Li 
   
Figure 22 Secondary foci from a 5 m concave mirror. 
(a) before and (b) after repolishing 
(Ehrenberg's X-ray method). 
than the critical angle method, providing sensitivities 
to height irregularities in the nanometer region, but 
the relative positions of the secondary concentrations 
are not easily related to the area of the reflector 
responsible. 
References 
12 Lindsey K and Penfold A B, "Production and 
assessment of supersmooth optical surfaces" 
Optical Engineering, 1976, 15, (3), pp 220-225. 
13 Ehrenberg W, "X-ray optics : imperfections of 
optical flats and their effect on the reflection 
of X-rays". Journal Optical Society of America, 
1949, 39, (9), pp 746-751. 
9 X-RAY STEREO MICRORADIOGRAPHY 
D J Pugh and P D West (14) 
An electron microscope can be used to provide a very 
small focused spot on a metal foil target, and, under 
suitable conditions, to yield a point source of X-rays. 
A scanning electron microscope technique is helpful in 
adjusting the electron beam focus, and can then be 
used to shift the X-ray source across the metal foil, 
Figure 23, to produce projections on the photographic 
plate from which stereoscopic measurements may be made. 
| | Electron beam 
/ - Metal foil target 
A est = 
~~ Thin specimen 
X-ray source 
  
Photographic plate 
P grap p 
7 
ee zz 
LI 2 
Figure 23 Outline of X-ray projection microscope 
95 
Distributions of voids, inclusions, and differences in 
elements throughout the thickness of the specimen are 
shown up well at all levels because of the small 
source size and short wavelength of the radiation, 
and there are several procedures which yield pairs of 
stereoradiographse 
(a) Tilting the specimen (as used also by D W Butler) 
(%) Translating the specimen 
and 
(c) Translation of the X-ray source by deflection of 
the electron beam. 
X-ray source positions 
  
Features in specimen 
  
  
i 
I AS. 
Specimen 
Position of images 
A. J- 
3 
  
  
Figure 24 Production of stereographic images by 
movement of the X-ray source. 
Method (c) is illustrated in Figure 24 and has been 
Successfully used with the Stereoscan and Microscan 
instruments. 
Reference 14 Pugh D J and West P D, "X-ray 
microradiography in the scanning electron microscope 
or microanalyser. Journal of Microscopy, 1975, 103, 
Pt 2, pp 227-238. 
10 MICROTOPOGRAPHIC MEASUREMENT WITH THE STEREO 
ELECTRON MICROSCOPE 
D W Butler (15) 
(National Maritime Institute) 
Diffraction gratings for use with X-rays are formed by 
etching grooves (300/mm) into highly polished silica 
surfaces. The grooves are, typically, 5 to 50 nm in 
depth and are intended to have flat bottoms, forming 
with the un—etched areas a series of shallow square 
waves in cross section. To improve reflectivity a 
layer of gold 10-100 nm thick is deposited under 
vacuum. To correlate X-ray performance with the 
physical dimensions it is necessary to have an 
accurate Inowledge of the grating profile, which 
governs the diffraction, and of the finish or micro- 
roughness of the surfaces, which affects the 
Scattering of X-rays. Height variations of only 
1-2 nm over surface features only 20 nm across may be 
significant in the latter case. 
The method used employs stereo transmission electron 
microscopy, and it is necessary to resort to the use