Thorsten Schumm: AcThor-Actinium approach to nuclear spectroscopy in 229Thorium

AcThor-Actinium Approach to Nuclear Spectroscopy in 229Thorium

The aim of this project is to observe for the first time the “optical gamma ray” that emerges in the radiative decay of the low-energy excited nuclear state (“isomer”) in Thorium-229. This will allow to measure the isomer energy and lifetime with unprecedent precision.

As a conceptually new approach, we will use the beta-decay of Actinium-229 as a means to populate the Th-229 isomer. Ac-229 can be produced in the ATI nuclear reactor through neutron-activation. Other than the commonly used alpha-decay of Uranium-233 (5 MeV alpha, 86 keV ion recoil), the Ac-229 beta-decay only transfers 2 eV to the recoil ion, not perturbing the host crystal structure. Furthermore, the population of the isomer is at least 14%, up to 94%, compared to 2% in U-233 decay.

This project is only possible due to the unique environment at Atominstitut (ATI): it combines the expertise in single-crystal doping with radioactive material (unique in Europe), neutron-activation in the on-site reactor, and rapid transfer (< 1 h) into a high-performance VUV spectrometer. The general feasibility of the project has been verified in pilot experiments; all required isotopes are in stock. Preparation of the experiment will take 9 months, the actual experiment 1 day.

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