Nuclear transitions have energy scale three-to-six orders of magnitude larger than atomic ones. This mismatch of energy scales prevented to use laser sources for excitation of nuclear levels.
In the 70th it was deduced that Th229 might have isomeric state at the energy of a few eVs. This observation stimulated a broad interest, in particular regarding a possible nuclear clock application allowing to enhance atomic clock precision. Many experiments aiming to determine Th229 isomer energy were performed, and concluded in 2017 with the first observation of electrons emitted from Th229m IC decay by the group of Munich University. In 2019 their energy was also measured, leading to Th229m energy of about 8 eV (150 nm). TORIO-229 experiment of INFN, proposed in 2016, aimed to measure Th229m energy directly by means of cryogenic Transition Edge Sensor (TES). In 2019 the experiment goal was extended to a search of Th229m gamma decay in large bandgap crystal ThF4. In this presentation current knowledge on lowest nuclear isomer Th229m will be presented as well as its possible future applications.
T. Marchi, G. Carugno