x-ray guides

The simplest but nevertheless a very effective application of capillary optics in material analysis is the use of monocapillaries for increasing the radiation intensity on the sample. Usually a pinhole is used for collimating the beam and obtaining the spot of the necessary size on the sample. A single-pinhole collimator (or a double-pinhole collimator) can be easily substituted for the monocapillary of the same diameter d. In this case monocapillary performs the following main functions:

1. it collimates the beam spatially (exit beam size ~ capillary diameter d);
2. it collimates the beam angularly (exit beam divergence ~ critical angle of the total external reflection q_cr);
3. it can produce significant intensity gain on the sample relative to conventional pinhole collimators.

The intensity gain obtained depends on the radiation energy and on the geometry of the experiment. The value of the gain may be different relative to a single-pinhole collimator or a double-pinhole collimator. Usually intensity gain relative to a double-pinhole collimator is much larger than intensity gain relative to a single-pinhole collimator if the source size is much larger than the pinhole (capillary) diameter d. The intensity gain relative to a single-pinhole collimator can be roughly estimated according to the expression

gain = (2 R q_cr / d)²,

where R is the distance between the source and the collimator (the end of the capillary), d - collimator (capillary) diameter , q_cr - critical angle of total external reflection. One must remember that intensity gain is achieved at the expense of larger beam divergence after the capillary: capillary captures radiation within double critical angle of total external reflection. But in many applications large total intensity on the sample is more important than some increase of the beam divergence. Monocapillaries can be successfully used in those cases when one has to get small irradiated spot on the sample while the distance between the source and the sample is large and cannot be reduced. This is a typical situation in diffractometry where the positions of the source and the sample are fixed by the diameter of the goniometer. In texture investigations one does not need very high angular collimation of the beam and angular divergence ~ 4 mrad for Cu Ka-line (E=8.0 keV) and ~ 2 mrad for Mo Ka-line (E=17.4 keV) obtained with glass capillary is quite acceptable.

Monocapillaries with different diameters were successfully used in diffractometers of all the main producers. They give intensity gain ~ 3-3.5 for Cu-anode fine-focus tube (effective anode spot 0.4 mm × 0.8 mm) when capillary diameters are 0.1 ÷ 1.0 mm.

Obtaining microspot beams with monocapillaries can meet certain difficulties. First of all, the number of reflections inside capillary may become large and radiation losses due to absorption may become significant. In this case a special combination of a monocapillary with a pinhole may be useful. One can use a monocapillary with a large diameter ~ 0.3 ÷ 1.0 mm and place a small pinhole < 0.1 mm at its end. A monocapillary captures radiation near a source and transports it to a large distance without spread while a small pinhole makes the final collimation of the beam. Usually such a combination works more efficient than direct application of monocapillary with a tiny diameter.