Let’s start right at the beginning: X-rays are electromagnetic waves with wavelengths in the range to 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!
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
When an electron is accelerated, it gains energy equal to the electronvolt; this energy can be calculated using:
This is the maximum energy that an X-ray photon can have.
Therefore, the maximum X-ray frequency , or the minimum wavelength , that can be produced is calculated using the equation:
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 (this will be given to you in the exam)
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!
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!
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