X-ray refractive lens
An X-ray lens is a lens that focuses X-rays . In addition to X-ray optics , such as X-ray mirrors based on Bragg reflection , multilayer systems and Fresnel zone plates , refractive X-ray lenses are used for this purpose.
Refractive X-ray lenses were considered technically impossible for a long time, since the refractive index for X-rays in all materials is very close to (and just below) one and X-rays are also relatively strongly absorbed in solids.
"It goes without saying that you cannot concentrate the X-rays with lenses."
It was only shown in 1996 that such a lens can still be built and used efficiently. Materials with a low atomic number such as beryllium , boron , aluminum or even polymer are used for this purpose . Snigirev et al. in 1996 proposed an arrangement of holes in a light material that has found its way into many devices.
Since the refractive index of these materials is <1, such concave cavities have a concentrating effect similar to a convex glass lens for visible light. Since the opening width ( aperture ) of the X-ray lenses is similar in size to their diameter, they do not have to be spherical but parabolic in shape for optical reasons, i.e. H. the cavities are cylindrical paraboloids or paraboloids of revolution . A large number of cavities / lenses are stacked one behind the other. The result is a compound refractive lens with a low focal length and a very small numerical aperture .
Lenses available on the market are suitable for X-rays from 6 to 120 keV. The optics made from it can resolve distances <50 nm. X-ray microscopy for objects in the submicrometer range is a practical area of application . X-ray telescopes , on the other hand, work with mirrors.
Types of x-ray lenses
There are several types of X-ray lenses. Such from rows of holes consist of an aluminum or beryllium cuboid in which two rows of perpendicular holes (to form a lens with a point focus) are drilled. These optics have two major disadvantages. On the one hand, the absorption was relatively high because the webs between two drill holes could not be arbitrarily thin. On the other hand, lenses of this type have strong spherical aberration due to the round drill holes. This can be counteracted by means of parabolic shaped holes (e.g. made by embossing). The manufacture of lenses made from bubbles in capillaries made of epoxy resin is quite simple, but these lenses suffer from spherical aberration. Alligator lenses (also known as "multi-prism lenses") consist of two opposing and approaching rows of microprisms. A point focus can be achieved by placing two alligator lenses one behind the other at right angles to one another. The focal length can be selected by changing the tilt angle of the two rows to one another. The image is free from spherical aberration. Rolled prism x-ray lenses consist of a structured polymer film that is rolled up to form a spiral.
Sources and individual references
- A. Snigirev, V. Kohn, I. Snigireva, B. Lengeler: A compound refractive lens for focusing high-energy X-rays . In: Nature . tape 384 , no. 6604 , 1996, pp. 49-51 , doi : 10.1038 / 384049a0 .
- ↑ Bruno Lengeler, Christian G Schroer, Marion Kuhlmann, Boris Benner, Til Florian Günzler, Olga Kurapova, Federico Zontone, Anatoly Snigirev, Irina Snigireva: Refractive x-ray lenses. In: Journal of Physics D: Applied Physics. 38, 2005, p. A218, doi : 10.1088 / 0022-3727 / 38 / 10A / 042 .
- ↑ x-ray-optics: Various CRLs , accessed on July 28, 2019
- ↑ Spectrum of Science: Lens System for X-rays - Spectrum of Science , accessdate: July 28, 2019