Microstructured optical arrays (MOAs) are instruments for focusing

x-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

s. MOAs use total external reflection at grazing incidence from an array of small channels to bring x-rays to a common focus. This method of focusing means that MOAs exhibit low absorption. MOAs are used in applications that require x-ray focal spots in the order of few micrometers or below, such as

radiobiology Radiobiology (also known as radiation biology, and uncommonly as actinobiology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation ...

of individual cells. Current MOA-based focusing optics designs have two consecutive array components in order to reduce comatic aberration.

Properties

MOAs are achromatic (which means the focal properties do not change for radiation of different wavelengths) as they utilize grazing incidence reflection. This means that they are able to focus chromatic radiation to a common point unlike zone plates. MOAs are also adjustable as the optic can be compressed to alter the focal properties such as focal length. Focal length can be calculated for the system in fig. 1 using the geometry shown in fig. 2 where it can be seen that changing the gap between the components (''d''+''D'' in the figure) or the radius of curvature (''R'') will have a large effect on the focal length. MOAs have been used in configurations shown in figs. 1 & 3 whereby one or both components can be adjusted. This has varying effects on the focal properties, in general it has been found that smaller focal spot sizes are apparent when MOAs are used as shown in fig. 1 with only the second component adjusted. The focal length of this system can be calculated using the geometry shown below:

Manufacturing

Current microstructured optical arrays are composed of

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...

and created via the Bosch process, an example of Deep reactive ion etching and not to be confused with the

Haber–Bosch process The Haber process, also called the Haber–Bosch process, is an artificial nitrogen fixation process and is the main industrial procedure for the production of ammonia today. It is named after its inventors, the German chemists Fritz Haber and ...

. In the Bosch process the channels are etched into the

using a plasma (

plasma (physics) Plasma ()πλάσμα
, Henry George Liddell, R ...

) in increments of a few micrometres. In between each etching the silicon is coated with a polymer in order to preserve the integrity of the channel walls.

Applications

The focal spot size is important in x-ray microprobe instrumentation where x-rays are focused onto a biological sample to investigate phenomena such as the bystander effect. To target a specific cell the focal spot size of the system must be around 10 micrometers, whereas to target specific areas of a cell such as the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

or the cell nucleus it should be no more than a few micrometers. Currently, only MOAs in the configuration shown in fig. 1 are thought to be able to achieve this. MOAs provide a good alternative to zone plates in microprobe use due to the adjustable focal properties (making cell alignment easier) and ability to provide focusing of chromatic radiation to a single point. This is particularly useful when considering the finding that different effects can be observed using radiation of different wavelengths.

References

5. {{cite web , author = Arndt Last , url = http://www.x-ray-optics.com/index.php?option=com_content&view=article&id=111&Itemid=127&lang=en , title = Microstructured optical arrays , access-date = 22 Jan 2010 , url-status = dead , archiveurl = https://web.archive.org/web/20091124075111/http://www.x-ray-optics.com/index.php?option=com_content&view=article&id=111&Itemid=127&lang=en , archivedate = 2009-11-24 X-ray instrumentation Optical devices