Bragg’s Spectrometer
Much of our knowledge about crystal structure and the structure of molecules as complex as DNA in crystalline form comes from the use of x-rays in x-ray diffraction studies. A basic instrument for such study is the Bragg spectrometer.
To obtain nearly monochromatic x-rays, an x-ray tube is used to produce characteristic x-rays. In order to eliminate as much of the bremsstrahlung continuum radiation as possible, matched filters are used in the x-ray beam to optimize the fraction of the energy which is in the K-alpha line. Such filters use elements just above and just below the metal in the x-ray target, making use of the strong “absorption edges” just above and below the K-alpha energy of the target metal.
The x-rays are collimated with apertures in a strong x-ray absorber (usually lead) and the narrow resulting x-ray beam is allowed to strike the crystal to be studied. The spectrometer arrangement couples the rotation of the crystal with the rotation of the detector so that the angle of rotation of the detector is twice that of the crystal. This satisfies the conditions of Bragg’s law for diffraction of the x-rays from the crystal lattice planes.
