An x-ray view of carbon


At the heart of the planets, we find extreme states: temperatures of several thousand degrees, pressures a million times higher than atmospheric pressure. They can therefore only be directly explored to a limited extent, which is why the expert community tries to use sophisticated experiments to recreate equivalent extreme conditions. An international research team including the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) adapted an established measurement method to these extreme conditions and tested it successfully: using flashes of light from the world’s most powerful X-ray laser , the team managed to take a closer look at the important element, carbon, as well as its chemical properties. As reported in the review Plasma physics, the method now has the potential to provide new information about the interior of the planets inside and outside our solar system.

The conditions inside Jupiter or Saturn ensure that the matter inside is as dense as a metal but, at the same time, electrically charged like a plasma. “We call this state hot dense matter,” explains Dominik Kraus, physicist at HZDR and professor at the University of Rostock. “It is a state of transition between solid state and plasma that is found inside planets, although it can also occur briefly on Earth, for example during meteor strikes.” Examining this state of matter in detail in the laboratory is a complicated process involving, for example, the firing of powerful laser flashes on a sample and, in the blink of an eye, its heating and condensation.

But what do the chemical properties of this hot, dense matter really look like? So far, existing methods have provided unsatisfactory answers to this question. So a team from six countries came up with something new, based on the world’s most powerful x-ray laser, the European XFEL in Hamburg. In a kilometer-long accelerator, extremely short and intensive X-ray pulses are generated. “We directed the pulses to thin carbon sheets,” says lead author Katja Voigt of the HZDR Institute for Radiation Physics. “They were made of graphite or diamonds.” In the leaves, a small proportion of the flashes of X-rays is diffused on the electrons and their immediate environment. The bottom line is that the scattered flashes can reveal the type of chemical bond that carbon atoms have formed with their surroundings.

After the doubts came the surprise

Known as Raman X-ray scattering, researchers in fields such as materials science have been using this method for some time. But for the first time, the team around Voigt and Kraus managed to equip it for experiments to probe hot dense matter. “Some experts doubted it would work,” Kraus explains. The detectors, in particular, which must pick up the x-ray signals emitted by the carbon sheets, must be both very efficient and at high resolution, a major technical challenge. But analysis of the measurement data clearly showed what bonding states the carbon had entered. “We were a little surprised that it worked so well,” Voigt says, visibly delighted. If they were to apply the method to dense, hot material, however, something was still missing: powerful laser flashes that would drive the carbon sheets at high pressures and temperatures of up to several 100,000 degrees. For this purpose, the international Helmholtz beamline for extreme fields (HIBEF), recently inaugurated under the auspices of HZDR at the European XFEL, comes into play. It is one of the most modern research facilities in the world. with high-performance lasers capable of performing the first Raman X-ray experiments in a matter of months. “I am really optimistic that it will work,” says Dominik Kraus.

Comet crash in the laboratory

The method could well facilitate a lot of different scientific knowledge: on the one hand, it is not known how many light elements like carbon or silicon are present in the Earth’s core. “The new method is not limited to carbon, but could be applied to other light elements,” Voigt explains. Another question to explore concerns the interior of so-called gas giants like Jupiter and ice giants like Neptune. Here, complex chemical reactions occur, as is the case in distant exoplanets of similar stature. It should be possible to reconstruct these processes in the laboratory using the Raman X-ray method. “Perhaps it would be possible to solve the puzzle of the reactions responsible for planets like Neptune and Saturn emitting more energy than they really should, “Kraus hopes.

In addition, this new method should allow scientists to simulate comet crashes on a miniature scale: if comets were actually transporting organic matter to Earth once, could the crash have triggered chemical reactions favoring the development of life ? And the method even has a potential for technical applications: in principle, it seems possible that, under extreme conditions, new materials can be formed and exhibit fascinating properties. An example would be a superconductor that operates at room temperature and does not need complicated cooling like existing materials. Such a superconductor at room temperature would be of great technological interest because it could conduct electricity without any loss without having to cool it with liquid nitrogen or liquid helium.

– This press release was originally published on the Helmholtz-Zentrum Dresden-Rossendorf website

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