Measuring the Th-229 isomer energy with a microcalorimeter

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The nuclear level scheme of the Thorium-229 isotope is expected to feature a long-lived isomer state, extremely close to the nuclear ground state. The corrently 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 Th-229 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.Progress in determining the isomer energy is triggered by advances in gamma detector technology. All available experimental data relies on measuring higher levels of the Th-229 nuclear structure in the 10-1000 keV regime, excited in the alpha decay of Uranium-233. The isomer energy on the eV level is then determined indirectly by substracting gamma energies corresponding to decay paths into the ground and isomer state respectively. Measuring high energies to derive a small difference obviously leads to large errors. Furthermore, some of the details of the decay paths (interband transitions) could not yet be resolved, so theoretical asumptions for specific branching ratios enter the energy determination, which is hence heavily disputed.Here we propose to use a state-of-the-art magnetic microcalorimeter to resolve the 29.19 keV doublet of Th229, 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 (project leader) and the University of Heidelberg. The Vienna team will produce and characterize the U-233 samples at the Institute for Atomic and Subatomic Physics, assist in the measuremens 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. As the project partners have demonstrated in a joint feasability study arXiv:1306.3069, the measurment can be successful with the already available detector technology, however an even more dedicated detector is currently under development. Combining the expertise and equipment available in Vienna and Heidelberg, the measurement can be performed in only 18 months and very little additional resources.