Types of X-ray Microscopes

Projection X-ray Microscopes

In a projection-based X-ray microscope, magnification is achieved by positioning the sample close to a point-source of X-rays. A magnified projection image of the object is formed on the detector with a magnification that is equal to the ratio of the source-detector and source-object distances (Ref: Figure 4). By rotating the sample, a series of projection images at different angles is acquired from which the internal 3-dimensional structure can be determined using tomographic reconstruction algorithms.

Figure 4: Operating principle of a projection based X-ray Microscope. A small spot source projects an image of the object on a detector screen with an optical magnification given by the ratio (a+b)/a.

The resolution (the ability to separate or "resolve" two objects that are very close together) of projection-based X-ray microscopy is limited by the size of the X-ray source and the resolution of the X-ray detector system (Ref: Figure 5).

Figure 5: Projection based X-ray microscope resolution as a function of X-ray spot size S and detector resolution r_D.

Lens-Based X-ray Microscopes:

By their very nature, projection-based X-ray microscope systems have important limitations. The most important one is that spatial resolution is limited by the point size of the X-ray source and/or the resolution of the detector to approximately 1 micrometer.

One can attain much higher resolution by building a microscope with an overall structure very similar to the standard light microscope: The X-rays emitted by the source are concentrated using a condenser lens onto the sample to be imaged, and the transmitted X-rays are imaged by an objective lens onto an area detector.

Xradia's unique nanoXCT™ 3D X-ray microscope utilizes high resolution zone plate (ZP) lenses to achieve a resolution below 50 nm (Ref: Figure 6) If phase contrast imaging is desired, a gold (Au) phase ring is inserted in the back focal plane of the objective.

Figure 6: Key components of the Xradia nanoXCT™ high resolution 3D X-ray microscope.

The use of a capillary condenser lens to condense X-rays from the X-ray source onto the sample provides high throughput, and short exposure time.

The Xradia microscope installed at SSRL. Courtesy of SLAC National Accelerator Laboratory

Scanning X-ray Microscopes:

In a scanning X-ray microscope, a zone plate lens is used to focus the X-ray beam to a fine spot (also called a "probe"), through which the specimen is raster-scanned. At each scan position, transmitted, fluorescent or diffracted X-rays can be detected, mapping the chemical, elemental or crystallographic phase properties of the specimen. Elemental composition is mapped by analyzing the fluorescent X-rays. Chemical state can be determined by NEXAFS spectroscopy. The local crystallographic phase and strain in polycrystalline materials can be determined by detecting the diffracted X-rays.

Due to the requirements for high intensity X-ray beams, the use of scanning X-ray microscopes is limited to synchrotron radiation laboratories.