Section 6.1.4.2. Focusing collimators for microfocus sources

In most arrangements that include conventional X-ray tubes, the planar angle of collection is very small. A more efficient use is always made of the radiation from the target by a focusing collimator, which forms an image of the source on the sample (Fig. 6.1.4.6). The angle of collection should be as large as possible, while the cross fire, i.e. the angle of convergence, is kept small, say, at about 10⁻³ rad. It is possible to design focusing collimators based on gold-surfaced toroids of revolution (Elliott, 1965), which afford a planar angle of collection of about three times the critical angle for total external reflection, that is, about 30 × 10⁻³ rad.

Figure 6.1.4.6 | top | pdf | Ellipsoidal mirror for use with a microfocus X-ray tube, where x 1 is ∼15 mm. The major axis, 2a, may be up to 600 mm, whereas the exit aperture, 2y2, lies in the region 0.8–1.4 mm. The angle γ determines the cross fire on the sample and is less than 1 rad.

Consequently, the mirror should magnify about 30 times, and if the image diameter, determined by a typical sample size, is to be 300 µm, the size of the focus should be about 10 µm. The solid angle of collection of such an imaging toroid is about 8 × 10⁻⁴ steradians, that is, more than 1000 times greater than the solid angle of a simple non-imaging collimator. The averaged mirror reflectivity achieved at present is about 0.3, so the microfocus tube and toroidal mirror combination produces a similar intensity at the sample as the conventional tube with a non-focusing collimator at about 300 times the power. Future increases of the reflectivity are likely as the surface roughness of the mirrors is improved.

A suitable microfocus tube has been described by Arndt, Long & Duncumb (1998); mirrors used with this tube were discussed by Arndt, Duncumb et al. (1998). The tube design allows the distance between the source and the mirror to be as little as 10 mm in order to achieve the necessary magnification without making the distance between the tube and the sample inconveniently long.