Micrometer-resolution imaging using MÖNCH: towards G2-less grating interferometry

Abstract

MÖNCH is a 25 µm-pitch charge-integrating detector aimed at exploring the limits of current hybrid silicon detector technology. The small pixel size makes it ideal for high-resolution imaging. With an electronic noise of about 110 eV r.m.s., it opens new perspectives for many synchrotron applications where currently the detector is the limiting factor, e.g. inelastic X-ray scattering, Laue diffraction and soft X-ray or high-resolution color imaging. Due to the small pixel pitch, the charge cloud generated by absorbed X-rays is shared between neighboring pixels for most of the photons. Therefore, at low photon fluxes, interpolation algorithms can be applied to determine the absorption position of each photon with a resolution of the order of 1 µm. In this work, the characterization results of one of the MÖNCH prototypes are presented under low-flux conditions. A custom interpolation algorithm is described and applied to the data to obtain high-resolution images. Images obtained in grating interferometry experiments without the use of the absorption grating G₂ are shown and discussed. Perspectives for the future developments of the MÖNCH detector are also presented.

Keywords: grating interferometry; hybrid detectors; interpolation; silicon detectors.

Figure 1

Simplified diagram of the basic pixel architecture of MÖNCH.

Figure 2

Spectrum of a single pixel [fitted using equation (1)], 2 × 2 pixel and 3 × 3 pixel clusters (fitted with a Gaussian) acquired at 16 keV.

Figure 3

(a) Flat-field image and (b) count distribution of the first supercolumn of MÖNCH0.2. Only two of the 4470 pixels plotted count too few photons and can be attributed to faulty bump-bonding. The estimated bump-bond yield is better than 99.95%.

Figure 4

Spectrum of a single pixel at different energies. The solid line shows the fit using equation (1), while the dashed line shows the Gaussian fit of the pedestals, which can be used to estimate the electronic noise.

Figure 5

Sketch of the cluster coordinate system compared with the physical pixels. The cluster is highlighted in red. It is centered at the corner between the four physical pixels (in black) and spans between their centers. The sub-cluster coordinates are also shown in relation to the main spatial coordinates .

Figure 6

Cumulative distribution for a flat-field measurement acquired at 16.7 keV, using a 320 µm-thick sensor biased at 90 V. The rendered grid shows the partitions of the bins resulting from the iterative algorithm described in §4.

Figure 7

Image of a kidney stone, (a) 25 µm resolution image and (b) using 0.5 µm binning after applying the interpolation algorithm.

Figure 8

Sketch of the G₂-less grating interferometer with the MÖNCH hybrid detector. The setup includes an X-ray source (synchrotron or X-ray tube), an optional source grating G₀ to increase the coherence, the sample, the phase grating G₁ and the MÖNCH detector which is placed at a Talbot distance from G₁.

Figure 9

Profile of the grating and sample images of Fig. 11 ▸ for pixel clusters (17127) and (17128) after flat-field normalization. The intensity is integrated in the direction parallel to the gratings. The fringes are visible only in the center of the pixel clusters, i.e. at the boundary between two physical pixels, where the spatial resolution is higher. The left-hand pixel is located in a flat region of the sample and therefore shows no phase shift between grating and sample profiles, while the right-hand pixel is located at the pyramid slope and shows a phase shift of about one sub-pixel (1 µm).

Figure 10

Retrieved (a) absorption and (b) differential phase contrast images with a pixel size of 25 µm for a polyethylene sphere with 700 µm diameter (left) and a nylon rod with 150 µm diameter (right).

Figure 11

(a) SEM image of the pyramid sample. Retrieved (b) absorption and (c) differential phase contrast images with a pixel size of 25 µm for the pyramids etched in Si.