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Date May 2014 Marks available 6 Reference code 14M.3.HL.TZ1.21
Level Higher level Paper Paper 3 Time zone Time zone 1
Command term Determine and Predict Question number 21 Adapted from N/A

Question

This question is about X-rays.

Define attenuation coefficient.

[1]
a.

The graph below shows the variation of attenuation coefficient μ with photon energy E for X-rays in an absorbing medium.

A beam of X-rays is incident on a sample of the medium with intensity I0. Using the graph,

(i) determine how far X-rays with energy equal to 0.1MeV travel inside the sample before their intensity reduces to 0.1I0.
(ii) predict whether X-rays of energy 10MeV are more penetrating than X-rays of energy 0.1MeV in this medium.

[6]
b.

Markscheme

probability of a single photon being absorbed in 1m of the material / reference to \(I = {I_0}{e^{ - \mu x}}\) with symbols defined;

a.

at 0.1MeV, μ=30(m-1);
\(0.1 = {e^{ - 30x}}\);
\(x = \frac{{\ln 0.1}}{{ - 30}}\);
x=0.077(m);

(ii) (at 10MeV,) μ is smaller (than at 0.1MeV);
so (10MeV X-rays are) more penetrating;
Award second mark only if first mark has been awarded.

b.

Examiners report

Candidates do not use precise enough wording in definitions, as seen in answers to (a).

a.

In (b), the majority of candidates had no problem with the determination of attenuation coefficient from the graph and used it in the calculation of the result.

b.

Syllabus sections

Option C: Imaging » Option C: Imaging (Additional higher level option topics) » C.4 – Medical imaging (HL only)
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