(a) basic structure of an X-ray tube; components – heater

The idea

An X-ray tube accelerates electrons and stops them at a metal target, producing X-rays. The heater, usually a filament, is heated by a low-voltage supply so that thermionic emission releases electrons from its surface. Those electrons are then accelerated towards the anode by a high voltage. The heater therefore controls the number of electrons available and so affects the tube current and the intensity of the X-ray beam. The heater does not accelerate the electrons; that is the role of the high voltage between cathode and anode.

What earns the marks

• The heater is a filament that is heated by a low-voltage supply.
• Heating the filament causes thermionic emission of electrons.
• The heater controls the number of electrons released and hence the tube current.
• The heater does not accelerate the electrons; the high voltage between cathode and anode does that.
• The heater is part of the cathode assembly in the basic X-ray tube.

Common mistakes and how to put them right

• Thinking the heater accelerates the electrons.

  Put it right: It releases them by thermionic emission, while the high voltage accelerates them.

• Confusing the heater with the target.

  Put it right: The target is the anode metal where X-rays are produced.

• Believing the heater produces X-rays directly.

  Put it right: X-rays are produced when accelerated electrons interact with the target.

• Assuming the heater uses a high voltage.

  Put it right: It is operated by a low-voltage supply.

Exam tips

• Link the heater to thermionic emission and electron release.
• Separate the roles of heater, cathode, anode and high-voltage supply in your answer.
• Use the phrase low-voltage supply for the heater and high voltage for acceleration.
• Check whether the question asks about electron number or electron energy before choosing.