The nuclear level scheme of the Thorium-229 isotope is expected to feature a long-lived isomer state, 229mTh, extremely close to the nuclear ground state. The currently most accepted isomer energy value is 7.8 eV, corresponding to a wavelength of 160 nm. Probably the lowest excited nuclear state of all isotopes, this 229Th isomer could be accessible to laser manipulation, creating an exciting link between atomic and nuclear physics. However, there is yet no unambiguous proof of the existence of this state, and the exact isomer energy remains elusive.

Here we propose to use a state-of-the-art magnetic microcalorimeter to resolve
the 29.19 keV doublet of 229Th, that only has a direct decay path into either the ground, or the isomer state. Resolving this doublet will provide ultimate proof for the existence
and measure the isomer energy without involving further assumptions and with an
accuracy, that will enable direct laser spectroscopy investigations.

The project will be carried out as an international collaboration between the Vienna
University of Technology and the University of Heidelberg. The Vienna team will
produce and characterize the 233U samples at the Institute for Atomic and Subatomic Physics, assist in the measurements in Heidelberg, and perform the data analysis. The Heidelberg team will provide the cryogenic microcalorimeter, optimize it for the project described here, and perform the measurement.