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We aim to perform direct optical spectroscopy of the Th-229 nuclear isomer tomeasure its energy and lifetime, and to demonstrate optical coupling to thenucleus. To this end, we develop Th-doped CaF2 crystals, which are transparentat the anticipated isomer wavelength. Such crystals are illuminated by tunableVUV undulator radiation for direct excitation of the isomer. We scan a 5 sigmaregion around the assumed isomer energy of 7.8(5) eV and vary the excitationtime in sequential scans between 30 and 600 seconds. Suffering from anunforeseen strong photoluminescence of the crystal, the experiment is sensitiveonly to radiative isomer lifetimes between 0.2 and 1.1 seconds. For thisparameter range, and assuming radiative decay as the dominant de-excitationchannel, we can exclude an isomer with energy between 7.5 and 10 eV at the 95%confidence level.

On an attempt to optically excite the nuclear isomer in Th-229