Exciting thorium-related research activity is currently going on in Japan! Out of a multitude of approaches to populate the isomeric state, a Japanese/Austrian initiative chose “optical” pumping with X-rays. Synchrotron radiation at SPring-8 is used to excite the Th-229 nucleus into the second excited state, located at 29 keV. This state may de-excite back into the ground state, or, with a probability of above 90%, fall into the isomeric state.
An unusually long beam time of 18 shifts (6 days in total), has been allocated to advance on this project. The currently ongoing measurement campaign is subdivided into two major parts: (i) finding the 29-keV nuclear resonance, and (ii) detecting the VUV gamma emitted upon de-excitation of the isomer.
The first experiment employs a “spotted” target with a layer of Th-229 deposited onto a substrate. The 29-keV gamma that is re-emitted as the nucleus de-excites into the ground or isomeric state is detected with a super-fast MCP.
The second experiment uses thorium-doped crystals produced by the Vienna group. The X-ray beam, tuned to the nuclear resonance, pumps population from the ground state into the isomeric state. After a few minutes, the beam is turned off, and the emission of the crystal is measured with a PMT and a VUV spectrometer.
The research is guided by Koji Yoshimura from Okayama University. Further partners include Kyoto University, Tohoku University, Osaka university, RIKEN (including Atsushi Yamaguchi, who had previously been working at PTB), and SPring-8. Measurements will last from December 14th through 20th.