Diffraction grating and alignment method thereof, and radiation imaging system

An alignment method of plural diffraction gratings used in an X-ray radiation imaging system for capturing a phase contrast image, the X-ray radiation imaging system including an X-ray radiation source for emitting an X-ray radiation, an X-ray radiation image detector for detecting an image carried on the X-ray radiation, and the diffraction gratings disposed between the X-ray radiation source and the X-ray radiation image detector, the alignment method comprising:

• disposing one of the plural diffraction gratings to be first aligned in a Z axis orthogonal to a detection surface of the X-ray radiation image detector;
• after the disposition of the diffraction grating to be first aligned, while the diffraction grating to be first aligned is rotated in a θx direction about an X axis orthogonal to the Z axis and in a θy direction about a Y axis orthogonal to the Z and X axes, measuring a dose of the X-ray radiation having passed through the diffraction grating to be first aligned, and locating the diffraction grating to be first aligned in such a position in the θx and θy directions that the dose of the X-ray radiation is maximized;
• after the location of the diffraction grating to be first aligned, disposing in the Z axis another one of the plural diffraction gratings to be aligned second or later, so that the X-ray radiation forms a moiré pattern;
• after the disposition of the diffraction grating to be aligned second or later, while the diffraction grating to be aligned second or later is rotated in the θx and θy directions, detecting the moiré pattern by the X-ray radiation image detector, and locating the diffraction grating to be aligned second or later in such a position in the θx and θy directions that a frequency of the moiré pattern becomes uniform;
• and
• adjusting at least one of a relative position among the plural diffraction gratings along a direction of the Z axis and a relative position among the plural diffraction gratings in a θz direction about the Z axis, so that the X-ray radiation image detector detects removal of the moiré pattern.