Growing crystals: a team achievement in pure form

The significance of crystals for science and technology was first recognized when the atomic structure, which is special because it is perfectly regular, was discovered along with the physical properties that this entails. Today, modern medicine has long since replaced the proverbial "look into the crystal ball" with powerful diagnostic techniques, some of them enabled by the use of crystalline materials in X-ray detectors.

Crystals not only look beautiful, they also play a decisive role in research into matter. After early researchers into quantum mechanics working with Schrödinger, Heisenberg etc. had clarified how individual atoms behave, the next step was obvious: What happens when a large number of atoms are "assembled"? Using X-rays, attempts were made to find out how solid bodies work at the smallest level. Ultra short-wave light, which cannot be perceived with the human eye, interacts with the regularly arranged atoms of a crystal in a special way.

Depending on the angle of incidence of the radiation, individual reflexes of each of the many trillion atoms are either added or cancel each other out. By analyzing the refractive images appearing on the screen, you can draw conclusions about the properties of a solid body from the arrangement and distance of the atoms from one another. For these pioneering studies, several Nobel Prizes were awarded in the first half of the 20th century, including to Max von Laue, William Henry Bragg, and his son William Lawrence Bragg. It was now recognized that a crystal owes its special properties to the absolutely regular arrangement of atoms and molecules in the material.

To illustrate the regular arrangement of the atoms in the solid body, consider pyramids of oranges stacked up in front of a fruit stall at a market. The arrangements of the fruit can be continued indefinitely – at least conceptually. Four fruits form a square in one plane, a fifth fruit is located on the center. Each orange in the large pile is also a vertex in the adjacent square and the apex of a small pyramid. It sounds like a children's game, but it is very significant. This is because the arrangement of building blocks, that is the atoms, is associated with fundamental properties: electrical conductivity, thermal conductivity, speed of propagation of sound waves.

Diamonds are one of the most valuable natural resources to this day, although they actually only consist of ordinary carbon. The absolutely regular arrangement of the atoms is what makes the material so special. As soon as an extraneous element, for example, a melon in the image of a stack of oranges, is sneaked into the pile, the regularity is disrupted. The pile then has a bend, the adjacent fruits are no longer arranged in a straight line in every direction, and the absolute purity is destroyed.

Cleanliness, patience, and perfect technique – these are the most important preconditions for growing perfect crystals. A single impurity in the molten substance can interfere with the growth process, which can take several weeks and months. Whereas a crystal arises randomly in nature, the experts at Siemens Healthineers must understand and control the growth process very precisely for industrial production. Processing ingredients of high purity is an art unto itself. Each vessel in which the ingredients are kept or processed must be absolutely clean – and "clean" means that not even a single atom of contamination must be overlooked.

The engineers at Siemens Healthineers had to overcome numerous obstacles on the way to series production. First of all, from 2011, a close partnership was entered into with Acrorad in Japan – one of the few companies in the world that masters the art of crystal growth with the highest quality. At the Forchheim location, a dedicated laboratory, headed by Christian Schröter, PhD, was set up to develop the new type of crystals: the "Crystal Center" which opened in 2020. The cadmium telluride grown there now has a degree of purity of 99.9999 percent, which is certainly a team achievement in pure form.

The subsequent processing of the finished crystals to form high-quality X-ray sensors is just as challenging a task. The new detector systems, including the highly specialized electronics for signal acquisition and processing, were also developed in Forchheim. Since 2014, three experimental prototype scanners are installed at selected clinical partners of Siemens Healthineers in Germany and the U.S., where they are being tested in radiological practice.

By 2025, the complete multi-stage process chain for building arguably the best X-ray detectors currently on the medical technology market will have been set up at the Forchheim location – starting with production of the high-purity raw materials and going on to crystal growth and processing of the cadmium telluride and subsequent integration into the data acquisition electronics.