A number of manufacturers offer LPSD systems where the detector can be scanned in [theta]-2[theta] mode with its centre maintained at the focus condition.
In LPSD systems, the receiving slit aberration is no longer relevant and the flat specimen aberration is replaced by an aberration function that embodies three effects which are folded together in the final function;
A full treatment of these aberrations and how they are modified by a scanning LPSD system is given in Cheary and Coelho .
4.7.1 LPSD Flat Specimen Error [J.sub.PSD]([epsilon]) Including Defocussing
In a conventional diffractometer defocussing and flat specimen error can be convoluted together independently, but this is no longer valid in a LPSD system.
22 for profile arising from a central ray entering the LPSD at 0.5[degrees] off-centre when set at a low 2[theta] (ie., [beta] = 0.5[degrees], 2[theta] = 20[degrees]).
When a diffracted x-ray photon enters a LPSD its path in the detector gas is not perpendicular to the anode wire, except at the centre of the detector, and additional broadening, known as parallax broadening, is introduced into off-centre diffraction lines.
The shape of the aberration profiles for parallax broadening [J.sub.[mu] PSD]([epsilon]) at an angle [beta] to the centre of the LPSD is given by,
The ultimate resolution of a LPSD is the spatial uncertainty of the position measurement of an individual x-ray photon incident normal to the anode wire of the detector.
Parallax broadening can be quite large for long detectors and is usually the dominant aberration when LPSDs are used for peaks above 2[theta] [approximately equal to] 40[degrees].