at the Advanced Photon Source

New Soller slit assembly commissioned at 16-ID-B

Optimizing the signal-to-background ratio is an enormous challenge in the context of static high pressure research.  Weak scattering—from samples with typical dimensions on the order of a few tens of microns or less—can be overcome by a strong background arising from the sample chamber’s surrounding material, often several millimeters thick.  One strategy to suppress the background is to incorporate the use of a multi-channel collimator (also commonly referred to as Soller slits, given their similarity to slit systems used to reduce axial divergence in laboratory diffractometers).  The multi-channel collimator (MCC) consists of two precisely aligned vertical slit assemblies each containing 101 tungsten carbide blades.  The MCC blocks all x-ray scattering from reaching the detector except for the scattering arising from a discrete sample volume defined by the spacing between blades and the distance between the two slit assemblies.  In practice the MCC sampling volume is aligned coincident to the high pressure sample position and rotated about the sample during x-ray exposure.  The scattering from the sample is able to reach the detector whereas the majority of the scattering from the surrounding material and air is blocked by the MCC, significantly increasing the signal-to-background ratio.  HPCAT’s multi-channel collimator is a custom, in-house design with a substantial increase in throughput and angular range (60 degrees in the horizontal plane, 30 degrees in the vertical plane) as compared to other existing assemblies.

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