Compound refractive lens
Encyclopedia
A Compound refractive lens (CRL) is a series of individual lenses arranged in a linear array in order to achieve focusing of X-rays in the energy range of 5-40 keV.
For all materials the refractive index
for X-rays is close to 1, hence a single conventional lens for X-rays has an extremely long focal length (for practical lens sizes, on the order of 1 mm). In addition, X-rays attenuate as they pass through a material so that conventional lenses for X-rays have long been considered impractical. The CRL gets its reasonably short focal length, on the order of meters, by using many lenses in series. To minimize the absorption the lens must be made from a material such as aluminium
, beryllium
, or lithium
.
CRLs were first demonstrated in the mid-1990s by a group of scientists at the ESRF. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders. In contrast, for visible light the index of refraction is larger than unity and focusing is done with a convex lens.
Scientists associated with the ESRF synchrotron have done much of the CRL's subsequent development, notably the parabolic CRLs pioneered by the Aachen group under Lengeler. Their signature material is beryllium: a group at the Advanced Photon Source APS
demonstrated the same lenses in lithium. These lenses have a direct counterpart in visible light.
The saw-tooth lens is a unique optical scheme suggested and demonstrated by Cederstrom. It approximates a parabolic lens much as a numerical computation on a grid approximates a smooth line, with a series of prisms that each deflect the X-rays over a minute angle. Lenses of this type have been made from silicon, plastic, and lithium.
For all materials the refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
for X-rays is close to 1, hence a single conventional lens for X-rays has an extremely long focal length (for practical lens sizes, on the order of 1 mm). In addition, X-rays attenuate as they pass through a material so that conventional lenses for X-rays have long been considered impractical. The CRL gets its reasonably short focal length, on the order of meters, by using many lenses in series. To minimize the absorption the lens must be made from a material such as aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, or lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
.
CRLs were first demonstrated in the mid-1990s by a group of scientists at the ESRF. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders. In contrast, for visible light the index of refraction is larger than unity and focusing is done with a convex lens.
Scientists associated with the ESRF synchrotron have done much of the CRL's subsequent development, notably the parabolic CRLs pioneered by the Aachen group under Lengeler. Their signature material is beryllium: a group at the Advanced Photon Source APS
APS
-Education:* Army Public School, group of Army schools in India under AWES* APS Netherlands, or APS International, a Dutch educational non-governmental organization* Abbottabad Public School and College...
demonstrated the same lenses in lithium. These lenses have a direct counterpart in visible light.
The saw-tooth lens is a unique optical scheme suggested and demonstrated by Cederstrom. It approximates a parabolic lens much as a numerical computation on a grid approximates a smooth line, with a series of prisms that each deflect the X-rays over a minute angle. Lenses of this type have been made from silicon, plastic, and lithium.