Page 520 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
508
a
Typically
50 to 200 kV –+
vacuum
rotating anode X-rays
b
Figure 32.3 a A general-purpose X-ray system. b A typical X-ray image produced by such a machine, showing the region around the pelvis.
film is used in the detection system. Figure 32.3b shows the resulting image.
Figure 32.4 shows the principles of the modern X-ray tube. The tube itself is evacuated, and contains two electrodes:
■■ Cathode – the heated filament acts as the cathode (negative) from which electrons are emitted.
■■ Anode – the rotating anode (positive) is made of a hard metal such as tungsten. (The anode metal is often referred to as the ‘target metal’.)
An external power supply produces a voltage of up to 200 kV between the two electrodes. This accelerates a beam of electrons across the gap between the cathode and the anode. The kinetic energy of an electron arriving at the anode is 200 keV. When the electrons strike the anode at high speed, they lose some of their kinetic energy in the
Figure 32.4 A simplified diagram of an X-ray tube.
form of X-ray photons, which emerge in all directions. Part of the outer casing, the window, is thinner than the rest and allows X-rays to emerge into the space outside the tube. The width of the X-ray beam can be controlled using metal tubes beyond the window to absorb X-rays. This produces a parallel-sided beam called a collimated beam.
Only a small fraction, about 1%, of the kinetic energy
of the electrons is converted to X-rays. Most of the incident energy is transferred to the anode, which becomes hot. This explains why the anode rotates; the region that is heated turns out of the beam so that it can cool down by radiating heat to its surroundings. Some X-ray tubes have water circulating through the anode to remove this excess heat.
X-ray spectrum
The X-rays that emerge from an X-ray tube have a range of energies, as represented in the X-ray spectra shown in Figure 32.5. Each spectrum has two components, the
electrons
filament (cathode)
0
characteristic X-rays
120kV
60kV 90kV
0 50 100 150 Energy of X-ray photon / keV
Figure 32.5 X-ray spectra for a tungsten target with accelerating voltages of 60 kV, 90 kV and 120 kV. The continuous curve shows the braking radiation while the sharp spikes are the characteristic X-rays.
Relative intensity