The core components of the X-ray Michelson interferometer require extremely high machining and fabrication accuracy, and hence, the measurement and characterization of device performance and working conditions are crucial for the final integration tests. For a working environment of the interferometer as close to the real one as possible, the fringe contrast and defocusing distance of the LLL interferometer and the exit-beam bandwidth and relative displacement of the MDCM are measured online at Shanghai Synchrotron Radiation Facility (SSRF).Results and Discussions The optimum fringe contrast of the LLL interferometer is 0.63, and the defocusing distance is 10.4 μm. The relative exit-beam bandwidth of MDCM is 1.3×10^-6, and the relative displacement of the exit beam from upper and lower channels is 37 μm. The measurement results show that the main factor affecting the contrast of the interference fringe is the defocusing distance of the LLL interferometer, while the main factor causing the exit-beam displacement of MDCM is the width inconsistency of the upper and lower channels, which is caused by the machining error in the crystal fabrication process. Next, we will attempt to use chemical corrosion to correct the defocusing distance of the LLL interferometer and the channel width of MDCM to obtain the LLL interferometer with better interference fringe contrast and the MDCM crystal with better consistency. They are required for the integration test of the Michelson interferometer and the measurement of the 14.4 keV M?ssburger nuclear resonance wavelength.

The development of synchrotron radiation (SR) technology has made a qualitative breakthrough in the luminance of M?ssburger sources. However, the traditional method based on the silicon lattice constant is still adopted in the experiment of wavelength measurement, and the measurement accuracy is affected by the uncertainty of the silicon lattice (2×10^-8). Since Bonse and Hart published their experimental results in 1965, the X-ray interferometer has been widely used for precision measurement of parameters, such as lattice constants, due to its extremely high accuracy. This interferometer technology can be used for accurate measurement of silicon lattice constants independent of X-ray wavelength values. The first report on the X-ray Michelson interferometer came from Appel and Bonse in 1991, who added a group of single channel-cut diffraction devices with adjustable optical paths into the space of the Laue-Laue-Laue (LLL) interferometer to form the structure of the interferometer. However, the Michelson interferometer based on this structure is not suitable for measuring the M?ssburger resonance wavelength at which its operating wavelength is not around 14.4 keV, and the adjustable range is limited (a few micrometers) as the optical path difference in the interferometer is formed by the rotation of the optical components, which can hardly achieve high-precision measurement. We design an X-ray Michelson interferometer, which can be used to measure 14.4 keV M?ssburger resonance wavelength. The LLL-interferometer and the monolithic double channel-cut monochromator (MDCM) that can accurately measure the optical path difference are fabricated. The key parameters such as the fringe contrast of the LLL-interferometer, diffraction bandwidth of MDCM, and relative displacement of the exit-beam position are measured online, which provides a technical basis and device foundation for the subsequent integration test of the Michelson interferometer.

The new design of the X-ray Michelson interferometer is shown in Fig. 1. The non-dispersive LLL-interferometer can be transformed into a dispersive Michelson interferometer when an MDCM that can pass through 14.4 keV photons is inserted into the space of the monolithic LLL-interferometer. The specially designed MDCM has two optical paths, upper and lower, each consisting of four Bragg reflections in two grooves. With the crystal plane combination with an appropriate index selected from monocrystalline silicon and ingenious structure design, 14.4 keV photons incident at the Bragg angle can pass through MDCM exactly after four consecutive reflections and keep the original direction of propagation. The application of certain pressure on the upper surface of the crystal can change the upper channel-cut width, which introduces an adjustable optical path difference between the upper and lower paths. At the same time, the optical path difference is accurately measured by the visible light interferometer, and X-ray wavelength measurement independent of lattice constants can be achieved by the comparison of the interference fringe orders between visible light and X-ray.

This paper introduces a new X-ray Michelson interferometer design that can be used for ultra-precise measurement of ⁵⁷Fe 14.4 keV M?ssburger nuclear resonance wavelength. The new design consists of a monolithic anti-symmetrical LLL-interferometer and an MDCM, which can match the X-ray with a wavelength of 14.4 keV. The performance of the first homemade LLL-interferometer in China and the working conditions of MDCM are measured online and characterized quantificationally by a 14.4 keV monochromatic X-ray at SSRF. The measurement results of the fringe visibility (0.37-0.63) of the LLL-interferometer and correction parameters of MDCM are obtained, which provide experience and a technical basis for the development and online characterization of X-ray optical elements with complex configurations in China.