A point focusing x-ray monochromator was designed and constructed for low angle scattering studies. The anastigmatic point focus is achieved by means of two cylindrically bent quartz crystals whose focal circles are mutually perpendicular. The beam, emanating from the copper target of an x-ray tube, is reflected in succession, first from the crystal defining the horizontal focal circle and, second from the crystal definingthe vertical focal circle following which it comes to a monochromatic point focus of wavelength 1.537 Å (Cu Kα1). The sample to be studied is placed between the second crystal andthe point focus, and the scattered beam is detected by means of a photographic plate placed at the point focus, at right angles to the undeviated beam, the latter being suppressed by means of an absorber.
Mathematical analysis, in which a ray was traced throughthe two crystal system, revealed correctly the shape and sizeof the point focus, and the possibility of reducing the latterin size by stopping down the beam emerging from the target.
The instrument and its lining up procedure is describedin detail, particularly the latter since the orientation ofthe two crystals relative to each other and the x-ray tubeinvolve twelve degrees of freedom.
Suggestions for improving the intensity of the instrumentare made. These consist of (1) substitution of topaz crystalsfor quartz, (2) use of helium atmosphere instead of air tosurround the x-ray beam (3) reorientation of the x-ray tubeto permit smaller angles of emergence of the beam.
As a trial run for the instrument, the scatteringpatterns of two kinds of carbon black (good scatterers) wereobtained from which the average particle sizes and sizedistributions were calculated.
