QUANTIFYING RADIATION DAMAGE IN LOW ALPHA DOSE ZIRCON

A number of properties of natural zircon are known to change with radiation damage from the decay of U and Th substituting in the zircon structure. Spectroscopic properties, including the full width half max (FWHM) of the v₃SiO₄ Raman stretching mode and the FWHM of several Dy³⁺ fluorescence peaks have been shown to linearly correlate with alpha dose over a range of _~2e¹⁴ to _~2e¹⁵ α/mg. Therefore, within this dose range, these spectroscopic proxies, coupled with measurements of parents to the α-dose (U and Th) can be used to determine a “damage age” with a closure temperature similar to the zircon U-Th/He system. At higher doses the zircon structure becomes highly disordered, and the relationship between spectroscopic proxies and thermal history is more complicated. In low dose zircon (<2e¹⁴ α/mg), changes in these spectroscopic proxies are not resolvable. The low-dose limit leaves many samples not amenable to “damage” dating. For example, a zircon with U = 200 ppm and Th = 100 ppm that has accumulated radiation damage for 100 m.y. yields a total dose of only ~8e¹³ α/mg. Here we report preliminary results on the use of yellow broadband luminescence in zircon as a radiation dosimeter at lower alpha doses.

Natural zircon, heat-treated natural zircon, and synthetic zircon were examined. Zircon compositions were determined by both LA-ICP-MS and by EPMA. LA-ICP-MS has the advantage of lower detection limits for the parent elements (U and Th), while EPMA provides a smaller spot size that more closely matches the spot size of spectroscopic analyses. The spectroscopic response of zircon was examined using both an XCLent hyperspectral cathodoluminescence (CL) detector that collected data simultaneously with the EPMA analyses and a Horiba XploraPlus that collected Raman and fluorescence data. Preliminary results show that for doses between _~1e¹³ to _~1e¹⁴ α/mg, Raman and Dy³⁺ FWHM values are invariant, as expected. In contrast, the hyperspectral CL data shows significant changes in the integrated intensity of the broadband yellow CL signal at the same dose. The signal initially increases, then falls before disappearing at _~1e¹⁴ α/mg. These preliminary findings show promise in using broadband yellow luminescence in zircon to quantify radiation damage at doses lower than resolvable via existing spectroscopic methods.