(a)(i) With the aid of a labelled diagram describe the mode of operation of a modern X-ray tube. (ii)State the energy transformations that take place during...

Answer Details

(a)(i) Mode of operation of a modern (Coolidge) X-ray tube

The tube is a highly evacuated glass envelope carrying a tungsten filament (cathode) at one end and a target-anode at the other, as shown in the labelled diagram below.

A low-voltage supply heats the tungsten filament, which by thermionic emission releases electrons. A concave focusing cup gathers these electrons into a narrow beam and directs them toward the target. A very high potential difference (the E.H.T., tens of kilovolts) is applied between the cathode and the anode, with the anode positive; this accelerates the electrons to a very high speed across the evacuated tube.

The fast electron beam strikes a small tungsten target embedded in a massive copper anode. On being suddenly stopped, the electrons give up their kinetic energy: only about 1 % is emitted as X-rays, which leave the tube through the window, while the greater part appears as heat. The copper anode and its cooling fins (aided in practice by circulating oil or water) conduct this heat away and prevent the target from melting.

The hardness (penetrating power) of the X-rays is controlled by the accelerating voltage across the tube, while the intensity (quantity of X-rays) is controlled by the filament heating current, which fixes the number of electrons produced.

(a)(ii) Energy transformations

\[\text{Electrical energy}\;\rightarrow\;\text{kinetic energy of the accelerated electrons}\;\rightarrow\;\text{X-ray (electromagnetic) energy}\;+\;\text{heat (thermal) energy at the target.}\]

(b) Definitions (as applied to X-rays)

(i) Hardness: the penetrating power of the X-rays. Hard X-rays have short wavelength and high frequency and penetrate deeply; the hardness increases with the accelerating potential difference across the tube.

(ii) Intensity: the energy radiated per unit area per unit time, i.e. the number of X-ray photons arriving per second on unit area. It depends on the number of electrons striking the target, and hence on the filament heating current.

(c)(i) Four uses of X-rays

1. Medical diagnosis (radiography) to reveal fractured bones and the condition of internal organs.
2. Radiotherapy, to destroy cancerous cells and tumours.
3. Detection of hidden flaws, cracks and air bubbles in metals and welded joints (industrial radiography).
4. Security screening of luggage at airports, and the study of crystal structure by X-ray crystallography.

(c)(ii) One hazard of over-exposure

Over-exposure to X-rays destroys living body cells, causing skin burns and cancer (for example leukaemia), as well as tissue damage, cataracts, sterility and genetic mutation.