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The design of B18 is based on three main optical elements: a collimating mirror, a water-cooled double crystal monochromator and a focusing mirror.
The two mirrors are in fixed positions and they are both coated on one half with Cr and on another half with Pt. This results in two parallel optical branches with energy-dependent performances. The two branches together cover the energy range from 2 to 35 keV .
The first mirror is a water-cooled, vertically collimating Si mirror, and is placed befor the monochromator to guarantee a parallel beam and good energy resolution. The second mirror is a double toroidal focusing mirror (one toroid per branch), placed after the monochromator and is used to focus the beam horizontally and vertically at sample position in experimental hutch.
The fixed exit double crystal monochromator is water cooled and capable of continuous scanning. It has two sets of crystals, Si(111) and Si(311), that can be translated horizontally under vacuum to align them to the optical branch desired.
Finally, a pair of smaller plane mirrors for harmonics rejection (HR) are placed in the Experimental Hutch. They are inserted in the beam path for low energy (<11 keV) operations.
The B18 collimating mirror is permanently fully exposed to the white beam, amd the selection of optical branch is done by means of a pair of secondary slits placed after the first mirror. This makes it quick to switch between optical branches, and reduces instabilities due to thermal drifts.
Si(111) crystal cut, in B18 can cover the energy range 2.1-22keV, and is the preferred choice for monochromator because of the higher flux. A pair of Si (311) crystals is also available, and can be used for higher energies that cannot be covered by Si111 (22-35keV) or from 4-22keV in case an improved energy resolution is needed but at the expense of flux.
The choice of optical branch is based on the absorption edge of interest. Cr coating provides better reflectivity adn therefore higher flux at lower energies while Pt coating is preferable at higher energies. Harmonic Rejection Mirrors are necessary to reject higher harmonics from the x-ray beam when working at low energies (<11keV). For example, at an energy of 6 keV, the third harmonic (18keV) will also be present in the beam, but it can be removed by selecting the appropriate angle of the harmonic rejection mirrors.
The combination of 2 optical branches, 2 crystal cuts for DCM, and use of HR results in 8 unique configurations listed in the table below, most commonly used configurations are highlighted (top lines) and the energy ranges where each of these configuration is preferable are reported in the last column. Gaps in energies covered because of absorption edges of the coating materials.
|Monochromator||Working Energy Range||HR Mirror
when commonly used
|Cr||Si111||2-5 keV||Yes (6-15 keV)||2-5 keV|
|Pt||Si111||4-11 keV||Yes (12-33 keV)||5-10 keV|
|Cr||Si111||8-22 keV||No (>24 keV)||10-15 keV|
|Pt||Si111||15-22 keV||No (>35 keV)||15-20 keV|
|Pt||Si311||15-35 keV||No (>35 keV)||20-35 keV|
|Cr||Si311||4-5 keV||Yes (12-33 keV)||only high Energy res.|
|Pt||Si311||4-11 keV||Yes (12-33 keV)||only high Energy res.|
|Cr||Si311||8-22 keV||No (>24 keV)||only high Energy res.|
The figure below shows all the possible optical configurations for the Cr branch (green) and the Pt branch (blue). Solid colored regions indicate the most commonly used configurations.
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