Xrays are about 1x10-8 cm, and of wavelength intermediate to Ultravoliet and Gamma radiation.
X-rays are produced when electrons "boiled" from a filament are caused to strike a target of atoms by the force of a high voltage field. Decelleration of electrons as they approach atoms in the target creates a "white" background of radiation called the Brehmstrallen radiation. Superimposed on this background are peaks of intense x-rays that have wavelengths that depend on the target-atom involved. The peaks of characteristic wavelength are produced when an atom looses an electron from an inner shell, due to a collision with an accellerated electron, and then compensates by having an electron from an outer shell fill the partly vacant inner shell. The peaks are labeled Ka, Kb, La, etc., depending on the energy involved. This is the principal behind an electron microprobe, which uses a beam of electrons to cause emission of characteristic x-rays from a material, and thus identify the elements in the material.
Two of the most fundamental aspects of a mineral, it's space group and unit cell dimension, can be determined from X-ray diffraction experiments. To understand these experiments, which we do in lab, we must explore the physics of diffraction. Diffraction, generally defined as a departure of a ray from the path expected from reflection and refraction, was first observed for light in the early 19th century. Sets of narrow slits and ruled gratings were observed to produce diffraction patterns when the spacing of the slits is similar to the wavelength of light used. Because all of the slits in a diffraction grating are illuminated by the same source of light, the set of slits may be considered to be a set of light sources all in phase with one another. Light rays travelling perpendicular to the diffraction grating will remain in phase. Light rays at an angle f to the perpendicular will not be in phase, except for special angles such that S sin f = n l, where S is the spacing of the slits, l is the wavelength of light, and n is an integer. We may use this expression to determine l for a laser or S for a diffraction grating from a measurement of the spacing of the diffraction pattern.
Diffraction of x-rays by crystals is possible because the spacing of planes of atoms in crystals is similar to the wavelengths of x-rays. The atoms in crystals behave like little x-ray sources as they scatter incident x-rays. Although an X-ray experiment can be designed to be very similar to an optical diffraction experiment (a Laue experiment), most experiments involve x-ray "reflections". For the diffracted X-rays to be in phase, the geometry of the experiment must satisfy Bragg's law:
n*lambda = 2 d(hkl) sin(theta)
d(hkl) is the spacing between the parallel planes of atoms with Miller index (hkl), theta is the complement of the incident angle, lambda is the wavelength of the x-rays and n is an integer. Note that each set of planes (hkl) may produce more than one diffracted ray, each with different values of n.
When the beam of monochromatic light strikes the powder mount, all possible diffractions take place simultaneously. If the orientation of particles in the mount is truely random, for each family of atomic planes with its characteristic interplanar spacing (d), there are many particles whose orientation is such that they make the proper theta angle with the incident beam to satisfy the Bragg Law. Different families of planes with different interplanar spacings will satisfy the Bragg Law at appropriate values of theta for different integral values of n, thus giving rise to separate sets of "reflected" rays.
To view diffraction from more than one set of planes (hkl), it is necessary to change both the angle theta and the orientation of the crystal. The buerger precession camera is designed to do both, so that diffracted x-rays from the set of all planes that belong to a single zone are recorded on the film. The precession camera also moves the film that records the diffraction in such a way that preserves the symmetry of the diffracting pattern.
The chart on which the record is drawn is divided into tenths of inches and moves at a constant speed, such as for example, 0.5 inch per minute. At this chart speed and a scanning speed of the detector of 1° per minute, 0.5 inch on the chart is equivalent to a 2-theta of 1° degree. The position of peaks on the chart can be read directly, and the interplanar spacings giving rise to the peaks can be determined by use of the Bragg equation.