# 27.4.2 Production & Use of X-rays

### Production of X-rays

• X-rays are short wavelength, high-frequency part of the electromagnetic spectrum
• They have wavelengths in the range 10−8 to 10−13 m
• X-rays are produced when fast-moving electrons rapidly decelerate and transfer their kinetic energy into photons of EM radiation

#### Producing X-rays

• At the cathode (negative terminal), the electrons are released by thermionic emission
• The electrons are accelerated towards the anode (positive terminal) 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:

Emax = eV

• This is the maximum energy that an X-ray photon can have
• Therefore, the maximum X-ray frequency fmax, or the minimum wavelength λmin, 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)
• c = speed of light (m s-1)

#### Worked example: Production of X-rays

Part (a)

• Photons are produced whenever a charged particle is accelerated towards a metal target
• The wavelength of the photons depends on the magnitude of the acceleration
• The electrons which hit the target have a distribution of accelerations, therefore, a continuous spectrum of wavelengths is observed

Part (b)

• The minimum wavelength is equal to

• This equation shows the maximum energy of the electron corresponds to the minimum wavelength
• Therefore, the higher the acceleration, the shorter the wavelength
• At short wavelengths, the sharp cut-off occurs as each electron produces a single photon, so, all the electron energy is given up in one collision

### Using X-rays in Medical Imaging

• X-rays have been highly developed to provide detailed images of soft tissue and even blood vessels
• When treating patients, the aims are to:
• Reduce the exposure to radiation as much as possible
• Improve the contrast of the image

#### Reducing Exposure

• 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

#### Contrast & Sharpness

• Contrast is defined as:

The difference in degree of blackening between structures

• 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
• Using a contrast media
• Sharpness is defined as:

How well defined the edges of structures are

• Image sharpness can be improved by:
• Using a narrower X-ray beam
• Reducing X-ray scattering by using a collimator or lead grid
• Smaller pixel size

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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