The Race for Nuclear Time – Scientists Make Important Advance
International Conference on Nuclear Physics
Nuclear clocks could potentially enable scientists to delve into the universe’s fundamental forces in future research endeavors. A critical step forward in this field has been taken by researchers at LMU as part of an international collaboration.
Atomic clocks provide measure time so precisely that they only gain or lose less than one second every 30 billion years. However, with so-called nuclear clocks, it would be possible to measure time even more accurately. Additionally, they could provide scientists with a more profound understanding of fundamental physical phenomena.
“We’re talking about the forces that hold the world together at its core,” says LMU physicist Professor Peter Thirolf, who has been researching nuclear clocks for many years. In contrast to conventional atomic clocks, this type of clock would register forces inside the atomic nucleus.
“This would open up a whole range of research fields that could never be investigated with atomic clocks,” adds Thirolf’s colleague Dr. Sandro Kraemer, who played a major role in driving the project forward while completing his doctorate at KU Leuven in Belgium.
In the race for nuclear time, Thirolf and Kraemer are in the leading pack. Working at the Chair of Experimental Physics in Garching, the two scientists have now made an important advance on the road to the first nuclear clock as part of an international team.
As they report in the journal Nature, they have managed to characterize the excitation energy of thorium-229 with great precision thanks to a new experimental approach. This atomic nucleus is to be used as the timekeeping element of nuclear clocks in the future. Precise knowledge of what frequency it needs for excitation is crucial for the feasibility of the technology.
The innermost clock
For a clock, you need something that periodically oscillates and something that counts the oscillations. A grandfather clock has a mechanical pendulum, the oscillations of which are registered by the clock’s mechanism. In atomic clocks, the atomic shell functions as the timekeeper. Electrons are excited and switch back and forth between high and low energy levels. Then it is a matter of counting the frequency of light particles emitted by the atom when the excited electrons fall back into their ground state.
In nuclear clocks, the basic principle is very similar. In this case, we penetrate to the nucleus of the atom, where various energy states can also be found. If we managed to excite them precisely with a laser and measure the radiation emitted by the nucleus when falling back into its ground state, then we would have a nuclear clock. The difficulty is that of all atomic nuclei known to science, there is only one that could lend itself to this purpose: thorium-229. And even that was purely theoretical for a long time.
What makes thorium-229 so special is that its nucleus can be put into an excited state using a relatively low light frequency – a frequency just about obtainable with UV lasers. Research stalled for 40 years, because although scientists suspected that an atomic nucleus with the right characteristics exists, they were unable to experimentally confirm this hypothesis.
And then in 2016, Thirolf’s research group at LMU made a breakthrough when they directly confirmed the excited state of the nucleus of thorium-229. This fired the starting gun on the race for the nuclear clock. In the meantime, many groups worldwide have taken up the topic.
To get a clock going, the timekeeping element and the clockwork need to be perfectly attuned to each other. In the case of the nuclear clock, this means that you need to know at what exact frequency the atomic nucleus of thorium-229 oscillates. Only then can you develop lasers that excite exactly this frequency.
“You can picture it as being like a tuning fork,” explains Kraemer. “As a musical instrument tries to match the frequency of the tuning fork, so the laser tries to hit the frequency of the thorium nucleus.”
If you were to try out all possible frequencies with different lasers, it would take forever. Not to mention that lasers would have to be laboriously developed first in the corresponding UV light spectrum. To narrow down the range in which the oscillation frequency of thorium-229 lies, the researchers, therefore, took a different tack. “Nature is sometimes merciful and offers us various routes,” says Thirolf. As it happens, lasers are not the only way of producing the excited state of the thorium nucleus. It also occurs when radioactive nuclei decay into thorium-229. “So we start with the grandparents and great-grandparents of thorium, as it were.”
ISOLDE is forging new paths
These ancestors are called francium-229 and radium-229. As neither are found readily in nature, they have to be manufactured synthetically. Currently, there are very few places in the world that are capable of doing this. One of them is the ISOLDE laboratory at the European Organization for Nuclear Research (CERN) in Geneva, which has made possible the old dream of the alchemists – of transforming one element into another.
To accomplish this, scientists bombard uranium nuclei with protons accelerated to extremely fast speeds, thereby producing various new nuclei – including francium and radium. These elements decay rapidly into the radioactive parent nucleus of thorium-229: actinium-229.
Kraemer, Thirolf, and their international colleagues embedded this elaborately manufactured actinium into special crystals, where the actinium decays into thorium in an excited state. When the thorium jumps back into its ground state, it emits the light particles whose frequency is so crucial for the development of the nuclear clock. Demonstrating this is no trivial task, however.
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