Jawaharlal Nehru Technological University Hyderabad B. Tech IV Year X-RAY METALLOGRAPHY Previous Question Paper (July/August - 2022 - R16)

A stereographic projection is a geometric method used to represent three-dimensional angular relationships between crystallographic planes and directions on a two-dimensional plane. It projects the poles of crystal planes onto a reference sphere and then onto a flat equatorial plane from a point on the sphere's surface (usually the North Pole).

The process involves:

1. Placing the crystal at the center of a reference sphere.
2. Drawing normals from the center to the crystal planes; where these normals intersect the sphere surface, they form 'poles'.
3. Projecting these poles onto the equatorial plane by drawing lines from the chosen projection point (e.g., the North Pole) through the poles on the sphere.

For example, in a cubic crystal, the (100), (010), and (001) poles are projected onto the reference circle. The (100) and (010) poles lie on the equator, while the (001) pole projects to the center of the circle, allowing for the visual determination of angles between planes using a Wulff net.

X-rays are produced in an evacuated tube containing a cathode (filament) and an anode (target). When a high voltage is applied, electrons are emitted from the heated filament (thermionic emission) and accelerated toward the metal target.

The production involves two primary mechanisms:

1. Bremsstrahlung (Continuous Spectrum): High-speed electrons are decelerated upon interacting with the strong electric field of the target nuclei. The loss in kinetic energy is emitted as X-ray photons with a continuous range of wavelengths.
2. Characteristic X-rays (Line Spectrum): Incident electrons knock out inner-shell electrons (e.g., K-shell) of target atoms. Electrons from higher energy levels (e.g., L or M shells) drop down to fill these vacancies, emitting X-rays with specific, discrete wavelengths corresponding to the energy difference between shells.

The intensity and quality of the X-ray beam are controlled by the tube current (filament temperature) and the accelerating voltage, respectively.