X-rays having a wavelength in the range of 10-100 angstroms are referred to as soft X-rays. The incident X-ray photons are elastically collided with electrons that are tightly bound to the nucleus in the crystal atoms. The radiated scattered waves only change direction without energy loss, and the wavelength and phase are unchanged. This phenomenon is called X-ray coherent scattering. Based on this principle, the detection of molecular and supramolecular structure types, as well as material analysis and medical diagnosis can be performed. Although the phenomenon of coherent X-ray scattering has been used for a long time, due to some limitations, it has not been universally applied to medical diagnosis and material analysis. Not only because of the limitation of the X-ray source, a pivotal factor in moving the technique forward is the development of detectors.
Most of the current soft X-ray beams, such as the SEXTANT beam at the SOLEIL sync center, use back-illuminated CCDs built into a vacuum environment. Scientists using the system are continually looking for improvements. It turns out that the current detectors still have some irritating shortcomings, such as long acquisition time, low image contrast, badly defined edges, and are prone to pixel overflow.
Commercially available CMOS is often used as an indirect detector component for X-rays for hard X-ray tomography or ptychography experimental platforms. But many applications of soft X-rays (with energy between tens and 2 keV) do not benefit, such as X-ray holographic imaging. At present, commercial sCMOS is mostly a front-illuminated structure, and although the pixels can increase the amount of light entering by adding microlenses, this is of no practical significance for X-rays with the energy lower than 1keV.
The Dhyana 95 (TUCSEN PHOTONICS) is a typical representative of the new generation of sCMOS, equipped with the latest back-illuminated sCMOS sensor, spectral response covering 180-1100nm, while having a large area array of 22.5mmx22.5mm, 11μmx11μm pixel size, 95% Quantum Efficiency. The high frame rate and low noise, make the camera an ideal choice for this application.
Kewin Desjardins, a scientist from the French SOLEIL synchrotron (synchronous accelerator development center) and his colleagues are studying how to improve the acquisition of soft x-ray coherent scatter signals. They used Dhyana 95 camera to do some preliminary test evaluation.
Figure 1. The Dhyana camera is mounted in the reflectometer cavity of the METROLOGIE beamline
The right image in Figure 2 is the diffraction image captured by the Dhyana95, its diffraction order reached to the maximum value, the 6th, and indicates that Dhyana95 has an excellent dynamic range.
Figure 2. A 5 micron pinhole diffraction spot of 186 eV recorded by Dhyana 95
The right picture of the Figure 3 is a composited radia-illumination image accumulated from 50 images and each with a 100 ms exposure time by the Dhyana95. The total acquisition time of 50 images is less than 10 s, while the same image effect achieved with a back-illuminated CCD camera is a couple of minutes. The comparison of the acquisition time highlights the advantage of the Dhyana 95 in productivity.
Figure 3. Irradiation reaction on a hexachromatic beam from 11 test masks with a diameter of 200 nm and an x-ray energy beam of 700 eV
Customer's comments: The distinctive features of Dhyana 95 make it an excellent alternative to back-illuminated CCDs which are commonly applied in soft X-ray applications of previous-generation synchrotrons, such as coherent scatter experiments. They would greatly benefit fully from the Dhyana95's high frame rates.
At this stage, the Desjardins team is very satisfied with Dhyana 95. Not only does it exhibit excellent linearity, dynamic range and quantum efficiency, it greatly reduces imaging time, and it is extremely cost-effective. As of now, Dhyana 95's excellent imaging capabilities have made their experiments progress very smoothly.
The preliminary research results of Kewin and his colleagues have been presented at the SRI 2018 Annual Meeting. Next, he and his team will explore the potential applications of back-illuminated sCMOS and Dhyana 95 camera in further X-ray related research and Tucsen will continue to offer them resources to achieve more advanced performance.
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Many thanks to Kewin Desjardins from SOLEIL synchrotron in France provided the above data and materials.
Kewin Desjardins, Horia Popesc1, Pascal Mercère, Claude Menneglier,Roland Gaudemer, Karina Thånel and Nicolas Jaouen. SRI2018.
Characterization of a back-illuminated CMOS Camera for soft x-ray coherent scattering
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