Date | May 2012 | Marks available | 3 | Reference code | 12M.3.HL.TZ2.20 |
Level | Higher level | Paper | Paper 3 | Time zone | Time zone 2 |
Command term | Explain | Question number | 20 | Adapted from | N/A |
Question
This question is about X-ray absorption.
A parallel beam of X-rays is incident on a section of tissue of thickness x. The constant incident intensity is I0 and the transmitted intensity is It.
The half-value thickness of the tissue is 4.0 cm.
On the axes below, sketch a graph to show the variation with tissue thickness x of \(\frac{{{I_t}}}{{{I_0}}}\).
Calculate the attenuation coefficient of X-rays for this tissue.
For a different type of tissue, the ratio \(\frac{{{I_t}}}{{{I_0}}}\) is smaller for the same thickness x of material.
Compare the attenuation coefficient of this tissue with that of the tissue in (b).
Barium has an attenuation coefficient that is much larger than that for human tissue.
Explain why a patient is asked to drink a liquid barium meal to help produce an X-ray image of the digestive system.
Markscheme
smooth curved decay line beginning at \(\frac{{{I_t}}}{{{I_0}}} = 1\) when x=0;
passing through other three points as shown;
\(\mu = \frac{{\ln 2}}{4}\);
0.17cm-1;
Watch for alternative correct answers such as 17m-1.
e-μx smaller/eμx is larger;
μ larger;
or
it has a shorter half value thickness;
and so μ larger;
stomach tissue has similar attenuation coefficient to nearby tissue (so does not show up on X-ray);
barium absorbs X-rays well (and lines the stomach after drinking);
barium allows stomach walls to show as contrast;
Examiners report