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)
Part (a)
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- 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)
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- The minimum wavelength is equal to
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- 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
- This equation shows the maximum energy of the electron corresponds to the minimum wavelength
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