5 minute read
DI Europe Summer 23
Guido Gebhardt
Every Photon Counts
In Japan, the current production facility for growing cadmium telluride crystals, an essential compound for the fabrication of photon counting detectors, is nearing its capacity limits. Siemens Healthineers is thus building a new factory in Germany to match the expected rising demand for photon-counting CT.
In 2019 Siemens Healthineers’ materials scientists began looking at growing cadmium telluride (CdTe) crystals to build another factory in addition to working with Acrorad’s crystal factory in Japan, to meet the growing demand for CTs with photon-counting detectors.
Conventional CT detectors have several disadvantages. They first convert incoming X-ray photons into visible light, which is then converted into an electrical signal by photodiodes. And the light pulses generated in the scintillator are emitted not only in the direction of the photodiode, but also in all spatial directions. Therefore, on the one hand, a grid is required to prevent pulses from neighboring pixels from being measured. On the other hand, only a small part of the light arrives at the photodiode.
More sensitivity
“Photon-counting detectors are much more sensitive to X-rays because they convert the energy of each photon directly into an electrical signal,” said Paul Heimann, development engineer in crystal growth, describing the advantage of the new technology.
Photon-counting detectors can not only count each individual photon, but also measure its energy, thereby eliminating scattered radiation. In addition, there is no need for a grid.
The greatest advantage of PCD, however, is that not only can scattered radiation be eliminated, but also different materials can be displayed, based on energy thresholds that can be set in different ways. It is assumed that future detectors will already be able to discriminate four or even six energy levels.
Cadmium telluride has proved to be an ideal detector material for photon-counting detectors. The crystal is particularly heavy and has a high density – both basic requirements for good x-ray absorption.
The big challenge, however, is that CdTe crystals of high purity are needed for CT detectors. This is why Siemens Healthineers decided to work with Acrorad, as the Japanese company has had the necessary know-how for growing the purest CdTe crystals for several years.
Complex manufacturing process
Siemens Healthineers expects the demand for photon-counting CT scanners to increase significantly in the future, and has therefore set up a laboratory as early as 2019 at its site in Forchheim to learn, start and optimize growing CdTe crystals. Following this successful test phase, ground was broken for a dedicated CdTe crystal factory in May 2023.
However, it is important to the Siemens scientists not to simply copy the production systems already available in Japan oneto-one, but to establish their own infrastructure and use production lines from European manufacturers. One reason for this decision is the different regulations that apply in Europe for the operation of electrical systems.
Now that the transfer of knowhow and the establishment of the crystal laboratory have been achieved, the processes are being scaled up in order to go into “series production” with the new plant, to grow crystals on an industrial scale over several weeks.
First, the cadmium, whose melting point is 320°C, must be homogeneously fused with tellurium, which liquefies at around 450°C, to form a polycrystalline ingot. The temperature required for proper “mixing” is about 1,100°C. Only then can crystal growth be initiated with the aid of an ideal seed in an evacuated glass cylinder coated with carbon, so that the finished CdTe crystal consists of perfect alternating layers of cadmium and tellurium.
More flexibility
"While the individual elements cadmium and tellurium are classified as hazardous materials, the final CdTe crystal is much less hazardous," Paul Heimann explained.
"As far as materials’ purity is concerned, we are a little paranoid. The clean room atmosphere is important to prevent even the smallest impurities during the crystal growth phase."
Only when cadmium telluride has solidified into a inhomogeneous gray block, the crystal growth can be started. To do this, the polycrystalline semiconductor is pulled through an oven for ten to twelve weeks so that the atoms, which go into solution again in the process, have enough time to rearrange themselves on the nucleation surface and the crystal can grow perfectly ordered layer by layer.
Siemens Healthineers has put a great deal of effort into setting up its own manufacturing process by growing CdTe crystals for photon-counting detectors. The company expects the two factories on different continents to provide maximum supply security and flexibility, to respond in the best possible way to the increasing demand for photon-counting CT equipment.
https://www.siemens-healthineers.com/
