (hydration and carbonation of the oxide) and that this change is not completely reversible after thermal decomposition of the reaction products. Therefore, special care was taken in the target handling at CERN-ISOLDE. From the sintering kinetics, studied in the range of 1000-1200°C, it was determined that this material’s microstructure degrades, with the reduction of the specific surface area and decrease of the powder porosity. At 1200°C, the specific surface area reduction is accentuated, reaching values of 50% of surface area reduction in 10h. These results suggest that the use of high temperatures, equal or higher than 1000°C must be avoided, if the microstructural characteristics of the targets are to be preserved. At CERN-ISOLDE, selected conditions for synthesis, handling of the target and target operation temperatures were chosen, based on the previous material research, and the obtained target material was tested under proton irradiation. From the online studies, the newly developed target proved to show better initial and stable over time release rates of almost all isotopes investigated and especially the exotic ones. These results are essentially due to the nanometric characteristics of the produced target and to the use of operation and handling conditions that decreased the degradation of the microstructural characteristics. Diffusion studies of Ar and Ne were also done in CaO through the application of a mathematical model, to the release curves of the respective isotopes at different temperatures, which enables the determination of the respective diffusion coefficients and activation energies.