$^{181}$\textit{Ta-}PAC Experiments with Zircon: The Case of the Lost Anisotropy

Perturbed angular correlation spectroscopy (PAC) is a nuclear technique often used to probe the hyperfine interaction of a nuclear moment with extra-nuclear fields. For example the electric field gradient (\textit{EFG}) at a $^{181}$\textit{Ta} probe nucleus in zircon (\textit{ZrSiO}$_{4})$ depends on the positions of the \textit{Zr}, \textit{Si}, and $O$-atoms and is very sensitive to structural rearrangements. In our PAC experiments with zircon we have shown that a very subtle rearrangement of \textit{Si}-atoms within the unit cell leads to a change in the temperature dependence of the \textit{EFG}.$^{1)}$ In addition to the \textit{EFG}, we also measure the anisotropy of the emitted $\gamma \gamma $-cascade in our PAC experiments. The anisotropy is a nuclear property but, because of finite sample and detector size, it also depends somewhat on the geometry of the detector arrangement. However, with a given nucleus and a fixed geometry one would not expect a substantial change in the anisotropy during a series of measurements, say as a function of temperature. Yet our PAC spectra of zircon show a consistent decrease of the anisotropy in the temperature range between 800 and 650$^{\circ}$ C. We will discuss possible reasons for this apparent loss in anisotropy. $^{1) }$H. Jaeger, K. S. Pletzke, and S. P. McBride, \textit{Perturbed Angular Correlation Study of Naturally Occurring Zircon with Very Small Impurity Concentrations}, accepted for publication in Hyperfine Interactions (2005).