Radiation damage in urania crystals implanted with low-energy ions

摘要

Implantations with low-energy ions (470-keV Xe and 500-keV La with corresponding ion range Rp ～ 85 nm and range straggling ΔRp ～ 40 nm) have been performed to investigate both radiation and chemical effects due to the incorporation of different species in UO_2 (urania) crystals. The presence of defects was monitored in situ after each implantation fluence step by the RBS/C technique. Channelling data were analysed afterwards by Monte-Carlo simulations with a model of defects involving (ⅰ) randomly displaced atoms (RDA) and (ⅱ) distorted rows, i.e. bent channels (BC). While increasing the ion fluence, the accumulation of RDA leads to a steep increase of the defect fraction in the range from 4 to 7 dpa regardless of the nature of bombarding ions followed by a saturation plateau over a large dpa range. A clear difference of 6% in the yield of saturation plateaus between irradiation with Xe and La ions was observed. Conversely, the evolutions of the fraction of BC showed a similar regular increase with increasing ion fluence for both ions. Moreover, this increase is shifted to a larger fluence in comparison to the sharp increase step of RDA. This phenomenon indicates a continuous structural modification of UO_2 crystals under irradiation unseen by the measurement of RDA.