Radiation-induced defects and amorphization in zircon

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The effects of self-radiation damage as a function of cumulative alpha-decay events in synthetic zircon doped with 238Pu and natural zircons damaged over geologic time are compared and interpreted in terms of the accumulation of both defects and amorphousness. The radiation-induced unit-cell expansion and amorphization result in macroscopic swelling that increases sigmoidally with cumulative decay events and saturates at a fully amorphous state. The derived amorphous fraction as a function of cumulative dose is consistent with models based on the multiple overlap of displacement cascades, indicating that amorphization in zircon occurs as a result of the local accumulation of high defect concentrations rather than directly within a displacement cascade. Annealing of point defects in the natural zircons suppresses initial swelling and delays the onset of amorphization. Full recrystallization of the zircon structure from the amorphous state occurs in two stages, with kinetics and activation energies consistent with the reported thermal stability of the amorphous state. This study further confirms that actinide doping is a viable accelerated technique to study or simulate radiation effects from alpha decay on geologic time scales.

I. INTRODUCTION

For well over a century, mineralogists have been studying the effects of alpha decay over geologic time on the structure and properties of natural minerals containing trace amounts of uranium and thorium isotopes. Radiation effects from alpha decay of the long-lived transuranic isotopes (and daughter products) present in nuclear waste will also accumulate over geologic periods of time and alter both structure and properties of the nuclear waste host. Consequently, the understanding of self-radiation damage from alpha decay has also become the subject of interest to scientists working on solutions for the safe storage of nuclear waste. Based on the similarities in damage processes and time scales, it is evident, as originally suggested by Ewing and Haaker,1 that a thorough understanding of radiation effects and damage mechanisms in both natural minerals and corresponding synthetic phases can help evaluate and model the long-term (up to 106 years) behavior of nuclear waste materials during geologic storage. In alpha decay, a high energy alpha particle (~4 to 6 MeV), an energetic recoil nucleus (—0.1 MeV), and gamma radiation are released. The gamma radiation generally results in negligible damage relative to the alpha particle and the recoil nucleus. Nearly all the energy of the recoil nucleus is lost through elastic collisions with atoms in the structure, producing highly localized damage (displacement cascade) with one to two thousand atomic displacements. The alpha particle, on the other hand, dissipates most of its energy by ionization processes, but still undergoes enough elastic collisions along its path to produce several hundred isoJ. Mater. Res., Vol. 5, No. 11, Nov 1990

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lated atomic displacements. In order to a