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Experiments with radioactive substances

Note to teachers

The studying of this topic requires skills of drawing and interpretating graphs, so although it is suitable to use a computer and collection and display of data electronically, time also needs to be spent ensuring that the work can be done by the pupils in an exam room too. Examination questions on the practical aspects of this topic are common.

Safety issues are very important when dealing with radioactive isotopes. See here.

 

You may want to use a simulation instead of real sources - See here.

It is a useful rule of thumb to know that the activity of a sample drops to less than 1% of its value in seven half lives (see Tc99-m)

Practical radioactivity investigations

The activity of a sample can be measured with a Geiger-Müller tube connected to a rate-meter or by connecting it to a scaler and timing how long you allow the scaler to count for.

 

The above photo is of the wonderful antique device we had at my school!

 

See here for how a Geiger-counter works

If the activity of a sample is plotted against time, an exponential curve is obtained.

(NB It must be the true activity - with the background count deducted from each reading. If you are given a 'corrected count rate' that has already been done for you!)

When plotting a graph, examiners like to see candidates:

Make maximum use of the graph paper (choose the best scale - have paper orientated the correct way so as to do this) Label axes with physical quantity and correct units

Mark the points clearly. A neat cross is better than a 'blob'.. most computer programs go for 'blobs'.

Draw the line of best fit. If the points indicate a curve, it should be smooth (not 'dot-to-dot' like in a puzzle book). If they indicate proportionality the line should be drawn with a ruler.

Work should be neat (sharp pencil, long ruler, axes in ink etc.)

When analysing data from a graph (whether drawn by them or given to them) candidates must clearly show how the graph was used.

 

When dealing with radioacive materials inside a human body we have to look at the effective half life, rather than just the physical one.