Describe the main parts of an X-ray tube.

An evacuated tube contains a heated filament acting as the cathode and a metal target acting as the anode, with a very large potential difference between them and a window through which the X-rays leave.

How are the electrons in an X-ray tube produced and given their energy?

They are boiled off a hot filament by thermionic emission and are then accelerated across the vacuum by a potential difference of tens of kilovolts, arriving at the metal target with a kinetic energy equal to eV.

Why do the sharp lines in an X-ray tube's output depend on which metal is used as the target?

Their photon energies are the differences between the inner energy levels of the target atoms, and those level energies are characteristic of the particular element.

An X-ray tube operates at 50 kV. Calculate the maximum energy of the X-ray photons it can produce.

The greatest energy is that of an electron that gives up all its kinetic energy, E = eV = 1.60 × 10⁻¹⁹ × 5.0 × 10⁴ = 8.0 × 10⁻¹⁵ J, which is 50 keV, and 8.0 × 10⁻¹⁵ ÷ 1.60 × 10⁻¹⁹ returns 5.0 × 10⁴ V.

CCEA A-Level Physics 1210

Physical Quantities, Scalars and Vectors

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Quantum Physics

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The Nucleus and Radioactive Decay

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Quantum Physics

X-ray production, radiography and CT scanning

3 things to know
Unit AS 2

Overview

Main parts of an X-ray tube, contrast medium and why is one used in X-ray imaging and physical principle on which a conventional X-ray image is formed.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain how X-rays are produced by electron movement between energy levels in the target atoms.

Numbers

An X-ray tube operates at 100 kV. Calculate the shortest wavelength in its output.

Key term

Why does an X-ray tube also produce a continuous range of photon energies as well as sharp lines?

Questions from this subtopic

Each one with the model answer underneath.

1
Describe the main parts of an X-ray tube.
Model answer
An evacuated tube contains a heated filament acting as the cathode and a metal target acting as the anode, with a very large potential difference between them and a window through which the X-rays leave.
2
How are the electrons in an X-ray tube produced and given their energy?
Model answer
They are boiled off a hot filament by thermionic emission and are then accelerated across the vacuum by a potential difference of tens of kilovolts, arriving at the metal target with a kinetic energy equal to eV.
3
Explain how X-rays are produced by electron movement between energy levels in the target atoms.
Model answer
A fast incoming electron knocks an electron out of an inner level of a target atom, and an electron from a much higher level then falls into the vacancy, emitting a photon whose energy equals the very large difference between those two levels, which lies in the X-ray region.
4
Why do the sharp lines in an X-ray tube's output depend on which metal is used as the target?
Model answer
Their photon energies are the differences between the inner energy levels of the target atoms, and those level energies are characteristic of the particular element.
5
Why does an X-ray tube also produce a continuous range of photon energies as well as sharp lines?
Model answer
Electrons are also slowed down by the electric fields of the target nuclei, and each electron gives up a different fraction of its kinetic energy as a photon, producing a smooth spread of energies.
6
An X-ray tube operates at 50 kV. Calculate the maximum energy of the X-ray photons it can produce.
Model answer
The greatest energy is that of an electron that gives up all its kinetic energy, E = eV = 1.60 × 10⁻¹⁹ × 5.0 × 10⁴ = 8.0 × 10⁻¹⁵ J, which is 50 keV, and 8.0 × 10⁻¹⁵ ÷ 1.60 × 10⁻¹⁹ returns 5.0 × 10⁴ V.

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