It is generally agreed, that the next generation frequency standards will be based on optical transitions in atoms. An interesting candidate for realizing such a novel frequency standard could be the low-energy isomer transition in the nucleus of Thorium-229. This unique long-lived (order seconds to minutes) isomer state is expected at an energy about 7.6 eV. It has not yet been directly observed experimentally.
One approach to determine the exact energy of the isomer state is to implant Th-229 into crystals, transparent for vacuum ultraviolet (VUV) light, and then perform spectroscopy. In this diploma thesis, initial steps in the characterization of VUV transparent crystals, especially CaF2 [CaF tief 2] were made. In particular, properties of Th-232-doped CaF2 [CaF tief 2] single crystals were characterized for the first time and compared to un-doped CaF2 [CaF tief 2] and various other possible host crystals.
For the characterization of the crystals we used radiological (neutron activation analysis and gamma spectroscopy), optical (transmittance and luminescence), and surface characterization methods (atomic force microscopy). We demonstrate that uniform doping with Thorium-232 can be successfully performed with a significant doping concentration 1018 [10 hoch 18] cm-3 [cm hoch -3] while maintaining VUV transparency of the crystal. The study reveals several material property correlations, like the dependence of the transmittance and the spectrally resolved luminescence performance on the crystal surface preparation or on impurities and crystal defects. We conclude how the transmittance and luminescence performance is affected by defects, crystal doping, and crystal surface preparation. In addition some sample preparation methods like cutting, polishing, and cleaving of crystals were tested. Further characterization steps should focus on the characteristic parameters of luminescence, like the decay time for different wavelength to unambiguously identify the microscopic processes. At this stage, Th-232-doped CaF2 [CaF tief 2] seems to be a promising candidate for precision spectroscopy of the Th-229 isomer state.