Apparatus:
GM-tube, counter and power supply, aluminium absorbers, special source and absorber holder, micrometer, beta source (strontium 90), local rules sheet, spill tray.
Procedure:
1. Read the sheet “Local rules for the use of sources of ionising radiations by students”
2. Connect the GM-tube to the digital counter/power supply and switch on.
3. Without the radioactive source present find the background count B (in counts per second) by noting the reading of the counter over a 100 second period.
4. Set up the apparatus as in the diagram (with the thinnest any aluminium absorber in place) with the VERY FRAGILE & EXPENSIVE front of the GM tube at a known constant distance (e.g. 8cm) from the source. All of the above should be placed on the spill tray.
5. Record the count received by the counter over a 100 seconds period.
Calculate the count rate (per second).
Subtract the background count rate B and so obtain a corrected count rate with the absorber C (1/s).
6. Use the micrometer to find the average thickness, x (mm) of the aluminium sheet.
7. Repeat stages 4 to 6 for other thicknesses of aluminium.
Note: (a) You can use more than one sheet at a time.
(b) Do not use any of the lead absorbers.
8. Plot a graph of corrected count rate C (1/s) against aluminium thickness x (mm).
9. Use your graph to determine the thickness of aluminium required to reduce the number of beta particles passing by half.
This is known as the half-thickness