JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year X-RAY METALLOGRAPHY Previous Question Paper (February/March - 2022 - R16)

Characteristic X-rays are produced when an incident electron with sufficient energy ejects an inner-shell electron (e.g., from the K-shell) of a target atom. This creates a vacancy that is immediately filled by an electron from a higher energy level (e.g., L or M shell). The energy difference between these shells is emitted as an X-ray photon with a discrete, wavelength-specific energy.

The most prominent lines are the K-alpha and K-beta series:

1. K-alpha lines: Result from an L-shell to K-shell transition. These are further split into K-alpha1 and K-alpha2 due to spin-orbit coupling, appearing as a strong doublet.
2. K-beta lines: Result from an M-shell to K-shell transition. These have higher energy and shorter wavelengths than K-alpha lines but lower intensity.

These lines are unique to the target material (e.g., Copper or Molybdenum) and provide the monochromatic radiation necessary for precise diffraction experiments, as they satisfy the Bragg's Law condition for specific lattice planes.

Filters are thin foils of material placed in the path of the X-ray beam to improve the spectral purity of the radiation by selectively absorbing unwanted wavelengths. In X-ray diffraction, the goal is to isolate the K-alpha line while suppressing the K-beta line and the continuous background radiation (Bremsstrahlung).

The selection of a filter material is based on its absorption edge:

1. The filter material is chosen such that its K-absorption edge lies between the K-beta and K-alpha wavelengths of the target anode.
2. Because the absorption coefficient of the filter material increases sharply just above its K-absorption edge, it strongly absorbs the K-beta radiation while allowing the K-alpha radiation to pass through with relatively low attenuation.
3. For example, a Nickel (Ni) filter is commonly used with a Copper (Cu) target. The K-beta line of Copper is absorbed by the Nickel foil, effectively producing a nearly monochromatic beam of K-alpha radiation.

Using filters enhances the signal-to-noise ratio in diffraction patterns, prevents the overlap of diffraction peaks caused by multiple wavelengths, and simplifies the interpretation of the resulting crystal structure data.