XRPD - Transmission vs. Reflective geometry

[Richard Osmond]

Hey guys,
i have read from various sources that the reflective geometry in X-ray powder diffraction (XRPD), provides higher resolution than the transmission geometry. My experiment uses a parallel beam produced by a polycapillary collimating optic, with very low divergence (3.5 mrad at Cu Ka), so i am ignoring divergence for simplicity. There are no receiving slit positioned between the sample and detector.

In the transmission geometry (incoming beam is orthogonal to sample), if the irradiating beam size is 50 microns, then 50 microns of the sample is irradiated and the diffracted beam size will be ~ 50 microns. In the reflective geometry (lets assume a 3 degree incident angle), if the irradiating beam is 50 microns, then ~ 1 mm of the sample is irradiated and the diffracted beams are also ~ 1mm. The amount of sample irradiated in the reflective geometry can be calculated as 'W/Sin(theta)', where W is the beam size and theta is the incident angle. Therefore, based on my calculations, the reflective geometry does not provide better resolution than transmission geometry. Have i got this completely wrong?

Also, does anyone know any papers comparing these two geometries?

Thanks, any help is appreciated.

[Marc]

You do not use a parallel beam in reflective mode really, typical is a para focussing setup which means that the incident beam diverges and the diffracted beam is ultimately focused on the detector. For transmission, I believe a focused incident beam is ideal.

[Richard Osmond]

Thanks for the reply Mark. I think your right. We are using a parallel beam in reflective mode, but it does not provide higher resolution than using a parallel beam in transmission mode. The literature i have read is referring to the parafocusing setup. In transmission mode, the self-focusing of the diffracted beam is not as precise as in Bragg-Brentano, therefore the resolution is lower in transmission mode. The resolution is much higher in transmission mode when using parallel beams, but more signal is achieved in reflective mode since the irradiated sample and detector are on the same side.

Thanks, think i've got it now (hopefully) !!