Absorption of Beta Radiation


GM-tube, counter and power supply, aluminium absorbers, special source and absorber holder, micrometer, beta source (strontium 90), local rules sheet, spill tray.


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