JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year X-RAY METALLOGRAPHY Previous Question Paper (September - 2021 - R16)

Consider a set of parallel atomic planes in a crystal with interplanar spacing d. When a monochromatic X-ray beam of wavelength λ strikes these planes at an incident angle θ, the rays are scattered by the atoms. Constructive interference occurs only when the path difference between rays reflected from successive planes is an integer multiple of the wavelength.

For constructive interference, the path difference must equal nλ, where n is the order of reflection. Thus, we obtain Bragg's Law:

Applications of Bragg's Law include: 1) Determination of crystal structures by calculating interplanar spacings from diffraction patterns. 2) Identification of unknown crystalline phases in materials. 3) Measurement of lattice parameters and strain in metals. 4) Analysis of preferred orientation or texture in polycrystalline materials.

X-rays are produced in an X-ray tube, which is a vacuum-sealed glass envelope containing a cathode (filament) and an anode (target). A high-voltage power supply is applied between the electrodes. The filament is heated by a low-voltage current, causing thermionic emission of electrons. These electrons are accelerated toward the target by the high potential difference. Upon striking the target material (typically tungsten or molybdenum), the electrons lose kinetic energy through two primary processes: 1) Bremsstrahlung (braking radiation), where electrons decelerate in the electric field of target nuclei, emitting a continuous spectrum of X-rays. 2) Characteristic radiation, where incident electrons eject inner-shell electrons from target atoms, and outer-shell electrons drop down to fill the vacancies, emitting discrete energy photons corresponding to the specific atomic energy levels.