Radiative decay of the nuclear isomer of thorium-229 observed for the first time / Observation paves the way for optical control of this atomic nucleus
24 May 2023
At present, the radioisotope thorium-229 is considered to be the only candidate for use in a nuclear clock. A nuclear clock of this kind would be considerably more accurate than the current atomic clocks. The timekeeper in this case would be the rate of oscillations in the nucleus of thorium-229, induced by laser light excitations. Researchers have now developed a new method to determine the excitation energy with significantly more precision. This represents an important quantum leap in the development of a functional nuclear clock.
The nucleus of the radioisotope thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of the nuclear state. It is for this reason that it is among the leading candidates for use in next-generation optical clocks. The last decade has been characterized by a number of experimental breakthroughs, such as the first direct and clear proof of the existence of the nuclear isomer, its characterization using laser spectroscopy, the measurement of its excitation energy, and X-ray pumping. However, there was still a lack of observation of radiative decay and of the precise determination of the light frequency for the development of the optical clock. Using a novel approach in which the isomer is populated with ionic beams at the ISOLDE facility at CERN, researchers have now been able for the first time to observe the elusive radiative decay of the isomer and substantially decrease the uncertainty of its energy and decay constant with the help of spectroscopic techniques.
The corresponding experiments were undertaken by an international team headed by researchers from KU Leuven. German members of the team came from Johannes Gutenberg University Mainz (JGU), LMU Munich, the Helmholtz Institute Mainz (HIM) and the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt. The researchers have recently published their results in Nature.
Vacuum ultraviolet spectroscopy of the nuclear isomer
The first proposal for an optical clock based on the excitation of a nuclear state to serve as an ultra-stable metrological instrument and quantum sensor was made long ago, but most recently caused a stir in the scientific world due to the direct detection of the thorium-229 isomer. The underlying idea is quite simple: Instead of measuring time based on the frequency of light needed to invoke electronic transitions in an atom, which is the method employed to date, the frequency of the light used to excite the atomic nucleus itself is employed for this purpose. The clear advantage would be that the atomic nucleus is a more compact structure compared with atomic shells and has small electromagnetic moments. It is thus less susceptible to external interference factors, meaning that the resultant clock will be of unparalleled accuracy. In addition, it is expected that the radiative transition of the isomer in the thorium-229 nucleus will be in the ultraviolet range of the electromagnetic spectrum, meaning that optical control should be possible using appropriately designed UV lasers. This, on the other hand, will only become possible when the corresponding nuclear transition has been observed using optical methods and analyzed in more detail.
The technique used to date was to populate the appropriate nuclear isomer with the help of the alpha decay of uranium-233. So far, the decay to the ground state did not lead to the emission of the characteristic light from the nucleus. In the experiments at CERN, the nuclear isomer was populated by means of the beta decay of actinium-229, which was previously implanted in crystals of calcium fluoride and magnesium fluoride at kinetic energy of 30 kiloelectron volts (keV). The researchers used vacuum ultraviolet spectroscopy to analyze the photon spectrum emitted by the crystals under favorable radioluminescence conditions and were finally able to identify the spectral line at a wavelength of 148 nanometers. "We have finally succeeded in observing a clear signature for the radiative decay of the thorium-229 nuclear isomer in our experiments. As a result, we have managed to measure its excitation energy with an accuracy improved by a factor of seven than previous results. And on the basis of our measurements, we have even been able to estimate the half-life of the radiative transition, which we put at about 10 minutes," said Dr. Mustapha Laatiaoui, junior research group leader at Johannes Gutenberg University Mainz, who was involved in the recent investigations.
Highly promising outlook
The reported results represent important steps towards the development of a nuclear clock. On the one hand, the decreased level of uncertainty in terms of the excitation energy constitutes a reduction of the potential search range and is a crucial preliminary parameter for the development of a suitable vacuum-ultraviolet laser control system. On the other hand, the observation of radiative decay in large-bandgap crystals shows that the creation of a solid-state nuclear clock with far greater stability than that of contemporary atomic clocks is feasible.
The practical realization of a clock that uses nuclear transition as a timekeeper would have an exciting range of potential applications – in applied and fundamental physics, geodesy and seismology through to trials to determine whether fundamental constants exhibit any time variations.