CP15 Investigate the Absorption of Gamma Radiation by Lead

Question 1

How?

Answer 1

Set up a Geiger counter and radioactive source, with a clamp and sheet of lead between them
Record the background radiation count on the GM tube over a long period (5-10 minutes
Find an average thickness of one sheet using a micrometre and multiple readings
Add each sheet one by one, recording the count over a much shorter amount of time than for the background count
Repeat 3 times
Graph thickness against count rate

Question 2

Safety?

Answer 2

Reduce exposure to the gamma source by keeping it in a lead-lined box when not in use
Handle the source with tongs
Wash hands after holding lead
Keep source and arm’s length away

Question 3

Reducing uncertainty?

Answer 3

Repeat readings as decay is random
Record over a long period
Uncertainty can be estimated as half of the range of the repeat readings

Question 4

Why use cobalt 60

Answer 4

The count rate is high enough for readings but low enough to be safe
The half-life of 5 years so doesn’t need to be replaced often