How are X-rays produced?

by fast electrons hitting a metal target. Some of the electrons' kinetic energy becomes X-ray photons.

The p.d. across an X-ray tube is increased. What happens to the minimum wavelength of the X-rays?

It decreases. Each electron brings more energy, so the most energetic photon has a shorter wavelength.

Which of these absorbs X-rays most strongly?

bone. It has the largest attenuation coefficient.

What is contrast in an X-ray image?

the difference in blackening between parts of the image. Without contrast, different tissues can't be told apart.

How does a CT scan differ from an ordinary X-ray picture?

It combines images from many angles to give a three-dimensional image. A computer does the combining.

X-rays and CT scans · Cambridge A Level Physics revision

In plain words

X-rays pass easily through flesh but are stopped by bone, so a shadow picture shows up a break at a glance. They are made by slamming fast electrons into a metal target. A CT scanner takes the idea further, combining shadows taken from every angle into a three-dimensional picture.

5 things to know

Production: electrons are accelerated through a large p.d. and hit a metal target. Some of their kinetic energy becomes X-ray photons.
The most energetic photon takes all of one electron's energy, eV. That gives the shortest wavelength produced: minimum λ = hc ÷ eV.
X-rays are absorbed as they pass through matter: I = I₀e^(−μx), where μ is the attenuation coefficient. It is large for bone and smaller for soft tissue.
Contrast is the difference in blackening between different parts of the image. Good contrast needs neighbouring tissues to absorb very different amounts.
CT scanning: X-ray images of one section of the body are taken from many angles and combined by a computer into a two-dimensional image of that slice. This is repeated along the body, and the slices are combined into a three-dimensional image.

Worked example

An X-ray tube works at a p.d. of 50 kV. Find the shortest wavelength of X-rays it produces.

Greatest photon energy = eV = 1.60 × 10⁻¹⁹ × 50 × 10³ = 8.0 × 10⁻¹⁵ J.
Minimum λ = hc ÷ E = 6.63 × 10⁻³⁴ × 3.00 × 10⁸ ÷ 8.0 × 10⁻¹⁵.
= 2.5 × 10⁻¹¹ m.

Tips and tricks

A higher tube voltage gives a shorter minimum wavelength, and X-rays that get through more.
Make sure μ and x are in matching units: if μ is in cm⁻¹, x must be in cm.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

X-rays and CT scans: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

How are X-rays produced?
- by fast electrons hitting a metal target (the answer)
- by heating a filament
- by radioactive decay
- by vibrating a crystal
Some of the electrons' kinetic energy becomes X-ray photons.
The p.d. across an X-ray tube is increased. What happens to the minimum wavelength of the X-rays?
- It decreases. (the answer)
- It increases.
- It stays the same.
- It doubles.
Each electron brings more energy, so the most energetic photon has a shorter wavelength.
Which of these absorbs X-rays most strongly?
- bone (the answer)
- muscle
- fat
- air
It has the largest attenuation coefficient.
What is contrast in an X-ray image?
- the difference in blackening between parts of the image (the answer)
- the sharpness of the edges
- the size of the image
- the time the exposure takes
Without contrast, different tissues can't be told apart.
How does a CT scan differ from an ordinary X-ray picture?
- It combines images from many angles to give a three-dimensional image. (the answer)
- It uses sound instead of X-rays.
- It uses no radiation.
- It uses a single image from one direction.
A computer does the combining.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 10 marks. Write your answers on paper, then check them.

X-rays and CT scans

Cambridge International AS & A Level Physics 9702 · 10 marks · papermunch.org

Name ______________________________ Date ______________

An X-ray tube works at 100 kV. Find the greatest energy of the photons it produces, in joules.[2]
Show answerHide answer
1.6 × 10⁻¹⁴ J (100 keV).
I got this one
An X-ray beam passes through 3.0 cm of tissue with an attenuation coefficient of 0.20 cm⁻¹. Find the fraction of its intensity that gets through.[3]
Show answerHide answer
0.55. e^(−0.20 × 3.0) = e^(−0.60).
I got this one
State what is meant by contrast in an X-ray image, and why bone shows up clearly against muscle.[2]
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Contrast is the difference in the degree of blackening between different parts of the image. Bone absorbs X-rays much more strongly than muscle, so far fewer reach the detector behind it.
I got this one
Outline how a CT scanner builds up a three-dimensional image.[3]
Show answerHide answer
X-ray images of one section of the body are taken from many different angles and combined by a computer to give a two-dimensional image of that section. This is repeated for sections all the way along the body, and the two-dimensional images are combined to give a three-dimensional image.
I got this one

Answers: X-rays and CT scans

1. 1.6 × 10⁻¹⁴ J (100 keV).
2. 0.55. e^(−0.20 × 3.0) = e^(−0.60).
3. Contrast is the difference in the degree of blackening between different parts of the image. Bone absorbs X-rays much more strongly than muscle, so far fewer reach the detector behind it.
4. X-ray images of one section of the body are taken from many different angles and combined by a computer to give a two-dimensional image of that section. This is repeated for sections all the way along the body, and the two-dimensional images are combined to give a three-dimensional image.