Counting electrons reveals thorium’s nuclear tick in a solid clock

Atomic clocks keep time by counting the ‘ticks’ of electrons moving between two energy levels. Physicists have long wanted to count a nuclear tick instead. A nucleus is more shielded than an atom’s outer electrons, so its energy levels are expected to be less sensitive to disturbances.

The main candidate for a nuclear clock is thorium-229 (²²⁹Th). Its nucleus has an unusual excited state that’s only about 8.4 electron-volts (eV) above the ground state: low enough for a vacuum-ultraviolet (VUV) laser light to excite it directly.

A practical problem has been detecting that excitation in solids. The most direct method is to look for the VUV photon emitted when the nucleus relaxes. But in many materials the thorium nucleus relaxes mainly by internal conversion: instead of emitting a photon, it transfers its energy to an electron in the host material, which may get kicked out.

Researchers from Germany, the UK, and the US recently reported a workaround by treating internal conversion itself as a signal. They used a laser-based method and ²²⁹Th in thorium dioxide, whose energy gap is about 6 eV, below the 8.4 eV nuclear energy. The idea was to excite thorium nuclei with VUV laser pulses, then count the electrons that escape when excited nuclei decay.

When the VUV pulse hits a surface, it produces a burst of ordinary photoelectrons that can overwhelm an electron detector. So the researchers used timed electric fields to suppress this burst, then extracted the delayed electrons associated with nuclear decay. They also used electric fields to guide electrons from the target to a detector.

The scan revealed a clear resonance at 2,020,407.5 GHz, consistent with previous studies. The team also inferred an internal conversion lifetime of 12.3 µs for their sample, implying the corresponding clock would be off by one second only every 15.8 billion years.

The study’s success has profound implications. In a commentary accompanying the paper, independent researchers from Texas A&M University wrote that the work “extends the available toolbox of materials for realising stable, high-precision nuclear clocks as well as sensors that can probe the nuclear environment of different materials.”

The findings also pave the way for substantial miniaturisation because a nuclear clock using this design can be monitored by measuring the current of the emitted electrons, rather than the complex techniques used today.