5.1 Debye-Scherrer

The original Debye-Scherrer camera placed a sample into a capillary tube and a ring of x-ray sensitive photographic film was placed around the sample. (The film needed to be loaded in a darkroom, as light would fog the film.) A measurement was made over hours or days and then the film was taken to a darkroom and developed. To convert to a pattern showing intensity vs. angle, a photo-densitometer was used to measure the darkness of the film. The collimation of the x-ray source and the size of the sample dictated the width of peaks on the film, which in general was not very good. A later development (1937) was the Guinier camera, which used a focusing monochromator to improve the resolution significantly. To this day, Debye-Scherrer geometry is used to describe this setup, where a capillary sample is placed in an x-ray or neutron beam. Since the beam must transverse some part of the sample to be observed (the amount varies with angle), this is also called transmission mode diffraction. The beam is larger than the width of the sample, so that the sample is fully illuminated. Diffracted radiation can be measured in a number of ways:

• With a discrete detector that is scanned to give a measurement as a function of angle. With synchrotrons, a perfect-crystal monochromator (crystal analyzer) is usually placed in front of the discrete detector, which tightly limits the angular acceptance range for the detector and strongly restricts the observed wavelength range. Recent work replaces the discrete detector with an area detector. The use of a post-sample monochromator provides for very high resolution and very low background levels. Multiple detectors are commonly used, as this type of measurement otherwise is very slow.
• With a linear position-sensitive detector (PSD) that measures many angles at a time, which may be scanned if the total angular range is small; These measurements can be very fast and have good resolution, but do not reject fluorescence and have have poorer collimation than post-sample monochromation leading to higher backgrounds.
• With a two-dimensional detector, which technology limits to almost being always flat, though curved would be preferred. The resolution is dictated by the size of the sample, the number of pixels in the detector, and the distance between the sample and detector. Count rates are faster than PSD’s, but collimation is even more poor. Count times are frequently seconds with a lab source and fractions of a second with a synchrotron.
• Film is pretty much a thing of the past. This is a good thing as it was very difficult to linearize the intensity scale from film.

A significant limitation of the Debye-Scherrer geometry is that if the sample absorbs appreciably (or scatters an appreciable fraction of the incident radiation in modes other than diffraction) then much of the scattering intensity will be lost. That amount depends on angle, so when there is significant intensity loss, this must be incorporated into the data reduction or fitting as an absorption correction. This is discussed further in §13.1.7.

It should be noted that the assumption is that the sample is placed exactly at the middle of the detector rotation circle. Any deviations from this produce a distortion between the measured \(2\theta \) angle and the actual diffraction angle. This must be compensated for in fitting, as will be discussed later in §12.2.

The original Debye-Scherrer cameras used a small motor to rotate the capillary along its long axis. This is still commonly done in Debye-Scherrer geometry measurements as it brings two advantages. First, it increases the effective number of crystallites that contribute to scattering. This is because a crystallite must be properly oriented with the beam and detector for diffraction to occur. By rotating the sample along an axis, one of the degrees of freedom in this alignment problem will always be satisfied and many more crystallites are now able to contribute to the pattern. This reduces the size of a sample that is needed to produce an accurate diffraction pattern significantly. The second improvement has to do with texture. If crystallites are placed into the capillary in a non-random way, the rotation will raise the symmetry of the texture, where the only possible orientational ordering would be along the capillary direction; this is called cylindrical texture.

Related to the Debye-Scherrer camera is a device that spins the sample on two different axes. This is known as a Ganolfi camera. It takes some care to design the spinning so that it performs the closest thing possible to optimal randomization in a non-random process and can produce a powder diffraction pattern from a single-crystal specimen.