CIE A Level Physics复习笔记24.1.6 Production & Use 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:

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 λ_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)
  • c = speed of light (m s^-1)

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

• 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