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Rand Ghassan Hazzam
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The increasing generation of radioactive waste from economic applications and radioactive elements presents a significant challenge in terms of safe disposal and storage. Fluorapatite crystals have been proposed as a potential solution for nuclear waste immobilization due to their ability to form a solid and insoluble matrix that can effectively contain radioactive elements. However, as a relatively new technology, further research is required to fully evaluate its long-term effectiveness and safety. Aim: This research aims to understand the effect of radiation induced by the decay of actinides in fluorapatite crystals (Ca5(PO4)3F), and to investigate the crystalline defects in natural apatite and their healing rates. Additionally, it is aimed to determine whether apatite can be used as a potential alternative matrix for embedding high-level radioactive waste (HLW), such as fission products and minor actinides, currently stored in nuclear glasses. Methods: An ex-situ ion implantation technique was used in this study, where pre-prepared natural fluoroapatite crystals (Ca5(PO4)3F), sourced from the Durango mine in Mexico, were implanted with 500 keV Bi3+ (heavy-ions) at the JANNuS facility located at IJCLab to simulate the decay of actinides. Different fluences ranging from 1012 to 1014 cm-2 were applied at different temperatures especially at room temperature and at 100 °C. The samples were then characterized using Rutherford backscattering spectroscopy/channeling (RBS/C) on the Ion Beam Analysis (IBA) beam line of the platform using 1.4 MeV He+ ions in channeling conditions. The RBS spectra data were simulated by using Mont Carlo simulations (McChasy RBS/C code). This adopted approach allows for a comprehensive analysis of the structural and compositional properties of the apatite crystals before and after irradiation, providing insight into the effects of heavy recoil nuclei and the simulation of actinide decay. Transmission electron microscope (TEM) was employed to monitor the evolution of damage induced by Bi3+ ions. Results: The RBS spectra data simulated by using Mont Carlo simulations (McChasy code) clarified the damage formation in the crystal induced by bombarding with Bi3+ ions, at different fluences. The damage distribution and its kinetics were extracted from RBS/C experiments and compared to transmission electron microscope (TEM) images to monitor the evolution of damage induced by Bi3+ ions, which deduced the presence of amorphous regions in apatite single crystals, supporting the main idea that apatite can be used as an alternative matrix for embedding HLW. Conclusions: The effect of radiation of alpha decay of actinides, on the crystalline structure of natural fluorapatite crystals Ca5(PO4)3F was studied using Bi3+ ions as a model of recoil nuclei. The ex-situ RBS/C technique was used to characterize 500 keV Bi implanted crystals at increasing ion fluence, simulating the effect of the damage generated by heavy recoil nuclei. The Monte Carlo McChasy code simulation fits very satisfactorily the Rutherford Backscattering by channeling mode experimental results. As the ion fluence increases the amount of damage that was modelled as RDA (Randomly Displaced Atoms) up to the final amorphization of the crystal at a fluence in the range 1014 cm-2. TEM characterization showed that the induced defects for the selected apatite samples increase with the increase in ion fluence, this is in accordance with RBS/C results. Keywords: Fluoroapatite crystals; Rutherford backscattering spectroscopy/channeling (RBS/C); ex-situ RBS/C; Monte Carlo simulation McChasy code; high level waste (HLW); Bi3+ ions.