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The first excited isomeric state of Th-229 has an exceptionally low energy ofonly a few eV and could form the gateway to high-precision laser spectroscopyof nuclei. The excitation energy of the isomeric state has been inferred fromprecision gamma spectroscopy, but its uncertainty is still too large tocommence laser spectroscopy. Reducing this uncertainty is one of the mostpressing challenges in the field.  Here we present an approach to infer the energy of the isomer fromspectroscopy of the electron which is emitted when the isomer de-excitesthrough internal conversion (IC). The experiment builds on U-233, which decaysto Th-229 and populates the isomeric state with a 2% fraction. A film of U-233is covered by a stopping layer of few-nm thickness and placed between an alphadetector and an electron detector, such that the alpha particle and the ICelectron can be detected in coincidence. Retarding field electrodes allow foran energy measurement. In the present design, the signal of the Th-229m ICelectrons is masked by low-energy electrons emitted from the surface of themetallic stopping layer. We perform reference measurements with U-232 and U-234to study systematic effects, and we study various means to reduce thebackground of low-energy electrons. Our study gives guidelines to the design ofan experiment that is capable of detecting the IC electrons and measuring theisomer energy.

Towards an energy measurement of the internal conversion electron in the de-excitation of the Th-229 isomer