Tough, light and reliable materials such as carbon composites are becoming increasingly common in advanced engineering. Identification of defects, impurities or cracks in modern ultralight materials demands modern, state-of-the-art technology. Standard imaging technology has been used for many years for detecting defects in traditional materials such as metals, but traditional X-ray imaging systems fail when being used with modern light materials such as composites. These new materials therefore call for novel inspection technologies and photon counting detectors are ideal for this purpose.
Advacam brings to the market a new range of X-ray imaging cameras that are optimised for composite material testing. As for biological applications, Advacam’s approach to non-destructive testing (NDT) is highly sensitive to low energies. In this way, light materials such as carbon fibres, epoxies, etc. are easily revealed in great detail. Hence, even challenging defects such as deep laminate wrinkles, kissing-bonds, delaminations, porosity, foreign objects and micro-cracks in the soft materials can be detected with a spatial resolution of 55 µm or better. Combining the sensitivity to low X-ray energy photons with the very high dynamic range of photon counting detectors creates a powerful tool for NDT in the aerospace industry and elsewhere.
Contrary to classical X-ray imaging setups, the robotic system produced by Radalytica gives nearly absolute flexibility of viewing angles. Therefore, robots are able to inspect using X-rays from different angles, allowing them to accurately determine the positions of defects. Moreover, robots further open the possibility of using 3D imaging techniques such as computed tomography and tomosynthesis. These are methods commonly used in X-ray imaging, but with limited applicability to large complicated shapes. Robots overcome this limit. The robotic systems could be used in quality control labs or built into production lines.
The leading detector technology, which Advacam uses for its products and solutions is based on Medipix hybrid pixel detectors. These devices were developed within international collaboration of universities and research laboratories lead by team at CERN during past 20 years. Advacam team’s members have been part of the Medipix Collaboration from it inception and have been contributing to the technology.
Photon Counting Technology
Advacam’s imaging cameras are direct conversion single photon counting pixel detectors that represent the cutting edge of current radiation imaging technology. The term “single photon counting” means that every single photon of X-ray radiation detected in individual pixel is processed and counted. The technology brings two major advantages in comparison to the conventional X-ray imaging – high contrast together with sharp images and spectral information of X-rays that allows material specific information to be displayed in colors.
In the direct conversion cameras each pixel of the semiconductor crystal is directly connected to the complex CMOS circuit using a conductive solder bump. In the indirect conversion cameras a scintillation layer is attached on top of a photodiode. The photodiodes manufactured on a simple CMOS circuit that enables fine pixel sizes.
Illustrative comparison of a single pixel of a direct conversion and indirect conversion cameras.
The term direct conversion refers to immediate conversion of the X-rays into electric charge within the semiconductor crystal. The principle is contrary to the conventional indirect conversion where the X-rays are first converted into visible light in the scintillation layer that subsequently is converted into electric charge in the photodiodes.
Illustration of the operation principles in a single pixel between the direct and indirect conversion cameras.
The photon counting principle of detection eliminates all other sources of noise that are present in CCD or flat-panel based cameras. This leads to considerably better signal-to-noise ratio and therefore detectability of more details in images. The images sharpness or the actual spatial resolution of the captured image is defined by the electric charge in the CMOS readout. Even thought the pixel size of of the direct converting cameras is larger than that of the conventional indirect conversion cameras, the signal of the detected X-rays is better focused into the pixels. The typical size of a direct conversion pixel ranges from few millimeters to tens of micro meters where Advacam represents the highest pixel density of the current industrial X-ray cameras with 55 um pixel size. The video below describes the differences between conventional indirect conversion, direct conversion charge integrating and photon counting cameras. It summarises the major differences in the captured image quality in terms of spatial resolution, image noise and material discrimination.
The energy sensitivity is as important advancement of the imaging technology as was the colour photography and film. Contrary to regular X-ray imaging cameras, the photon counting cameras can discriminate or even directly measure energy (wavelength) of incoming photons. Since each element of the sample has different X-ray attenuating properties, it is possible to estimate material composition of the sample if the energy of the photons is measured. The spectral sensitivity offers major improvement over the conventional X-ray imaging cameras