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A2 Daily A Level Physics question

2026-06-11 OCR A Medical imaging I: X-rays — attenuation & HVT (M6.2) Module 6.2.2 Medical imaging — X-rays: attenuation and half-value thickness (HVT) Module 6.2.3 Medical imaging — qualitative dose and image contrast considerations

In a quality-control test of an X-ray beam at fixed kVp, two 8.0 mm-thick inserts are compared. For this beam, material A has a half-value thickness (HVT) of 2.0 mm and material B has an HVT of 4.0 mm. Which statement must be true about the transmitted X-ray fraction through the inserts?

  1. A Material A transmits the larger fraction, because the smaller HVT means less attenuation per millimetre.
  2. B Both transmit the same fraction, because the inserts have the same physical thickness.
  3. C Material A would transmit more if the tube current (mA) were increased during the exposure.
  4. D Material B transmits the larger fraction; a larger HVT means fewer halvings across 8.0 mm. (correct)

Answer

The correct answer is D.

Correct: D — Material B transmits the larger fraction; a larger HVT means fewer halvings across 8.0 mm. A smaller HVT means the material attenuates more strongly per unit thickness, so A is wrong. B is wrong because equal physical thickness does not imply equal transmission; the material’s HVT matters. C is wrong because changing tube current alters the number of photons emitted, not the fraction that gets through a given thickness. D is correct because 8.0 mm is four HVTs for A (about 1/16 transmitted) but only two HVTs for B (about 1/4 transmitted).

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