Perturbed Angular Correlation Study of Naturally Occurring Zircon with very Small Impurity Concentrations

We used 181Ta-PAC spectroscopy to characterize the electric field gradient at Zr sites of zircon samples from the Harts Range in Australia. Zircons from this region contain very low levels of radioactive impurities. The PAC spectra between room temperature and 1100 deg. C are axially symmetric and show little damping. The quadrupole frequency decreases linearly with increasing temperature, and the slope of the νQ vs. T graph increases above 900 deg. C. This is attributed to a rearrangement of the Si-O coordination, which affects the Zr-O coordination. The spectra of all zircons show a distinct decrease of the anisotropy between 800 and 600 deg. C. This behavior is also seen with samples that do not exhibit the change in slope in the νQ vs. T data. We believe that the decrease in the anisotropy is due to an aftereffect following the β-decay of 181Hf.