GCSE Perfect
Explain — 4 marks
A hospital uses Technetium-99m (Tc-99m) as a radioactive tracer in medical imaging. Tc-99m has a half-life of 6 hours. A patient is injected with an initial activity of 800 MBq at 09:00. The medical team needs to understand how the activity changes over time to plan when scans should be performed and when it is safe to discharge the patient.
- (a) Define what is meant by the half-life of a radioactive isotope. [1 mark]
- (b) Calculate the activity of Tc-99m remaining in the patient's body at 21:00 (12 hours later). Show your working. [2 marks]
- (c) Explain why Tc-99m is suitable for use as a medical tracer, considering its half-life and the type of radiation it emits (Tc-99m undergoes gamma decay). [1 mark]
Show mark scheme
- (a) The time taken for the activity/number of radioactive nuclei to decrease to half its original value
- (b) Correct identification that 12 hours = 2 half-lives (6 hours each)
- (b) Correct calculation: 800 ÷ 2 ÷ 2 = 200 MBq (or equivalent working showing halving twice)
- (c) Half-life of 6 hours is long enough to allow time for imaging/diagnosis but short enough to minimize radiation dose to the patient / Gamma radiation is penetrating so it can be detected outside the body but less ionizing than alpha/beta, reducing harm to healthy tissue