AQA A-Level Physics: Photomultiplier Tube and Gamma Camera Basics — mark scheme explained

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

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  1. The photon strikes the photocathode, causing it to emit one or more electrons through the photoelectric effect.

• S2

  2. These photoelectrons are accelerated towards the first dynode by a high voltage.

• S3

  3. The impact of these electrons on the dynode causes secondary emission, releasing multiple electrons for each incident electron.

• S4

  4. This process is repeated at each subsequent dynode, exponentially increasing the number of electrons.

• S5

  5. Finally, the multiplied electrons are collected by the anode, producing a detectable current.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   1. The photon strikes the photocathode, causing it to emit one or more electrons through the photoelectric effect.

2. S2

   2. These photoelectrons are accelerated towards the first dynode by a high voltage.

3. S3

   3. The impact of these electrons on the dynode causes secondary emission, releasing multiple electrons for each incident electron.

4. S4

   4. This process is repeated at each subsequent dynode, exponentially increasing the number of electrons.

5. S5

   5. Finally, the multiplied electrons are collected by the anode, producing a detectable current.

6. 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.

Common mistakes

• Confusing the role of the photocathode with that of the anode in a PMT. — 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. — 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. — 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. — 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. — 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. — 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

• Full worked solution (all marking points)5 marks

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