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We have recently described a novel method for the construction of asolid-state optical frequency reference based on doping $^{229}$Th into highenergy band-gap crystals. Since nuclear transitions are far less sensitive toenvironmental conditions than atomic transitions, we have argued that the$^{229}$Th optical nuclear transition may be driven inside a host crystalresulting in an optical frequency reference with a short-term stability of$3\times10^{-17}<\Delta f/f <1\times10^{-15}$ at 1 s and a systematic-limitedrepeatability of $\Delta f/f \sim 2 \times 10^{-16}$. Improvement by$10^2-10^3$ of the constraints on the variability of several importantfundamental constants also appears possible. Here we present the results of thefirst phase of these experiments. Specifically, we have evaluated several highenergy band-gap crystals (Th:NaYF, Th:YLF, Th:LiCAF, Na$_2$ThF$_6$, LiSAF) fortheir suitability as a crystal host by a combination of electron beammicroprobe measurements, Rutherford Backscattering, and synchrotronexcitation/fluorescence measurements. These measurements have shown LiCAF to bethe most promising host crystal, and using a $^{232}$Th doped LiCAF crystal, wehave performed a mock run of the actual experiment that will be used to searchfor the isomeric transition in $^{229}$Th. This data indicates that ameasurement of the transition energy with a signal to noise ratio (SNR) greaterthan 30:1 can be achieved at the lowest expected fluorescence rate.

Progress towards fabrication of Th:229-doped high energy band-gap crystals for use as a solid-state optical frequency reference