What you need to know before proceeding:
- ALARA - what it means and how it applies to this lab.
- CAUTION: some of the aluminum sheets are very thin - please DO
NOT TOUCH THEM - handle by the white plastic holder only - we do not
want more holes in this expensive equipment.
What to do:
- Use the scaler provided. Write your
data on paper or in Excel in well labelled tables and make copies for each person in
your group.
- Connect the GM tube to the scaler, ensure the voltage is set to 400 V,
and then take a 2 minute background reading. (Record on paper).
- Insert the Ra-226 source into the holder as shown at right (use ALARA!)
and take a 60 s counts with the sample holder and GM tube holder touching.
(This is your "null count" - no added shielding is present and so makes
a good comparison for your later values.)
- Chose a range of 8 different thicknesses of aluminum and take a 60 s reading of
the total counts for each thickness. Record the area density for
each shield you use - it is printed in grams per square centimetre: g/cm2
(but incorrectly printed as "gm" for grams! Careful - some are
correctly printed as mg/cm2
meaning milligrams per square centimetre!)
- Take a 60 s reading for 8 of the lead shields. Record the
area density with your count.
- Take a 60 s reading for paper - create your own shields by cutting a
piece of paper and inserting into the holder using the empty white barrier
holder, using 1 sheet, then 2 sheets and so on up to 8 sheets.
- Work out the area density of your paper...
- If time: Gently drape a rubber glove over the GM tube
(see photo at right) so that there is only a single layer of glove
between the sample and the GM tube. Obtain a 60 s count. Put the glove back in the box
for re-use when
done.
- Clean up:
put the source back in its lead castle, add the lead top and close the
lid. Leave all equipment on the desk.
Turn off the scaler.
- Homework:
a) Use your data to produce graphs with activity (in Bq) on the
vertical axis and area density of each type of shield material on the
horizontal (in g/cm2). You should do this in Graphical Analysis rather than Excel as
it is better at curve fitting.
b) Put all your data together and plot on a single set of axes (note:
the area density units must be the same - you'll have to convert some)
c) Fit a best fit curve on each graph... (hint: fit the simplest line that
has the lowest RMSE value.)
d) Print off and bring your 4 graphs to the NEXT CLASS. This
will be checked in class and will then be used to do further analysis.
After next class:
d) What evidence exists in your data to support the idea that there are three types of
radiation present? (This is more difficult than it first seems.
Hint: you will need to use your "null count" - the one with no shielding
present.)
Note - you are not being asked to look up what type of radiation the
sample emits (although you can certainly do that) but rather are being
asked to look for evidence in your graphs that three types of radiation
are emitted... or not...
e) Explain the importance of this lab as it relates to ALARA and radiation safety.
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Setup:
Handling the radioactive source:
1. Wear gloves and use pliers to remove the source.
Keep it pointed away from you at all times.
2. Insert into the holder:
Putting a glove over the GM tube (#8):
What to hand in:
Answers to the homework questions. Insert your Graphical Analysis graph into a Word
document. (Copy & paste!)
Each student must do their own report.
Print off and put in The Box within 7 days of the day the graphs are
taken up. GLC
eligible.
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