(a)(i) By means of a labelled diagram, describe the mode uf operation of a modern X-ray tube. (ii) State the energy transformation which takes place during ...

(a)(i) Modern X-ray (Coolidge) tube

The tube is highly evacuated. A low-voltage supply heats the tungsten filament at the cathode. The hot filament emits electrons by thermionic emission. A very high potential difference between the cathode and the anode accelerates and focuses the electrons towards the tungsten target.

When the high-speed electrons strike the tungsten target, they are suddenly decelerated. A small fraction of their kinetic energy is emitted as X-rays, which leave through the window. Most of the energy becomes heat; hence the tungsten target is mounted in copper and the anode is cooled.

(a)(ii) Energy transformation

Electrical energy is converted to kinetic energy of the electrons, and then mainly to heat energy and partly to X-ray electromagnetic radiation:

\[\text{electrical energy} \rightarrow \text{kinetic energy of electrons} \rightarrow \text{heat energy} + \text{X-ray energy}.\]

(b) Hardness and intensity of X-rays

• Hardness is the penetrating power, or quality, of an X-ray beam. Hard X-rays have high frequency, short wavelength and high photon energy, and are therefore more penetrating. Their hardness is increased by increasing the accelerating potential difference across the tube. X-rays produced at lower voltage are softer and less penetrating.
• Intensity is the quantity of X-rays emitted per second, that is, the number of X-ray photons in the beam. It is increased by increasing the filament heating current, since more electrons are then emitted and strike the target.

(c)(i) Uses of X-rays

1. Radiography of the body, for example to locate fractured bones.
2. Detecting internal cracks and defects in welds, metal castings and other engineering materials.
3. Studying crystal structure by X-ray diffraction.

(c)(ii) Hazard and precautions

Hazard: Over-exposure damages living cells and tissues and may cause radiation burns, mutations or cancer.

Safety precautions:

1. Use lead shielding, such as lead-lined screens or a lead apron, between the operator and the X-ray source.
2. Keep exposure time as short as possible and operate the equipment from a safe distance, outside the direct beam.