A teacher investigates how the count rate from a gamma source changes with distance. Fig. 1 shows the source, a radiation detector and a lead box used for s...
Question 1 Report
A teacher investigates how the count rate from a gamma source changes with distance. Fig. 1 shows the source, a radiation detector and a lead box used for safe storage. At 0.50 m, the detector records 520 counts in 20 s. Background radiation is 40 counts in 20 s. The source and detector are kept at the same height.
(a) Calculate the count due to the gamma source in 20 s at 0.50 m. [2]
(b) Calculate the count rate due to the source, in counts per second. [2]
(c) Use the inverse-square relationship to calculate the expected source count rate at 1.0 m. [3]
(d) Explain why a measured value at 1.0 m may differ slightly from the calculated value. [2]
(e) State why a gamma source is suitable for this investigation rather than an alpha source. [1]
Answer Details
(a) The recorded count includes background radiation:
\[520-40=480\,\text{counts}\]
The count due to the gamma source is 480 counts. [2]
(b)
\[\text{count rate}=\frac{480\,\text{counts}}{20\,\text{s}}=24\,\text{counts s}^{-1}\]
The source count rate is 24 counts s-1. [2]
(c) The distance doubles from 0.50 m to 1.0 m. By the inverse-square relationship, the count rate becomes one quarter:
\[\frac{24}{4}=6\,\text{counts s}^{-1}\]
The expected source count rate is 6 counts s-1. [3]
(d) Radioactive decay and count measurements are random, so repeated counts are not identical. Background radiation may also fluctuate, causing a measured value to differ slightly from the calculated value. [2]
(e) Gamma radiation is suitable because it can travel through air over this distance. Alpha radiation is strongly absorbed by air. [1]
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