Emerald, ruby, amethyst and many other gemstones have one thing in common: they consist of a perfectly regular crystal structure into which foreign atoms are incorporated in low concentrations. From a physical point of view, these foreign atoms are actually "disturbances", imperfections in the crystal. But they are precisely what give the gemstone its color. Amethyst, for example, has the atomic structure of a simple quartz crystal. However, the addition of a few iron atoms gives it its characteristic violet color. A completely perfect crystal, in which the same arrangement of atoms is repeated exactly over and over again, usually looks quite bland and pale.

The situation is very similar with the crystals that were grown at TU Wien in order to find the long-sought thorium transition: The decisive factor here is also the careful and precise incorporation of foreign atoms - in this case radioactive thorium. Of all the research groups worldwide working on the thorium transition, the team at TU Wien is the only one that can produce such thorium-containing crystals on their own. In the end, this was also the key to their success.

Thorium atoms in the crystal

"If you want to excite thorium atomic nuclei with a laser, you basically have two options," explains Prof. Thorsten Schumm. "Either you use thorium ions, which you trap and hold with electromagnetic fields, or you build the thorium atoms into a solid." Only a very small number of atoms can be trapped in ion traps, so Thorsten Schumm soon realized that he wanted to pursue the solid-state approach. However, there are major technical challenges to overcome.

"The starting material must be completely transparent for the laser. The laser should only have an effect on the built-in thorium atoms," emphasizes Thorsten Schumm. Glass or similar materials that are rather irregular on an atomic level are out of the question, as they are not transparent enough. Only extremely regular crystals, such as calcium fluoride, can be used.

Melting and re-solidifying

But how is it possible to incorporate thorium atoms into an extremely regular calcium fluoride crystal? "It took years to develop this process," says Thorsten Schumm. "We start with a tiny, very regular crystal, to which we add thorium and place it in an ultra-high vacuum. Oxygen would destroy the process immediately." The crystal is then heated in the vacuum chamber and partially melted. This creates a liquid mixture of thorium, calcium and fluorine, while part of the crystal underneath is still solid. The temperature is then lowered again, and the mixture is allowed to solidify – precisely along the geometric pattern defined by the solid crystal underneath.

"There are many technical details that have to be precisely controlled, but if you do everything right, you get a very regular crystal with built-in thorium atoms, a few millimetres in size."

Rare combination of knowledge from different areas

Originally, Thorsten Schumm did not necessarily plan to produce the crystals himself. "There are research institutes and companies that specialize in growing crystals. I had a lot of discussions looking for partners who could produce such crystals, but it was more difficult than I thought," says Schumm.

Most manufacturing processes are optimized for the largest possible crystals. However, small crystals are required to excite the thorium transition: the laser beam used for the experiments only hits a small area of the sample, any material beyond this would only contribute to interference. "When producing small crystals, you have to face completely different difficulties. Surface tension, for example, plays a much more important role on a small scale. It also took us years to go from excellent centimetre-sized crystals to excellent millimetre-sized crystals."

In addition, there are hardly any institutes that have the necessary knowledge and equipment to handle radioactive thorium. "This is of course a great advantage for us at the Institute for Atomic and Subatomic Physics at TU Wien," says Thorsten Schumm. "We are certified for this, we have radiation protection expertise and the necessary equipment to work with thorium." A seemingly simple step such as polishing the crystal becomes a major problem if you have no experience with radioactive material. You can't simply generate thorium dust in the laboratory, which can then perhaps be inhaled.

A completely new field of research

Thorsten Schumm found himself in the strange situation of having to build up world-class expertise in an area that was not really his true field of research: the aim was always to excite atomic nuclei with a laser, i.e. to combine quantum physics and nuclear physics. But as a means to this end, a breakthrough was also needed in materials research – which is actually a completely different area of physics.

"I would never have thought that I would also become a solid-state physicist," says Schumm. "But ultimately that was the key to success: precisely because we produced, characterized and measured the crystals ourselves, we had the decisive edge and, together with our colleagues in Braunschweig, were ultimately the team that achieved the crucial breakthrough."

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