Search for the ultra-low energy isomer state of thorium-229 nuclei*

The nuclear isomeric first excited state of Thorium-229 (^229mTh) has drawn attention from many fields. In general, nuclei are insensitive to ambient disturbance due to the shielding by their core electrons. In addition, the extraordinary-low energy level (7.8 eV) of ^229mTh can be accessed by laser spectroscopy techniques. Since its insensitiveness may enable us to achieve longer decoherence time in comparison to atomic states, it attracts much interest from atomic clock, fundamental physics, and other fields. However large energy uncertainty of 0.5 eV makes the laser spectroscopy difficult. In spite of much effort, a transition between the isomer state and the ground state has not been observed directly by optical methods.
The aim of our experiment is a determination of the energy of the isomeric transition precisely by nuclear resonant scattering (NRS) with synchrotron X-ray beam in SPring-8. We have succeeded in developing a fast X-ray detector system and a dense ²²⁹Th target which are the essential parts of the experiment. We will present a detail of the developed system and the current status of the experiment especially the latest beam-time result taken in April 2018.