When matter is heated to millions of degrees, it turns into plasma — a mix of free electrons and ions, like a boiling soup of charged particles. You can't measure the temperature of such a 'soup' with an ordinary thermometer, but you can shine X-rays through it. X-ray Thomson scattering works like a flashlight in fog: the rays collide with electrons and scatter, and by how their energy changes, you can reconstruct the temperature and density of the plasma.
Since the early 2000s, this method has become the primary tool for studying extreme states of matter in the lab. It is used at powerful laser facilities, where a short pulse compresses a sample, and at X-ray free-electron lasers, which produce ultra-bright flashes. The authors of the new review brought together results from all such experiments: which elements were studied, at what temperatures and pressures, at what angle the rays were directed, and how the data were processed afterward. The result is a detailed map — from light hydrogen to heavy metals.
The review also shows that different analysis methods yield slightly different results, and this is important to keep in mind. For example, to extract temperature, you need to know exactly how free electrons scatter the rays, and there are subtleties there. In the future, more accurate models and brighter X-ray sources will allow even more detailed studies of plasma — down to the behavior of individual electrons.
X-ray Thomson scattering is named after J. J. Thomson, who discovered the electron: he was also the first to describe how a free electron scatters an electromagnetic wave.
In Larry Niven's novel 'Ringworld', the protagonists use a similar principle, scattering of radiation, to study the properties of unfamiliar materials on the ring.