How to Improve X-Ray Spectrometers

Directly resolved micro-meter interference fringes help reveal subtle phase contrast in the sample (Source: Kagias / PSI)

Directly resolved micro-meter interference fringes help reveal subtle phase contrast in the sample (Source: Kagias / PSI)

Researchers at the Paul Scherrer Institute’s Swiss Light Source in Villigen, Switzerland, have developed an x-ray grating inter­ferometry setup which does not require an analyzer grating – exhibiting fabri­cation diffi­culties especially for high energies – to ultimately improve the flux efficiency by a factor of two and lower overall inter­ferometer production costs.

X-ray grating inter­ferometry is an extremely useful tool for investi­gating the structures of unknown biological samples. The inter­ferometers work by firing x-rays through a phase grating that introduces an inter­ference fringe at specific distances down­stream. When an object such as a bio­medical or material science sample is placed in the beam’s path, it modifies the observed inter­ference pattern via absorp­tion, refraction, and small-angle scattering. Once these signals picked up by the detector, technicians can algorith­mically determine the sample’s proper­ties.

In order to achieve a high sensi­tivity the period of the generated fringe needs to be in the micro­meter range. Such fringes are chal­lenging to be recorded directly over a large field of view. To work around this, one widely-used method known as Talbot-Lau inter­ferometry employs an absorption grating G2 placed right before the detector, and senses the distor­tions by a proce­dure known as phase stepping. Here, the absorp­tion grating is scanned step by step for one or more periods of the inter­ference fringe, each time recor­ding an image which results in an intensity curve at each pixel. This way the inter­ference fringe can be sensed indi­rectly and absorption, differential phase and small-angle scattering signals are obtained for each pixel.

This technique carries the advantage of decoupling the inter­ferometer’s sensitivity from the detector’s reso­lution but ulti­mately causes a lower system dose effi­ciency due to photon absorp­tion by G2. Addi­tionally, a slurry of manu­facturing costs and mechanical comple­xities regarding G2, such as the area and aspect ratio of the gratings, further compli­cate its implemen­tation. To remedy this, researchers at the Insti­tute for Biomedical Enginee­ring in Zurich and the Swiss Light Source (SLS) have developed an inter­ferometer which eschews the G2 grating, instead opting to directly exploit the fringe inter­ference via higher resolution.

“We can perform diffe­rential phase contrast imaging with high sensi­tivity without the need for a G2 grating or a detector with small pixel size in order to resolve the fringe,” said Matias Kagias, a PhD student in the labora­tory of  Marco Stampanoni. The researchers’ experi­mental setup consisted of an x-ray source, a single phase grating, and a GOTTHARD micro­strip detector developed by the SLS detector group, a signi­ficantly simpli­fied version of the tradi­tional Talbot-Lau inter­ferometer. The GOTTHARD detector uses a direct conversion sensor, in which x-ray photons are absorbed, the charge generated from one absorp­tion event is collected by more than one channel for small channel sizes.

“The key point to resol­ving the fringe is to acquire single photon events and then inter­polate their positions using the charge sharing effect, which is usually considered as a negative effect in photon counting detectors,” Kagias said. By inter­polating the position of many photons, a high reso­lution image can then be acquired.

When the researchers implemented the appro­priate algorithm to analyze this recorded fringe, they found that fringes of a few micro­meters could be acquired success­fully while still retrieving the diffe­rential phase signal. According to Kagias, this ulti­mately increases the inter­ferometer’s flux effi­ciency by a factor of 2 compared to a standard Talbot-Lau inter­ferometer, which may lead to faster acqui­sition times and poten­tially a dose reduction. Future work for Kagias and his colleagues involves moving to large area pixel detectors, and improving the reso­lution and sensi­tivity of their setup. (Source: AIP)

Reference: M. Kagias et al.: Single shot x-ray phase contrast imaging using a direct conversion microstrip detector with single photon sensitivity, Appl. Phys. Lett. 108, 234102 (2016); DOI: 10.1063/1.4948584

Link: Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland

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