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A hospital uses the radioactive isotope technetium-99m as a tracer - AQA - A-Level Physics - Question 1 - 2021 - Paper 5
Question 1
A hospital uses the radioactive isotope technetium-99m as a tracer. Technetium-99m is produced using a Molybdenum-Technetium generator on site at the hospital. 1.1 ... show full transcript
Worked Solution & Example Answer:A hospital uses the radioactive isotope technetium-99m as a tracer - AQA - A-Level Physics - Question 1 - 2021 - Paper 5
Step 1
1.1 Explain why the value of the half-life of technetium-99m: - makes it suitable for use as a tracer - means that it must be produced in a generator on site.
Answer
The half-life of technetium-99m is approximately 6 hours, which is long enough to allow the procedure to take place without the patient being subjected to excessive radiation. This short half-life also ensures that there is minimal radiation exposure to the patient after the tracer has been used, as it will decay quickly, thereby making it suitable for medical diagnostics. Additionally, this short lifespan necessitates that technetium-99m is produced on site, as it cannot be transported over long distances without losing effectiveness due to decay.
Step 2
1.2 Explain why this makes technetium-99m suitable for use as a tracer.
Answer
Since technetium-99m emits only gamma rays, it is able to pass through the body tissue without causing significant damage. This reduces the risk of ionization damage compared to other forms of radiation. The energy and frequency of gamma rays are similar to that of medical X-rays, making them effective for imaging purposes while ensuring safety for the patient.
Step 3
1.3 Describe how the current produced by the photocathode is amplified in the photomultiplier tube.
Answer
When gamma radiation hits the crystal scintillator, it generates visible light photons. Each visible light photon released leads to the emission of an electron from the photocathode. These electrons are then accelerated towards the dynodes, where they collide with them, producing even more electrons upon impact. This process results in a cascading effect, amplifying the current significantly as more electrons are generated with each collision.
Step 4
1.4 Determine whether the patient can be safely released from hospital after 10 days.
Answer
The effective half-life is calculated considering biological elimination. The initial activity is 3.2 GBq, and after 8 days (one physical half-life), it will be approximately 1.6 GBq. After another 8 days (16 days total), it can be calculated that the activity will reduce further. Since the patient is monitored for 10 days, after these 10 days, it should still exceed 1.1 GBq, thereby indicating that the patient cannot be safely released until a full 16 days have passed.