An X-ray interferometer was proposed for the apparent extremely small beam size measurement in the FCC-ee. The wavelength information is necessary to determine the beam size via interferometry. Certain monochrometer such as band-pass filter is used for identify the wavelength in ordinal interferometer using the visible SR. the K-edge filter is proposed as getting quasi monochromatic X-ray for the X-ray interferometer, but due to wide band width, the effective wavelength is not clear. Since this reason, we propose a method for determine the wavelength by interferometer itself. Since one of the function of interferometer is absolute measurement of wavelength. we can determine the wavelength of input X-ray from pitch of interference fringe. By applying this method, we can determine beam size from the data of interferometer self consistently.
We propose a test of X-ray interferometer with this method at X-ray montor line in the SuperKEKB. The X-ray monitor line is used for beam size measurement mainly with coded aperture, and it has a long range X-ray beam line which is suitable for testing the X-ray interferometer.