• Tue. Feb 27th, 2024

Race for Nuclear Time – Scientists Make Significant Advances

Race for Nuclear Time – Scientists Make Significant Advances

Nuclear clocks, which are more accurate than atomic clocks, could offer scientists new ways to explore the fundamental forces of the universe. An international team, including LMU researchers, has made significant progress towards this, accurately characterizing the excitation energy of the element thorium-229, the timekeeping component in nuclear clocks. (Artist’s idea.)

Nuclear clocks will enable scientists to probe the fundamental forces of the universe in future research efforts. As part of an international collaboration, researchers at LMU have taken a crucial step in this field.

Atomic clocks measure time by less than one second every 30 billion years. However, with so-called nuclear clocks, it is possible to measure time more precisely. In addition, they can provide scientists with a deeper understanding of fundamental physical phenomena.

“We’re talking about forces that hold the world together at its core,” says LMU physicist Professor Peter Thirolf, who has been researching nuclear clocks for years. Unlike traditional atomic clocks, this type of clock would record the forces inside the atomic nucleus.

“This will open up a whole range of research areas that could never be explored with atomic clocks,” said Thirolf’s colleague Dr. Sandro Kramer adds.

Peter Thirolf

Peter Thirolf has been researching nuclear clocks for years. Credit: Stephan Höck / LMU

In the race for nuclear time, Thirolf and Kramer are leading the pack. Two scientists working at the Chair of Experimental Physics in Garching have made a significant breakthrough on the way to the first nuclear clock as part of an international team.

As they report in the journal Nature, thanks to a new experimental approach, they were able to characterize the excitation energy of thorium-229 with great precision. This atomic nucleus will be used as the timekeeping component of nuclear clocks in the future. A precise knowledge of the frequency required for excitation is critical to the feasibility of the technique.

The innermost clock

For a clock, you need something that oscillates periodically and counts the oscillations. A grandfather clock has a mechanical pendulum whose oscillations are registered by the clock’s mechanism. In atomic clocks, the atomic shell acts as the timekeeper. Electrons become excited and switch back and forth between higher and lower energy levels. Then it’s a matter of counting the frequency of the light particles it emits the atom When the excited electrons fall back to their ground state.

Sandro Kramer

Sandro Kramer researched the nuclear clock as part of his Ph.D. Currently continuing his work at LMU. Credit: Stephan Höck / LMU

In nuclear clocks, the basic principle is very similar. In this case, we penetrate into the nucleus of the atom, where the various energy levels can be found. If we can precisely excite them with a laser and measure the radiation emitted by the nucleus as it falls to Earth’s surface, we will have a nuclear clock. The difficulty is that of all the atomic nuclei known to science, there is only one that lends itself to this purpose: thorium-229. It was also purely theoretical for a long time.

A nucleus like no other

What makes thorium-229 so special is that its nucleus can be excited using a relatively low frequency of light. Although scientists suspected the existence of an atomic nucleus with the correct characteristics, they could not confirm this theory experimentally, because research stopped for 40 years.

Then in 2016, Thirolf’s research team at LMU made a breakthrough when they directly confirmed the excited state of the thorium-229 nucleus. It fired the starting gun in the race for the nuclear clock. In the meantime, many groups around the world have taken up the issue.

For a clock to work, the timing element and the clockwork must be in perfect harmony with each other. In the case of a nuclear clock, this means that you need to know the exact frequency at which the atomic nucleus of thorium-229 oscillates. Only then can you develop lasers that excite this frequency.

Sandro Kramer Research Machine

Only one known atomic nucleus has the right properties to develop a nuclear clock: thorium-229. To make the clock tick, you need to find the exact frequency of light that can excite it. Credit: Stephan Höck / LMU

“You can picture it like a tuning fork,” Kramer explains. “Like a musical instrument trying to match the frequency of a tuning fork, the laser tries to hit the frequency of the thorium nucleus.”

If you try all possible frequencies with different lasers, it will take forever. Needless to say, lasers in the corresponding UV light spectrum had to be developed first. To lower the thorium-229’s oscillation frequency range, the researchers adopted a different strategy. “Nature is sometimes merciful and offers us various ways,” Thirolf says. As it happens, lasers are not the only way to produce an excited state of a thorium nucleus. This also happens when radioactive nuclei decay into thorium-229. “So we start with the great-grandparents of thorium.”

ISOLDE is forging new paths

These progenitors are called francium-229 and radium-229. Since both of these are not readily available in nature, they have to be manufactured artificially. Currently, very few places in the world can do this. One of them is the ISOLDE laboratory of the European Organization for Nuclear Research (CERN) in Geneva, made possible the old dream of alchemists – to transform one element into another.

To do this, scientists rapidly accelerate uranium nuclei with protons, thereby producing a variety of new nuclei, including francium and radium. These elements rapidly decay into thorium-229: the radioactive parent nucleus of actinium-229.

Kramer, Thirolf, and their international colleagues embedded this elaborately produced actinium into special crystals, where the actinium decays into thorium in an excited state. As thorium plunges to its surface, it emits light particles at a frequency critical to the development of the nuclear clock. However, proving this is not trivial.

Laboratory of Sandro Kramer and Peter Thirolf

The next experiments to study the nucleus of thorium-229 in the laboratory of Sandro Kramer and Peter Thirolf will begin in the summer. Credit: Stephan Höck / LMU

“If the nuclei don’t sit in the right place on the crystal, we don’t stand a chance,” Kramer says. “Electrons in the environment absorb energy, and nothing we can measure puts it out.”

Previous attempts to introduce uranium into the crystal lattice instead of actinium fell at this hurdle. “When uranium-233 decays to thorium-229, a recoil is produced that damages the crystal,” Thirolf explains. Decaying actinium into thorium causes very little damage, which is why researchers in collaboration with CERN chose this challenging path for the new study.

The hard work and patience paid off: with their new method, the team was able to determine the energy of the state transition very precisely. They also demonstrated that a nuclear clock based on thorium embedded in a crystal was feasible. Such solid-state-based clocks have an advantage over other approaches because they work with more atomic nuclei and can provide faster measurement results.

A matter of time

“We now know the approximate wavelength we need,” Thirolf says. The next task is based on new findings to progressively narrow down the exact transition energy. First, researchers will create an excitation using a laser. Then, they can home in on increasing frequency accuracy With a more precise laser. To keep it from taking too long, they don’t use tweezers to find a needle in a haystack, so to speak, but a rack.

This ‘rake’ is called a “frequency comb” and was developed by Thirolf’s LMU colleague Professor Theodor Hansch, who was awarded the 2005 Nobel Prize in Physics for this achievement. Scientists can use this comb to scan hundreds of thousands of wavelengths simultaneously until they find the right one.

Some challenges remain on the road to nuclear clocks. Scientists need to better understand thorium isomers, develop lasers, and formulate theories. “But it’s worth staying the course,” Thirolf reckons. “In the long term this project opens up a wealth of new application possibilities, making all the experimental effort worth it,” Kramer adds.

These new possibilities include not only basic physics research but also practical applications. Using a nuclear clock, scientists can detect the smallest changes in Earth’s gravitational field, such as when tectonic plates shift or before volcanic eruptions. With new victories the prize is within reach. The first prototypes will be here in ten years. “We may have them ready in time for the second redefinition in 2030,” the two physicists hope. They refer to plans to come up with a new, more precise standard definition of a second, for which scientists will use state-of-the-art atomic clocks – and perhaps even the first nuclear clocks.

Reference: Sandro Kramer, Jani Mons, Michael Athanasakis-Kaklamanakis, Sylvia Bara, Kjeld Beeks, Premaditya Chhetri, Katerina Chrysalidis, Jokos M. Coles, Joss Kolsio, Jossio, Jossio Korsesmo, Jossio, Jossio , “Observation of the Radiative Decay of the 229th Nuclear Clock Isomer,” by Jossio Corcesero, Sandro Kramer, Jani Mons, and Michael Athanasakis-Kaklamanakis. De Witte, Rafael Ferrer, Zarina Geldof, Reinhard Heinke, Nyusha Hosseini, Mark Husey, Ulli Koster, Yuri Kudryavtsev, Mustafa Latiau, Razvan Lika, Gole Magchiles, Vladimir S. Mania, Clement Merkels, Ratten Sebastian, Lino Sebastian, Lino Sebastian Peter G. Thirolf, Shandirai Malven Tunhuma, Paul van den Berg, Piet van Dauppen, Andre Vantom, Matthias Verlinde, Renan Villarreal, and Ulrich Wahl, 24 May 2023, Nature.
DOI: 10.1038/s41586-023-05894-z

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