'A' Level Medical Option Questions - X-Rays


(a) An X-ray tube operates with a pd across the tube of 80 kV. The figure above shows the X-ray spectrum emitted. Explain why the spectrum has spikes at specific photon energies.

The spikes are specific to the anode target element. The energy level transitions of electrons within the atoms are fixed. That is why they are called characteristic spikes.

(2 marks)

(b) The pd across the tube is increased to 90 kV. Sketch on the figure above the X-ray spectrum produced at this new pd.

See diagram:

    • entire Bremsstrahlung curve has more intensity
    • stops at 90 kV
    • Characteristic spikes in same position

(3 marks)

(c) At the working pd of 80 kV, the anode current was 120 mA. The X-ray tube has an efficiency of 0.70 %. Calculate the rate of production of heat at the anode.

0.70% of the energy is converted to X-Rays - so, 99.3% is heat.

P = IV

= 120 x 10-3 x 80 x 103

= 9600 W

Rate of heat production

9600 x 0.993 = 9.5 kW

(3 marks)

(Total 8 marks)