Characterization of the Oxidation State of $^{\mathrm{229}}$Th Recoils Implanted in MgF$_{\mathrm{2}}$ for the Search of the Low-lying $^{\mathrm{229}}$Th Isomeric State

The low-lying (7.8 eV) isomeric state in $^{\mathrm{229}}$Th has the potential to become a nuclear frequency standard. $^{\mathrm{229}}$Th recoils from $^{\mathrm{233}}$U decays have been collected in MgF$_{\mathrm{2}}$ for use in the direct search of the transition. Of interest is the oxidation state of the implanted $^{\mathrm{229}}$Th atoms as this can have an influence on the decay mechanisms and photon emission rate. Too determine the oxidation state of the implanted $^{\mathrm{229}}$Th recoils we have employed laser induced florescence (LIF), and plan-wave pseudopotential DFT calculations to search for emission from thorium ions in oxidation states less than $+$4. Our search focused on detecting emission from Th$^{\mathrm{3+}}$ ions. The DFT calculations predicted the Th$^{\mathrm{3+}}$ state to be the most likely to be present in the crystal after Th$^{\mathrm{4+}}$. We also calculated the band structure for the Th$^{\mathrm{3+}}$ doped MgF$_{\mathrm{2}}$ crystal. For LIF spectra a number of excitation wavelengths were employed, emission spectra in the visible to near-IR were recorded along with time-resolved emission spectra. We have found no evidence for Th$^{\mathrm{3+}}$ in the MgF$_{\mathrm{2}}$ plates. We also analyzed the detection limit of our apprentice and found that the minimum number of Th$^{\mathrm{3+}}$ atoms that we could detect is quite small compared to the number of implanted $^{\mathrm{229}}$Th recoils. The number of implanted $^{\mathrm{229}}$Th recoils was derived from a $\gamma $-ray spectrum by monitoring emission from the daughters of $^{\mathrm{228}}$Th. These were present in the MgF$_{\mathrm{2}}$ plates due to a $^{\mathrm{232}}$U impurity, which decays to $^{\mathrm{228}}$Th, in the source. LA-UR-16-20442