The table below gives data from a laboratory check of a radon monitor in the basement of a library. Radon is a radioactive gas that can enter buildings from...
Question 1 Report
The table below gives data from a laboratory check of a radon monitor in the basement of a library. Radon is a radioactive gas that can enter buildings from some rocks and soil. The monitor records alpha particles from radon decay products. Each result is the mean of several readings made for the same time. The facilities manager compares the values before and after a ventilation fan is installed. Windows are kept shut during both sets of measurements so that the test is controlled.
| Condition | Mean count rate / counts per minute |
|---|---|
| Fan off | 94 |
| Fan on | 39 |
| Outdoor background | 22 |
(a) What is the corrected count rate with the fan on? [1]
(b) Which condition gives the greatest radon level? [1]
(c) When the fan is used, explain why the count rate decreases but does not become zero. [2]
A geologist takes a rock sample from a volcanic ash layer beneath an old settlement. Fig. 1 is a simplified graph from the laboratory report. It shows the percentage of potassium-40 remaining in minerals in the sample. The geologist compares the graph with the age of pottery found above the ash. The graph uses percentage rather than mass because different samples began with different masses of potassium-40. The radioactive decay process is random for one nucleus, although the trend for a large number of nuclei is predictable.
(a) What percentage of potassium-40 remains after 1.25 billion years? [1]
(b) What is the half-life shown by Fig. 1? [1]
(c) Sketch the shape of a graph of count rate against time for this sample. Include labelled axes. [3]
Answer Details
Radon monitor
(a) Corrected count rate \(=39-22=\mathbf{17}\) counts per minute. Background must be removed because it is not caused by radon in the basement. [1]
(b) Fan off gives the greatest radon level because it has the greatest measured count rate. [1]
(c) Ventilation removes or dilutes some radon, so fewer alpha particles are detected. The count does not become zero because background radiation remains and/or some radon remains in the building. [2]
Potassium-40 dating
(a) At 1.25 billion years, 50% of the potassium-40 remains. [1]
(b) A half-life is the time for the amount remaining to fall to 50%. The graph gives a half-life of 1.25 billion years. [1]
(c) Count rate is proportional to activity, so it has the same exponential decay shape: high initially, decreasing, becoming less steep, and approaching zero. [3]
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