| Date | May 2014 | Marks available | 1 | Reference code | 14M.3.HL.TZ1.21 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Define | Question number | 21 | Adapted from | N/A |
Question
This question is about X-rays.
Define attenuation coefficient.
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.
Markscheme
probability of a single photon being absorbed in 1m of the material / reference to \(I = {I_0}{e^{ - \mu x}}\) with symbols defined;
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.
Examiners report
Candidates do not use precise enough wording in definitions, as seen in answers to (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.
