X-Ray Machines: The Cool Secrets Every Physics Student Should Know 

Nearly all of us will have an X-ray image taken of a body part at some point in our lives, but this common-place hospital tool is powered by some seriously cool science

To create this week’s blog post, we’ve handpicked some of the best content from our CIE A Level Physics Revision Notes so that we can help you get to grips with the mechanisms by which X-ray machines work. 

Written by experienced teachers and examiners, our Revision Notes are the perfect tool for homeschoolers looking to get ahead of their classmates. Did we mention they include worked example questions too? 

Remember, if you’re an A Level Physics student or you’re hoping to study medicine at university, you’ll need to become very familiar with the science behind the X-ray machine. Read on to find out the facts and smash this section of the syllabus!

What are X-rays?

Let’s start right at the beginning: X-rays are electromagnetic waves with wavelengths in the range 10^{-8} to 10^{-13} m. This puts them in the short wavelength, high-frequency part of the electromagnetic spectrum

X-rays are produced when fast-moving electrons rapidly decelerate and transfer their kinetic energy into photons of EM radiation. 

X-rays can be classified into soft X-rays and hard X-rays, with the hardness defined as “the measure of the penetrating strength of a beam”. The greater the energy of the ray, the greater the hardness and the penetrating strength!

How do X-ray machines work?

X-ray machines contain a vacuum tube that converts an electrical input into X-rays. This tube consists of a cathode (negatively charged electrode) and rotating anode (positively charged electrode) inside a vacuum chamber. 

  • At the cathode, the electrons are released by thermionic emission (the process by which free electrons are emitted from the surface of a metal when a heat source is applied
  • The electrons are accelerated towards the anode at high speed
  • When the electrons bombard the metal target, they lose some of their kinetic energy by transferring it to photons
  • The electrons in the outer shells of the atoms (in the metal target) move into the spaces in the lower energy levels

As they move to lower energy levels, the electrons release energy in the form of X-ray photons

Energy, frequency and wavelength

When an electron is accelerated, it gains energy equal to the electronvolt; this energy can be calculated using:

E_{max}=eV

This is the maximum energy that an X-ray photon can have.

Therefore, the maximum X-ray frequency f_{max} , or the minimum wavelength \lambda_{min} , that can be produced is calculated using the equation:

Where:

e = charge of an electron (C)

V = voltage across the anode (V)

h = Planck’s constant (J s) (this will be given to you in the exam)

c = speed of light m s^{-1} (this will be given to you in the exam)

What are the applications of X-ray machines?

The most commonly-known applications are for medical imaging (often called radiography). 

The penetrative power of X-rays allow them to pass through soft tissue whilst being absorbed by denser materials like bones or teeth. 

Because of this, when X-rays that have passed through a body strike a photographic plate or a fluorescent screen, they create an image of the body’s interior structure of the body. This is a very good way for doctors to observe broken bones or the location of foreign objects inside the body. 

X-rays can also be used to kill diseased cells, to detect flaws in metallic products, and even to examine antiques and paintings!

Important considerations for Medical Radiography

  1. Contrast & Sharpness

    Contrast is defined as the difference in degree of blackening between structures, and contrast allows a clear difference between tissues to be seen. Image contrast can be improved by using the correct level of X-ray hardness: hard X-rays for bones, soft X-rays for tissue.

    Sharpness is defined as how well defined the edges of structures are, and image sharpness can be improved by using a narrower X-ray beam, reducing X-ray scattering by using a collimator or lead grid, and by using a smaller pixel size.
  2. Safety

    X-rays are ionising, meaning they can cause damage to living tissue and can potentially lead to cancerous mutations.

    Therefore, healthcare professionals must ensure patients receive the minimum dosage possible.

    In order to do this, aluminium filters are used…
  • This is because many wavelengths of X-ray are emitted
  • Longer wavelengths of X-ray are more penetrating, therefore, they are more likely to be absorbed by the body
  • This means they do not contribute to the image and pose more of a health hazard
  • The aluminium sheet absorbs these long wavelength X-rays making them safer

Fascinating stuff, right?! If you’re ready to learn more about medical imaging, ultrasounds and X-rays, dive right in to our full collection of CIE A Level Physics Revision Notes.

Then, test yourself with our Topic Questions!

Remember, if you’ve got a question for our Revision Experts, get in touch via our social media channels (@SaveMyExams).

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