The search for the existence of 229mTh at IGISOL

An overview of preparatory work aiming at the identification of the low-lying 7.6 eV isomer in 229Th through a measurement of its hyperfine structure is presented. A 233U recoil gas cell has been developed and has undergone several iterations in order to improve the efficiency of extracting a low-energy beam of 229Th+ ions. Spectroscopic studies on stable 232Th have been carried out to establish an efficient laser ionization scheme. The latter will be applied in connection with the gas cell in order to improve the extraction efficiency by accessing the neutral atomic fraction. In addition to that of the 229Th ground state, the hyperfine structure of the isomer can then be determined either by collinear laser spectroscopy on the extracted and accelerated ions or by direct in-source/in-jet high-resolution Resonance Ionization Spectroscopy (RIS). The first experiments using high-resolution RIS have been performed and isotope shifts of 228Th, 229Th and 230Th relative to 232Th were measured on an atomic 7s2 → 7s7p ground-state transition at 380.42 nm. A template of the ground-state hyperfine structure of 229Th has been established for a 261.24 nm UV transition. This is an important step towards identification of the isomeric state.