A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Production and Control of X-rays in Medical Physics — mark scheme explained
The short answer
In medical physics, the production and control of X-rays are crucial for diagnostic imaging. This section covers the physical principles behind the generation of X-rays, including their energy spectrum, methods to control beam intensity, photon energy, image sharpness and contrast, and patient dose.
The question
An X-ray tube operates at a voltage of 120 kV. Calculate the maximum energy of the bremsstrahlung X-rays produced. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
The maximum energy of bremsstrahlung X-rays is equal to the kinetic energy of the incident electrons, which depends on the applied voltage (kV).
- S2
The formula for the maximum energy in electron volts (eV) is: E max = kV × 1000 eV/kV.
- S3
Substitute the given values into the formula: E max = 120 × 1000 eV = 120,000 eV.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
The maximum energy of bremsstrahlung X-rays is equal to the kinetic energy of the incident electrons, which depends on the applied voltage (kV).
- S2
The formula for the maximum energy in electron volts (eV) is: E max = kV × 1000 eV/kV.
- S3
Substitute the given values into the formula: E max = 120 × 1000 eV = 120,000 eV.
Final answer: 120,000 eV
Common mistakes
- Confusing bremsstrahlung radiation with characteristic X-rays — Review the definitions and characteristics of both bremsstrahlung radiation (continuous spectrum) and characteristic X-rays (discrete lines).
- Incorrectly stating that the maximum energy of bremsstrahlung X-rays is independent of the applied voltage — Emphasize that the maximum energy of bremsstrahlung X-rays is directly proportional to the applied voltage (kV).
- Failing to explain why a rotating-anode X-ray tube is used — Highlight that the rotating anode distributes heat over a larger area, reducing overheating and allowing for longer exposure times and higher power settings.
- Confusing methods to control beam intensity with methods to control photon energy — Clarify that beam intensity is controlled by adjusting the tube current (mA), while photon energy is primarily determined by the applied voltage (kV).
- Not mentioning the role of collimators in improving image sharpness and contrast — Explain that collimators limit the X-ray beam to the region of interest, reducing scatter and improving both sharpness and contrast.
- Failing to discuss patient dose minimization techniques — Emphasize that using the lowest possible kV and mA settings, employing collimators, and using digital image processing are all important for reducing patient dose.
Where the marks go
- Full worked solution (all marking points)3 marks