Naturally occurring zircon contains varying amounts of U and Th impurities which radioactively decay and expose the host material to bombardment of energetic $\alpha$-particles and recoil nuclei. Over geologic time periods this results in lasting structural damage which may range from slightly disordered to entirely aperiodic or amorphous. We performed PAC experiments to characterize the radiation damage and recovery after annealing in natural zircon with low and high U/Th concentrations.¹ PAC proved to be very effective in identifying three distinct probe environments in the low U/Th zircon; environments corresponding to distorted zircon (site-I) and aperiodic zircon (site-II) were present before annealing, while an undamaged zircon (site-III) appeared only after annealing at 900^oC, and its fraction increases to almost 20% after annealing at 1150^oC.

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The high U/Th zircon did not show an undamaged zircon site, however, both site-I and site-II were seen there as well. Initially only site-II was present; site-I begins to appear after annealing at 600^oC and increases to about 92% after annealing at 1150^oC. The annealing behavior of the high U/Th sample differed from that of the low U/Th sample in two points; (i) after the final annealing step at 1150^oC the interaction frequency and damping of site-I in the high U/Th sample had reached the values observed for synthetic zircon, while the site-I frequency of the low U/Th sample remained about 9% below the value of synthetic zircon; (ii) the aperiodic site fraction dropped below 10% after annealing at 1150^oC, while in the low U/Th sample about 35% of probes were still in an aperiodic environment after equivalent annealing. In addition we found the site-II fractions in the low U/Th sample to be unexpectedly large for stage-I zircon.² We suspect this to be specific to the PAC technique since others using PAC spectroscopy report similar results.³

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1. H. Jaeger, L. Abu-Raddad, and D. J. Wick, Appl. Radiat. Isot. 48(1997) 1083. [Abstract]   [ Article (PDF, 565 kB)]

2. T. Murakami, B. C. Chakoumakos, R. C. Ewing, G. R. Lumpkin, and W. J. Weber, American Mineralogist 76 (1991) 1510.

3. P. Lacentre and M. C. Caracoche, Radiation Effects and Defects in Solids 129 (1994) 181.