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A2 Daily A Level Physics question

2026-04-30 OCR A Medical imaging (M6.3) 6.3.1 Medical imaging: X-rays and CT — attenuation and half-value thickness (HVT); simple ratio calculations 6.3 Medical imaging — image contrast and dose (qualitative)

In a lab calibration of an X-ray detector, aluminium sheets are stacked in the beam. For this beam, the half-value thickness (HVT) in aluminium is 3.0 mm. If 9.0 mm of aluminium is placed in the beam, what percentage of the initial intensity I0 reaches the detector?

  1. A 12.5% (correct)
  2. B 25%
  3. C 33%
  4. D 50%

Answer

The correct answer is A.

Correct: A — 12.5%. A is the only one consistent: 9.0 mm is three half-value thicknesses, so the beam halves three times to 1/8 = 12.5%. B would be correct for two HVTs (6.0 mm), not three. C wrongly assumes a linear one-third drop rather than successive halvings. D corresponds to just one HVT (3.0 mm), not 9.0 mm.

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