Response of Zircon to Electron and Ne + Irradiation
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ABSTRACT Zircon (ZrSiO 4) is an actinide host phase in vitreous ceramic nuclear waste forms and a potential host phase for the disposition of excess weapons plutonium. In the present work, the effects of 800 and 900 keV electron, and 1 MeV Ne' irradiations on the structure of single crystals of ZrSiO 4 have been investigated. The microstructural evolution during the irradiations was studied in situ using a high-voltage electron microscope interfaced to an ion accelerator at Argonne National Laboratory. The results indicate that electron irradiation at 15 K cannot amorphize ZrSiO 4 even at fluences an order of magnitude higher than that required for amorphization by 1.5 MeV Kr÷ ions. However, the material is readily amorphized by I MeV Ne+ irradiation at 15 K. The temperature dependence of this amorphization is discussed in light of previous studies of radiation damage in ZrSiO 4. INTRODUCTION Nuclear waste management, including the disposition of plutonium from weapons, is one of the most daunting challenges facing the United States Department of Energy. Zircon (ZrSiO 4), which is a durable mineral that occurs in nature with U and Th in its structure, has been proposed as a host phase for the immobilization of Pu and other actinides [1]. a-decay of these actinides results in an a-particle with an energy of about 5 MeV and a recoil nucleus that has an energy of about 80 keV. The a-particle loses most of its energy by ionization, while the recoil nucleus comes to rest mainly by elastic collisions with the lattice atoms that result in a damage cascade. Previous studies have examined the crystalline-to-amorphous transition due to recoil damage in zircon using heavy-ion irradiation (such as MeV Kr+ and Xe+) [2,3]. A systematic investigation of the irradiation temperature dependence of the amorphization of zircon by lighter ions is needed to gain an understanding of particle mass effects. Moreover, it has been shown that ionization can significantly influence the microstructural evolution of irradiated ceramics [46]. However, the effects of ionizing radiation on the structural evolution of irradiated zircon are not well understood. The present work addresses these issues by examining the structural changes brought about in ZrSiO 4 by energetic electron and Ne+ irradiation. EXPERIMENTAL DETAILS The crystal structure of zircon is schematically shown in Fig. 1. Single crystals of synthetic ZrSiO 4 , grown by a flux technique [7], were irradiated with 800 and 900 keV electrons, and 1 MeV Ne+ and examined in situ in the HVEM Tandem Facility at Argonne National Laboratory. This facility consists of a 1.2 MeV modified Kratos/AEI EM7 electron microscope interfaced to two National Electrostatics Corporation ion accelerators [8]. The details of the experimental setup have been discussed elsewhere [6]. The zircon samples were prepared by mechanical polishing and liquid nitrogen-cooled ion milling with 5 keV Ar ions at about 100 to 419
Mat. Res. Soc. Symp. Proc. Vol. 481 ©1998 Materials Research Society
the sample surface. During
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