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Radiative Decay Study of the low-lying thorium-229 isomer

Project description

High-precision measurements of the lowest nuclear-excited state

Thorium-229 has a remarkably low-lying metastable isomer. Due to its low excitation energy, the isomer should allow the development of a nuclear clock of unprecedented accuracy. Increasing understanding of the isomer’s properties would have far-reaching implications for metrology, dark matter research and geodesy. Funded by the Marie Skłodowska-Curie Actions programme, the RaDTho project plans to use two complementary techniques to accurately determine the isomer’s properties. Researchers will use a novel technique of producing the thorium-229 isomer via the beta decay of actinium-229, which is expected to increase sensitivity analysis fivefold. Spectroscopy techniques should also help measure the hyperfine structure of the ground state and low-energy isomer of thorium-229.


The existence of a low-lying nuclear isomer of thorium-229 at 8.28 eV was suggested several decades ago but was only recently identified via the observation of its decay signal. The low energy and an estimated relative decay width of around 10^{-19} open new possibilities for the development of a nuclear frequency standard-a nuclear clock, which can outperform the existing atomic clocks. This will have far-reaching consequences, such as in metrology, dark matter research, geodesy and time variation of fundamental constants. However, the isomer's properties are unknown or known with insufficient accuracy to exploit its far-reaching opportunities.
This proposal aims to determine the properties of the isomer using two complementary techniques.

1. The isomer is populated using a novel mechanism via the beta-decay of actinium-229 implanted in a suitable crystal at the ISOLDE-CERN facility. VUV spectrometry of the implanted crystal will allow measurement of the isomer's excitation energy with a precision of < 0.1 nm as well as its radiative-decay half-life. This production scheme increases the sensitivity by at least a factor of five and allows for improved control of experimental conditions and a reduced background signal.

2. Magnetic dipole and electric quadrupole moment as well as the nuclear charge radii of the singly charged ground state and isomer of thorium-229, produced via alpha-decay of uranium-233, will be measured using the in-gas-jet laser ionization and spectroscopy technique at KU Leuven. This technique allows the necessary efficiency, sensitivity and spectral resolution to measure the hyperfine structure of the singly charged thorium-229 ground state and isomer.



Net EU contribution
€ 166 320,00
Oude markt 13
3000 Leuven

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Vlaams Gewest Prov. Vlaams-Brabant Arr. Leuven
Activity type
Higher or Secondary Education Establishments
Other funding
€ 0,00