Elusive nuclear-clock transition spotted in thorium-229

Photograph of the experimental set-up used to make the first direct detection of the excited nuclear state Th-229m
Beamline: discovery of nuclear transition could lead to better clocks

A nuclear clock that is more precise than any atomic clock available today could soon be a reality, thanks to a discovery made by physicists in Germany. The team is the first to detect a crucial low-energy transition in the thorium-229 nucleus, which could be used to create a new frequency standard. Although the transition must be located more precisely before it can form the basis of a clock, the results provide the first direct experimental confirmation that the elusive transition exists at roughly the same energy it was predicted to have.

The best atomic clocks available today could keep time to within one second if they were left running for 13 billion years – the current age of the universe. These clocks work by keeping a laser in resonance with electronic transitions between energy levels in atoms or ions – with the “ticks” of the clock being the frequency of the laser light. The most important limitation on clock performance is how susceptible the device is to interference from stray electromagnetic fields. Nuclei are hundreds of thousands of times smaller than atoms and bound together much more tightly – and this makes nuclear transitions less sensitive to external electromagnetic fields.

It has long been a goal of some in the metrology community to produce a “nuclear clock” by locking a laser to a nuclear transition. The problem is that nuclear transitions tend to occur at energies that are thousands or even millions of times greater than the photons produced by today’s lasers. However, the transition between the ground state of the thorium-229 nucleus and an excited state (called Th-229m) is expected to have only around 7.8 eV energy. This corresponds to the energy of ultraviolet photons, which can be laser-generated.

Existence doubted

Unfortunately, physicists trying to excite the nucleus using lasers with photon energies around 7.8 eV have so far found no evidence for the transition. This had led some researchers to ask whether the transition may have a different energy – and some had even doubted its existence. “People started to say ‘Is there something wrong with the theory?’,” says Lars von der Wense of Ludwig Maximilian University of Munich.

Now, Wense and colleagues have performed experiments that show the transition does exist, and that its energy is roughly what it is expected to be. Their measurements involve guiding a beam of thorium-229 ions onto a micro-channel plate detector. The nuclei are in the Th-229m excited state but do not decay significantly while in the beam. However, when the ions hit the detector, they are converted to neutral atoms and the nuclei decay within 1 s, producing a cloud of electrons. This, conclude the researchers, shows that the excited nucleus in a thorium-229 atom is able to decay via a rapid process called internal conversion. However, the excited nucleus in a thorium-229 ion cannot follow this decay path. This is because internal conversion involves the emission of an electron and will only occur if the energy released by the nuclear decay is greater than the ionization energy needed to separate the electron from the atom. Because the ions are positively charged, they have higher ionization energies than atoms.

Putting all of this together, the researchers could conclude that the transition has an energy between the first and third ionization energies of thorium – putting it in the 6.3–18.3 eV range.

Caught in a trap

The researchers now hope to nail the energy down more precisely by measuring the kinetic energies of the electrons produced by internal conversion as well as the lifetimes of the ions. “We are aiming to build a so-called Paul trap in a cryogenic environment,” explains Peter Thirolf, who led the research. “We will be allowed to store our ions and observe them for the full span of their lifetimes.” The researchers believe they may be able to narrow the energy range to a few milli-electronvolts, which should allow the laser physicists to start designing an appropriate laser to excite the transition. “We are doing this as part of a larger, European research consortium called nuClock, involving experimental groups like us and also laser people aiming at taking over our results,” says Thirolf.

Atomic physicists Kyle Beloy of the National Institute for Standards and Technology in Boulder, Colorado, and Marianna Safronova of the University of Delaware, both believe the result is important. “When you have these big, expensive experiments where people are trying to go out and search for this transition, any useful information – like saying ‘Yes, it exists’ – is very comforting,” says Beloy.

“I think I’m more excited to see that they plan to do the next step and actually measure the transition energy. If they, for example, say that it’s not 7.8 + 0.5 but 8.5 + 0.3, it will give people guidelines as to where they should be looking when they’re laser exciting,” adds Safranova.

The research is described in Nature.

comments powered by Disqus