Solutions: Radioactivity Questions
Q15.
(a) In a radioactivity experiment, background radiation is taken into account when taking corrected count rate readings in a laboratory.
One source of background radiation is the rocks on which the laboratory is built.
Give two other sources of background radiation.
Any TWO from: 
the sun,
cosmic rays,
radon (in atmosphere),
nuclear fallout (from previous weapon testing),
any radioactive leak (may be given by name of incident)
nuclear waste,
carbon-14
(1 mark)
(b) A γ ray detector with a cross-sectional area of 1.5 × 10–3 m2 when facing the source is placed 0.18 m from the source. A corrected count rate of 0.62 counts s–1 is recorded.
(i) Assume the source emits γ rays uniformly in all directions.
Show that the ratio is about 4 × 10–3
The ratio of area of detector to the surface area of sphere = (1.5 × 10–3)/(4π x 0.182)
ratio = 1.5 × 10–3/0.407
= 3.7 x 10-3 which rounds to 4 × 10–3
(2 marks)
(ii) The γ ray detector detects 1 in 400 of the γ photons incident on the facing surface of the detector.
Calculate the activity of the source.
State an appropriate unit.
0.62 counts s–1 is recorded so 0.62 x 400 counts s–1 are incident on the detector.
The detector only intercepts 0.0037 of the gamma rays so the activity of the source is:
0.62 x 400/3.7 x 10-3 s–1
6.7 x 104 Bq or s–1
(3 marks)
(c) Calculate the corrected count rate when the detector is moved 0.10 m further from the source.
I0.18 x 0.182 = I0.28 x 0.282
I0.28 = 0.62 counts s–1 x 0.182/0.282
I0.28 = 0.256 counts s–1
I0.28 = 0.26 counts s–1
(3 marks)
(Total 9 marks)
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