Center for Radiochemistry and Nuclear Materials
Department of Chemistry
The detection of radioactive contamination is a crucial skill whether working in a laboratory, hospital, industrial plant or the wider environment. This practical provides an introduction to contamination monitoring and the assessment of external gamma dose rate. By selection and use of the appropriate monitoring method it is possible to infer the type of radioactive contamination present, approximate activity levels and external dose. The lessons learnt in this session will be carried through to subsequent experiments.
Explanation and Exercise Guide
Six potential sources (A-F) are set out on the lab benches. You are provided with four different contamination monitors. Using the monitors record the background and counts per second (cps) for each source. DO NOT ALLOW THE MONITOR TO TOUCH THE SOURCE.
2. Dose rate
Using the dose rate meter provided determine the ambient dose rate in the laboratory. Record the dose rate at increasing distances from Source F. Plot a graph of dose rate against distance.
3. Wipe testing
Take ¼ piece of filter paper and holding it with tweezers soak with 10% acetone. Place the 10 cm2
template on the bench and thoroughly wipe within the whole of this area. Place the filter paper in a plastic scintillation vial and add 10 mL of scintillation cocktail. Firmly cap the vial and shake on the whirlimix for 10 seconds. Write your initials on the cap and place in the scintillation counter rack. The vials will be counted in the scintillation counter using the wipe test protocol during the practical and the results given back to you.
4. Simulation monitoring
One bench has been ‘contaminated’ using a simulant spray. Using the STS802 radiation simulation monitor, identify as many “contaminated” areas as you can. In order to detect the ‘contamination’ the monitor must be moved extremely slowly and close to the bench. The simulant is not easily detectable at distances of greater than 1 cm or if the probe is moved too rapidly over the area, and can be easily missed if the area is given a cursory inspection. This type of monitor is used for training purposes, as it provides practice in how monitoring should be done in the case of a real radiation spillage. Record the locations of any contamination detected on the plan of the lab provided.
Questions For students:
1. What can you say about the radioactive emissions produced by each source?
2. Which monitor is the most suitable in each case?
3. What factors affect the readings obtained?
4. Derive an expression for the dose rate as a function of distance from Source F.
1.Wear lab coat, safety glasses and gloves.
2.Handle filter papers used for wipe testing with tweezers.
3. Do not touch or attempt to move the radioactive sources.