At first glance, one might think that the situation is quite simple: a thorium nucleus can be in different states - in the ground state or in a metastable state, both of which have almost the same energy. If you hit the thorium with a laser beam whose wavelength exactly matches the energy difference between these two states, you can make the thorium nucleus switch from one state to the other.
The problem is that for this to work, you have to know exactly how much energy is required. You will only see an effect if you hit the bullseye exactly. You have to work with a precision in the range of millionths of an electron volt, otherwise the laser beam simply has no effect.
This becomes even more difficult due to the fact that it is not possible to tell immediately whether the right energy has been hit or not. "If you excite the atomic nucleus, switching it from the lower state to the higher state, it initially remains in this higher state for a while," says Thorsten Schumm. "The excited state has a lifetime of over ten minutes. Only then does the atomic nucleus return to the ground state and emit light, which can then be measured."
Reliably testing a specific wavelength can take hours. "For a long time, it was therefore considered impossible to simply try out all conceivable energy values one after the other," says Thorsten Schumm. "If you don't know where to look, the search is hopeless." So the question is: how can you narrow down the energy range to be searched?
Skyscrapers versus the kerb
As early as the 1970s, uranium nuclei, which transform into thorium nuclei through radioactive decay, were investigated. The radiation from this decay revealed that thorium-229 should have two energy states that are extremely close together – with an energy difference of less than 100 electron volts. It was not possible to be more precise at the time.
In the 1990s, efforts were made to measure this energy difference more precisely. A special trick was used for this: the thorium nuclei were put into a different state with much higher energy than the two states that were actually to be investigated (in the range of many thousands of electron volts). After a while, the thorium nucleus will then switch to one of the two closely neighboring low-energy states – the ground state or the closely neighboring metastable state. The distance between the two low states can be indirectly calculated from the radiation that is produced.
However, the accuracy of this method is limited. "It's a bit like measuring the height of a kerb by first going much higher and dropping a ball onto the street from the roof of a skyscraper," says Thorsten Schumm. "The ball can fall onto the street, or onto the kerb. The difference between these two possibilities is tiny, compared to the height of the skyscraper. But if you measure both distances very precisely, you can in principle learn something about the kerb."
A race for the highest precision
Initial results using this method yielded extremely low values for the energy of the sought-after thorium transition: 3.5 electron volts was an early assumption, then in 2005 a value of 5.5 electron volts was published – with an estimated accuracy of one electron volt.
Research teams from Europe, the USA and Japan competed for the best results, with the estimated value shifting slightly upwards: in 2009, a value of 7.8 ± 0.5 electron volts was measured at the Lawrence Livermore Lab (USA); in 2019, a team from LMU Munich with the participation of Thorsten Schumm finally arrived at 8.28 ± 0.17 electron volts; in 2020, Schumm and colleagues from Heidelberg published data indicating 8.10 ± 0.17 electron volts.
The accuracy had increased over the years, but the results were still far too imprecise to hope to actually hit the energy transition exactly with a laser.
An essential step in improving these measurements was the production of thorium-containing crystals – a difficult task that Thorsten Schumm and his team at TU Wien solved. In this way, a large number of thorium nuclei can be examined simultaneously.
First light from the transition itself
In 2023, a collaboration between LMU Munich and TU Wien succeeded for the first time in directly measuring the sought-after thorium transition using crystals of this kind: It was no longer necessary to indirectly deduce the energy of the thorium transition via another, much higher energy state; it was possible to directly measure the radiation produced during the sought-after transition. This suddenly made a much higher precision possible: 8,338 ± 0.024 electron volts was the new result.
For the first time, there was now hope of being able to excite the state specifically with a laser. Step by step, the now narrowly defined area was scanned until success was finally achieved and a clear signal was obtained – at an energy of 8.355743 ± 0.000003 electron volts.
In five decades, "less than 100 electron volts" thus became a precision in the microelectron volt range. This has finally opened the door to a new field of research with many technical applications.
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