Abstract
It is demonstrated that in a zone plate based scanning X-ray microscope, used to image low absorbing, heterogeneous matter at a mesoscopic scale, differential phase contrast (DPC) can be implemented without adding any additional optical component to the normal scheme of the microscope. The DPC mode is simply generated by an appropriate positioning and alignment of microscope apertures. Diffraction from the apertures produces a wave front with a non-uniform intensity. The signal recorded by a pinhole photo diode located in the intensity gradient is highly sensitive to phase changes introduced by the specimen to be recorded. The feasibility of this novel DPC technique was proven with the scanning X-ray microscope at the ID21 beamline of the European Synchrotron Radiation facility (ESRF) operated at 6 keV photon energy. We observe a differential phase contrast, similar to Nomarski's differential interference contrast for the light microscope, which results in a tremendous increase in image contrast of up to 20 % when imaging low absorbing specimen.
Highlights
Since the early development of microscopes for X-rays, it became rapidly obvious that phase sensitive techniques using the imaginary part of the refractive index should be preferred absorption based methods because (i) materials consisting of low Z-elements, especially organic matter have a low absorption, and (ii) it is predicted by several theoretical models that a lower radiation dose is applied to the specimen at similar or higher image contrast [1].Phase sensitive techniques were introduced to X-rays by Bonse and Hart, who invented X-ray interferometry based on a double crystal in Laue geometry [2]
The differential phase contrast (DPC) mode is generated by an appropriate positioning and alignment of microscope apertures
The signal recorded by a pinhole photo diode located in the intensity gradient is highly sensitive to phase changes introduced by the specimen to be recorded. The feasibility of this novel DPC technique was proven with the scanning X-ray microscope at the ID21 beamline of the European Synchrotron Radiation facility (ESRF) operated at 6 keV photon energy
Summary
Since the early development of microscopes for X-rays, it became rapidly obvious that phase sensitive techniques using the imaginary part of the refractive index should be preferred absorption based methods because (i) materials consisting of low Z-elements, especially organic matter have a low absorption, and (ii) it is predicted by several theoretical models that a lower radiation dose is applied to the specimen at similar or higher image contrast [1].Phase sensitive techniques were introduced to X-rays by Bonse and Hart, who invented X-ray interferometry based on a double crystal in Laue geometry [2]. Since the early development of microscopes for X-rays, it became rapidly obvious that phase sensitive techniques using the imaginary part of the refractive index should be preferred absorption based methods because (i) materials consisting of low Z-elements, especially organic matter have a low absorption, and (ii) it is predicted by several theoretical models that a lower radiation dose is applied to the specimen at similar or higher image contrast [1]. Due to the complementarity of full-field imaging and scanning type microscopes, their X-ray source and detection requirements, different techniques have been developed in the past. Rudolph et al proposed and proved a Zernike type phase contrast setup for full-field imaging type or transmission X-ray microscopy (TXM) [3]. Due to the versatility of detection modes in scanning X-ray microscopes (SXM), several phase techniques were developed independently for this type of X-ray microscope. A differential interference contrast technique similar to Nomarski interference contrast based on twin zone plates for X-rays was investigated, with the advantage of being applicable for both, SXM and TXM [8,9]
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