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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For explanation questions, ensure you cover all key points and provide clear, concise descriptions. For calculation questions, show your working step-by-step and use appropriate units. For discussion questions, consider multiple perspectives and provide balanced arguments.
What the command words demand
- Explain
- Provide a detailed account of how or why something happens.
- Describe
- Give a detailed account of the characteristics or features of something.
- Calculate
- Perform a mathematical operation to find a numerical answer.
- Discuss
- Consider and write about the various aspects of a topic, including advantages and disadvantages.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes for each question in this section to ensure you have enough time to provide detailed and accurate answers.
- The maximum energy of bremsstrahlung X-rays is equal to the kinetic energy of the incident electrons, which depends on the applied voltage (kV).1 mark
- The formula for the maximum energy in electron volts (eV) is: E max = kV × 1000 eV/kV.1 mark
- Substitute the given values into the formula: E max = 120 × 1000 eV = 120,000 eV.1 mark
Final answer: 120,000 eV
Work through every step correctly and you earn all 3 marks.
Another worked example
Explain the difference between bremsstrahlung radiation and characteristic X-rays.
- Bremsstrahlung radiation is a continuous spectrum of X-rays produced when high-speed electrons are decelerated by the positively charged nucleus of atoms in the anode. The maximum energy of these X-rays is equal to the kinetic energy of the incident electrons, which depends on the applied voltage (kV).2 marks
- Characteristic X-rays are discrete lines in the X-ray spectrum that occur when inner-shell electrons are ejected from atoms and outer-shell electrons fall into the vacancies, emitting X-rays with specific energies. The energy of these characteristic X-rays depends on the atomic number (Z) of the anode material.2 marks
Final answer: Bremsstrahlung radiation is a continuous spectrum of X-rays produced by deceleration of high-speed electrons, while characteristic X-rays are discrete lines in the spectrum emitted when inner-shell electrons are ejected and outer-shell electrons fall into the vacancies.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing bremsstrahlung radiation with characteristic X-rays
Why it happens: Students often mix up the two mechanisms of X-ray production, leading to incorrect descriptions and explanations.
Fix: 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
Why it happens: Students may overlook the direct relationship between the applied voltage and the maximum energy of the X-rays.
Fix: 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
Why it happens: Students might not fully understand the benefits of using a rotating anode, leading to incomplete or incorrect answers.
Fix: 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
Why it happens: Students may mix up the different controls available in an X-ray tube, leading to confusion in their answers.
Fix: 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
Why it happens: Students might overlook the importance of collimators in controlling the X-ray beam, leading to incomplete answers.
Fix: 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
Why it happens: Students might focus only on image quality and overlook the importance of minimizing patient exposure to radiation.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate maximum energy | Find the maximum bremsstrahlung X-ray energy from the tube's operating voltage. | 3 |
| Explain X-Ray Production | Distinguish the two X-ray production mechanisms and what each spectrum's energy depends on. | 4 |
| Explain Anode Design | Explain why a rotating anode is needed to produce a consistent X-ray beam. | 3 |
| Explain Image Sharpness | Describe two methods that improve the sharpness of an X-ray image. | 4 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.