A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Photomultiplier Tube and Gamma Camera Basics — mark scheme explained
The short answer
In the realm of medical physics, particularly in nuclear medicine, understanding the basic structure and workings of a photomultiplier tube (PMT) and a gamma camera is crucial. These devices play a pivotal role in detecting and imaging radioactive emissions from within the body, which are essential for diagnosing various conditions.
The question
Explain the role of each component in a photomultiplier tube (PMT) when it detects a single photon.
[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 questions on photomultiplier tubes and gamma cameras, marks are typically awarded for correctly identifying and explaining the function of each component. Ensure that your answers are clear, concise, and step-by-step to maximize your score.
What the command words demand
- Explain
- Provide a detailed account of how something works or why it happens.
- Describe
- Give a detailed account of the structure or appearance of something.
- Identify
- Recognize and name specific components or processes.
- Compare
- Highlight similarities and differences between two or more items or concepts.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes for a question worth 6-8 marks on this topic. This allows time for careful reading, planning, and writing a well-structured response.
- 1. The photon strikes the photocathode, causing it to emit one or more electrons through the photoelectric effect.1 mark
- 2. These photoelectrons are accelerated towards the first dynode by a high voltage.1 mark
- 3. The impact of these electrons on the dynode causes secondary emission, releasing multiple electrons for each incident electron.1 mark
- 4. This process is repeated at each subsequent dynode, exponentially increasing the number of electrons.1 mark
- 5. Finally, the multiplied electrons are collected by the anode, producing a detectable current.1 mark
Final answer: The photocathode emits electrons when struck by a photon, which are then amplified through multiple stages of dynodes before being collected by the anode to produce a measurable current.
Work through every step correctly and you earn all 5 marks.
Another worked example
Describe the process by which a gamma camera detects and images radioactive isotopes in the body.
- 1. A patient is administered a radioactive tracer that accumulates in specific tissues or organs.0 marks
- 2. Gamma rays emitted by the tracer pass through the collimator, which filters and directs them to the scintillation crystal.1 mark
- 3. The gamma rays interact with the scintillation crystal, producing flashes of light (scintillations).1 mark
- 4. The PMTs detect these scintillations and convert them into electrical signals.2 marks
- 5. The electronics process these signals to determine the position and intensity of each detected gamma ray.1 mark
- 6. The data is compiled to create a two-dimensional image that shows the distribution of the radioactive tracer within the body.1 mark
Final answer: The gamma camera detects gamma rays emitted by a radioactive tracer in the body, converts them into light signals using a scintillation crystal, and processes these signals with PMTs and electronics to create an image.
Work through every step correctly and you earn all 6 marks.
Common mistakes
Confusing the role of the photocathode with that of the anode in a PMT.
Why it happens: Students may mix up the functions of different components due to similar-sounding names or roles.
Fix: Review the specific function of each component: the photocathode emits electrons when struck by photons, while the anode collects the multiplied electrons.
Forgetting that dynodes in a PMT amplify the electron signal through secondary emission.
Why it happens: The process of secondary emission might be overlooked if students focus only on the initial and final stages of detection.
Fix: Emphasize the role of each dynode in amplifying the electron signal by releasing multiple electrons for each incident electron.
Misunderstanding the function of the collimator in a gamma camera.
Why it happens: The concept of filtering and directing gamma rays might be confusing if not clearly explained.
Fix: Clarify that the collimator filters and directs gamma rays to ensure they reach the detector from specific angles, helping to localize the source of radiation.
Thinking that a scintillation crystal directly converts gamma rays into electrical signals.
Why it happens: Students might overlook the intermediate step where light is produced and then detected by PMTs.
Fix: Explain that the scintillation crystal produces flashes of light when gamma rays interact with it, and these light signals are then detected by PMTs to produce electrical signals.
Believing that a gamma camera can only detect one type of radioactive isotope.
Why it happens: Students might assume that the device is limited to a single application without understanding its versatility.
Fix: Highlight that gamma cameras are used for various diagnostic procedures and can detect different types of radioactive isotopes, depending on the tracer administered.
Failing to explain how the electronics in a gamma camera process signals from PMTs.
Why it happens: The role of electronics might be overlooked if students focus only on the detection and conversion processes.
Fix: Emphasize that the electronics process the signals from PMTs to determine the position and intensity of each detected gamma ray, which is essential for constructing an image.
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 |
|---|---|---|
| Explain PMT Components | Describe the function of each photomultiplier tube component during single-photon detection. | 5 |
| Gamma Camera Operation | Describe how a gamma camera detects gamma rays and forms an image. | 6 |
| Explain Component Function | Explain how the collimator directs gamma rays to localise the radiation source. | 3 |
| Explain Component Function | Describe how the scintillation crystal converts incoming gamma photons into detectable light within a gamma camera. | 4 |
| Total across these question types | 18 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.