Development and application of X-ray 4D phase tomography

In the measurement of X-ray phase tomography, we normally assume that a sample is stationery during a scan. However, when we study on not only its structure but also function, we need to visualize the dynamics of the sample. 4D tomography is known as a technology that measures four-dimensional image (a movie of three-dimensional image) with a time axis added to conventional tomography. We are developing in this direction also with X-ray phase tomography.

An X-ray Talbot interferometer functions with X-rays even of a broad spectrum bandwidth. Synchrotron radiation has a continuous spectrum, and a monochromatic X- ray beam extracted from the spectrum through a monochromator is used normally. But, when ‘white’ (therefore intense) synchrotron radiation is used as it is, it is possible to speed up the scan of phase tomography with a Talbot interferometer (Fig. 1) [1].

However, radiation damage on samples and gratings caused by white synchrotron radiation is problematic because we cannot distinguish between the dynamics in sample and the change due to radiation damage. Based on the same idea as Rayleigh’s 1/4 wavelength law, one can find that the performance of a Talbot interferometer with X- rays of a 10% bandwidth is also the same as that with monochromatic X-rays. Therefore, we installed a multilayer mirror at BL28B2 to generate a so-called pink beam that has a central energy of 25 keV and a 10% bandwidth. As a result, the problem of the radiation damage is relaxed, keeping the resultant image quality almost unchanged.

Pink-beam 4D phase tomography was applied to a polymer sample (polypropylene) under irradiation of laser, as a model of laser processing (Fig. 2) [2]. Melting frontier due to laser irradiation is visualized, thanks to the sensitivity of phase tomography.