Fig. 1 shows a beta thickness gauge used as paper moves through a printing factory. A beta source is above the paper and a detector is below it. The detecto...
Question 1 Report
Fig. 1 shows a beta thickness gauge used as paper moves through a printing factory. A beta source is above the paper and a detector is below it. The detector is connected to a control box. When the count is lower than the value set by the operator, the control box moves the rollers closer together. With normal paper the detector count is 800 counts per minute. A test with unusually thick paper gives 600 counts per minute.
(a) State why beta radiation, rather than alpha radiation, is used in this gauge. [1]
(b) Explain why beta radiation is more suitable than gamma radiation for this use. [3]
(c) Describe how the control box responds when paper becomes too thick. [2]
(d) Calculate the percentage decrease in detector count for the thick paper. [2]
(e) Explain why thicker paper gives a lower count. [2]
(f) State two precautions used to reduce radiation exposure from the source. [2]
(g) Complete the statement: the activity of a source is the number of nuclear decays per _____. [1]
Answer Details
(a) Beta is used because it can pass through paper, whereas alpha radiation is stopped by paper or by air. [1]
(b) Beta is partly absorbed by paper, so a change in thickness produces a measurable change in detector count. Gamma is too penetrating, so the same thickness change would cause a much smaller count change. [3]
(c) When paper becomes too thick, the detector count falls below the set value. The control box moves the rollers closer together, making the paper thinner. [2]
(d)\[\text{decrease}=800-600=200\text{ counts min}^{-1}\]
\[\text{percentage decrease}=\frac{200}{800}\times100=25\%\]
The percentage decrease is 25%. [2]
(e) Thicker paper contains more material. More beta particles are absorbed or stopped before reaching the detector, giving a lower count. [2]
(f) Any two precautions are acceptable: shield the source when not in use, use a sealed source, minimise time nearby, keep workers at a distance, or use remote operation. [2]
(g) Activity is the number of nuclear decays per second. [1]
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