A water treatment plant considers using radiation to check for leaks in a buried pipe. Engineers would add a very small amount of a gamma-emitting tracer to...
Question 1 Report
A water treatment plant considers using radiation to check for leaks in a buried pipe. Engineers would add a very small amount of a gamma-emitting tracer to the water and use detectors above the ground. A sudden increase in count rate at one position could show where tracer has escaped. The tracer must have a short enough half-life that it does not remain radioactive for a long time, but long enough for the survey to be completed. The plant compares safety rules before approving the method.
(a) What would a high count rate above one section of ground suggest? [1]
(b) Which radiation is suitable because it can pass through soil and pipe material? [1]
(c) When selecting the tracer, explain why a very short half-life could cause a problem for the survey. [2]
A nuclear medicine engineer checks a syringe shield used when preparing a gamma-emitting tracer. Fig. 1 shows the syringe inside a tungsten sleeve with a small viewing window. Tungsten is dense and absorbs much of the radiation. The window is made from leaded glass so the liquid level can be seen without removing the shield. The engineer uses tongs to keep the source away from hands and prepares the dose only shortly before it is needed.
(a) What material is used for the viewing window? [1]
(b) Which safety principle is illustrated by using tongs? [1]
(c) When the sleeve is made thicker, explain what happens to the gamma count outside it. [2]
(d) What does a shield reduce: activity of the source or radiation reaching a person? [1]
Answer Details
Pipe tracer
(a) A high count rate above one section suggests a leak: tracer has escaped from the pipe there. [1]
(b) Gamma radiation is suitable because it can pass through the soil and pipe material. [1]
(c) With a very short half-life, activity would fall rapidly. Too little radiation may remain to detect before the survey is finished. [2]
Syringe shield
(a) The viewing window is made of leaded glass. [1]
(b) Using tongs applies the safety principle of increasing distance from the source. [1]
(c) A thicker sleeve gives a lower gamma count outside it because more gamma radiation is absorbed by the thicker shielding. The shielding does not change the source activity. [2]
(d) A shield reduces the radiation reaching a person, and hence dose, not the activity of the source. [1]
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