German researchers take us a step closer to making nuclear clocks

The clock's accuracy would be as high as one second for every 300 billion years.
Ameya Paleja
Representative image of an atomic nucleus
Representative image of an atomic nucleus


A collaboration between researchers from various institutes in Germany has brought us a step closer to building the first-ever nuclear clock. In experiments carried out at the European Organization for Nuclear Research (CERN), the researchers measure the radiative decay of thorium-229 nuclear isomer, the first instance of having achieved this feat and a critical component for building nuclear clocks.

For years atomic clocks have been our standard of accuracy when it comes to clocks. The best optical atomic clocks have a precision rate of 10-18, which is equivalent to an inaccuracy of one second every 30 billion years.

A nuclear optical clock is expected to be at least 10 times more precise. While it was first proposed more than two decades ago, researchers have only been able to make some advances in its direction after some major findings in recent years.

Toward the world's first nuclear clock

According to a press release from Johannes Gutenberg University Mainz, the discovery that caused quite a stir in scientific circles was the direct detection of thorium-229 isomer. An isomer is an atom whose nucleus is in a higher energy state. This can be achieved using light of a specific frequency.

For atomic clocks, researchers use the frequency of light used to invoke transitions in the atom as a measure of time. For nuclear clocks, however, the frequency of light used to excite the atomic nucleus alone is used. Since the atomic nucleus is a more compact structure and has small electromagnetic moments, it is less susceptible to external interference.

German researchers take us a step closer to making nuclear clocks
Representative image of atoms with its electron orbits

When the isomer goes back to its ground state, it emits a photon, a process that scientists called radiative decay, which is crucial to the measurement. However, researchers had not previously managed to measure the decay accurately. This hurdle was recently overcome through a collaborative effort by researchers from various German research organizations.

The experiments were conducted at the ISOLDE facility at CERN where actinium-229 atoms were implanted in calcium fluoride or magnesium fluoride crystals and left to decay into thorium-229. The research team measured photons with an ultraviolet wavelength of 148 nm and transition energy of 8.338 electronvolts.

This is the most precise measurement of the isomer's energy and the accuracy has seen improvements by a factor of seven when compared to previous results, the researchers claim. A lot more work needs to be done before a nuclear clock can be built but the research shows that Thorium-229 is our best bet to building one.

The research findings were published in the journal, Nature.

Study abstract:

​​The radionuclide thorium-229 features an isomer with an exceptionally low excitation energy that enables direct laser manipulation of nuclear states. It constitutes one of the leading candidates for use in next-generation optical clocks. This nuclear clock will be a unique tool for precise tests of fundamental physics. Whereas indirect experimental evidence for the existence of such an extraordinary nuclear state is substantially older, the proof of existence has been delivered only recently by observing the isomer’s electron conversion decay. The isomer’s excitation energy, nuclear spin and electromagnetic moments, the electron conversion lifetime and a refined energy of the isomer have been measured. In spite of recent progress, the isomer’s radiative decay, a key ingredient for the development of a nuclear clock, remained unobserved. Here, we report the detection of the radiative decay of this low-energy isomer in thorium-229 (229mTh). By performing vacuum-ultraviolet spectroscopy of 229mTh incorporated into large-bandgap CaF2 and MgF2 crystals at the ISOLDE facility at CERN, photons of 8.338(24) eV are measured, in agreement with recent measurements and the uncertainty is decreased by a factor of seven. The half-life of 229mTh embedded in MgF2 is determined to be 670(102) s. The observation of the radiative decay in a large-bandgap crystal has important consequences for the design of a future nuclear clock and the improved uncertainty of the energy eases the search for direct laser excitation of the atomic nucleus. 

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