X-ray dark-field imaging is usually performed with a Talbot-Lau interferometer, but the quality of its analyser grating is limited by the fabrication process. This makes it difficult to achieve a large area and high aspect ratio. In this study, the analyser grating is abandoned and dark-field imaging is realized using only one absorption grating. The optimal grating position is derived theoretically in the single absorption grating x-ray dark-field imaging, and the conclusion is partially verified by the experiment. Meanwhile, the optimal object position for dark-field imaging has also been obtained from both the theory and experiment. In the process of comparing x-ray dark-field imaging with x-ray refraction imaging, it is found that both of them can be explained by geometric optics, showing the intrinsic relationship between them.
In this paper, we propose a new method to prepare X-ray absorption gratings. The thermal composite absorption gratings with periods of 80 μm and 140 μm are successfully fabricated by using the difference in X-ray absorption between aluminum and silver film. The production process and the use of equipment are simple and easy to implement. A number of absorption gratings can be produced in one production, which greatly reduces the cost of gratings. Finally, the X-ray single absorption grating imaging experiment is used to verify the effectiveness of the thermal composite absorption grating.
Over the last two decades, the grating-based phase-contrast imaging has aroused the interest of a number of researchers. It could provide an access to three complementary signals simultaneously: the conventional absorption contrast, the differential phase contrast related to refraction of incident wave, and the dark-field contrast that relates to ultra small angle scattering in a sample. The grating-based phase-contrast signals have higher contrast sensitivity for some types of soft samples than the absorption signals. Dark-field signals have better diagnostic effects in the detection of lung tumors, pneumothorax and the identification of microcalcifications in breast. There are two main phase retrieval methods in grating-based X-ray phase-contrast imaging, i.e. phase stepping method and Fourier transform method. The phase signals retrieved by phase stepping is high precise and has low noise. But the sample suffers high dose due to at least three exposures. The phase signals retrieved by Fourier transform is low-dose due to the fact that only one image with sample is needed, but it is easily affected by artifacts when the size of the filtering window is too large. However, when the size of the filtering window is too small, the high-frequency information of the phase-contrast image will be lost, and the image will become blurred. A trade-off between definitions of the image and artifacts should be made. Since the phase-contrast signal and the dark-field signal of the sample are modulated by carrier fringes, the frequency spectrum of the detected image consists of many different harmonics. The artifacts in the retrieved signals originate from the spectrum aliasing between primary peak around zero spatial frequency and first-order harmonic peaks. Therefore, the subtraction between two images with phase difference can remove the primary peak, and the artifacts in the phase-contrast signals and dark-field signals will be suppressed. In order to further suppress the artifacts, we increase the frequency of carrier fringes, which results in a larger distance between first-order harmonic peaks in frequency domain. We finally attain artifact-free phase-contrast images and dark-field images while maintaining high definition of the images. The method proposed here is not only applicable to incoherent imaging system, but also to Talbot-Lau interferometer, and it would be useful in fast and low-dose X-ray phase-contrast and dark-field imaging.
A Monte Carlo model is developed and implemented to calculate the characteristics of x-ray induced secondary electron (SE) emission from a CsI photocathode used in an x-ray streak camera. Time distributions of emitted SEs are investigated with an incident x-ray energy range from 1 to 30 keV and a CsI thickness range from 100 to 1000 nm. Simulation results indicate that SE time distribution curves have little dependence on the incident x-ray energy and CsI thickness. The calculated time dispersion within the CsI photocathode is about 70 fs, which should be the temporal resolution limit of x-ray streak cameras that use CsI as the photocathode material.
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