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Nuclear physics

Nuclear clock could be one tick closer

23 Apr 2018 Hamish Johnston
Thorium spectroscopy
Search continues: team member Johannes Thielking with the system used to study the thorium-229 nucleus (Courtesy: PTB Braunschweig)

The internal structure of the thorium-229m nuclear state has been studied in detail for the first time by physicists in Germany. Thorium-229m is a metastable (or isomer) excited state of thorium-229 that decays via the emission of an ultraviolet (UV) photon. This photon has much lower energy than most nuclear emissions and could form the basis of a “nuclear clock” that would be much more precise than existing atomic clocks.

Atomic 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. Although the best atomic clocks available today could keep time to within one second if they were left running for 13 billion years, physicists are still keen on boosting this performance. Clock performance is limited by the effects of stray electromagnetic fields on atomic energy levels – and this is where nuclei can help. 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.

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 (thorium-229m) is expected to have only around 7.8 eV energy. This corresponds to the energy of ultraviolet photons, which can be laser-generated.

Narrow transition

The spectral width of this transition is extremely narrow, which is good for clock performance. However, this narrowness has made it very difficult to determine the actual energy of the transition. A breakthrough came in 2016, when  Lars von der Wense of Ludwig Maximilian University of Munich and colleagues throughout Germany compared the decays of thorium-229 atoms and ions, which allowed them to conclude that the energy of the ultraviolet photons is in the 6.3–18.3 eV range. Subsequently, the researchers were also able to measure the lifetime of the thorium-229m state – and important piece of information for those aiming to build a nuclear clock.

In this latest research, von der Wense and colleagues – including Christoph Düllmann of Johannes Gutenberg University Mainz – have taken a much closer look at the thorium-229 nucleus to further characterize its potential as a nuclear clock. They began their experiments by storing thorium-229 ions in an ion trap. Some of these are in the thorium-229m state, and the team used laser spectroscopy to measure the hyperfine structure of the ions. Hyperfine structure arises from interactions between atomic electrons and the nucleus and can provide important information about the structure of a nucleus.

From the spectroscopic studies, the team worked out the charge radius of the thorium-229m state as well as its magnetic dipole and electric quadrupole moments. These quantities are important because they define how the state interacts with external electric and magnetic fields. The value of the electric quadrupole moment, for example, suggests that a clock based on a crystalline solid doped with thorium-229 would have to deal with a substantial shift of the nuclear transition frequency caused by electric-field gradients in the crystal.

Testing a constant

Writing in Nature, the physicists say that if such a nuclear clock could be built, its timekeeping would be extremely sensitive to the value of the fine structure constant, which measures the strength of the electromagnetic interaction. This could allow physicists to test whether this constant is indeed constant, or if its value changes under certain circumstances.  If discovered, variations in the fine structure constant could point to physics beyond the Standard Model of particle physics.

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