Paper 3: Required practical 12 - Inverse square law for gamma radiation

Question 1

What safety precautions must be taken when working with a radioactive source?

Answer 1

• Limit time of exposure
• Warning signs should be displayed
• Keep source at arm’s length
• Only handle the source using long-handled tongs

Question 2

How should a radioactive source be stored safely?

Answer 2

• Locked away in a sealed lead-lined container
• Hazard symbol should be visible on the container as well as at the location where it is stored

Question 3

What device can be used to measure a radioactive count rate?

Answer 3

Geiger-Muller tube connected to a scaler

Question 4

Why is cobalt-60 a suitable source for this experiment?

Answer 4

• Safe for use in schools and has a half-life or around 5 years, meaning it can be reused for a number of years
• Activity is low enough to be safe, but high enough for measurements to be taken easily

Question 5

How can you make the measured count-rate more accurate?

Answer 5

The longer the period of time it’s recorder, the lower the measurement of uncertainty will be

Question 6

How to convert a count to count rate?

Answer 6

Divide the count by the length of time over which it was taken

Question 7

What should you do prelininarily?

Answer 7

• Take recordings to find the max distance from the source that you cang get a measureable count
• Take recordings closer to the source to determine a sutiable time period over which to take readings

Question 8

What graph should be plotted to confirm that the inverse square law has been obeyed?

Answer 8

A graph of 1/√C against x should be plotted - should form a straight line graph

Question 9

Why should a graph of 1/√C against x be plotted instead of a graph of C agaisnt 1/x^2?

Answer 9

Removes any systematic error in the distance measurements

Question 10

Why is there likely to be a systematic error in the distance measurements in this experiment?

Answer 10

The location of the radioactive source within the sealed capsule, and in the precise location of the ionisation in the GM tube are not known to a high degree of precision

Question 11

How can you obtain the systematic distance error from a graph of 1/√C against x?

Answer 11

• The straight line plotted won’t cross the x axis at the origin
• The difference between x=0 and the point at which the straight line crosses the x axis will be the systematic error

Question 12

Why may an old gamma source be unsuitable for this experiment?

Answer 12

• The activity may have fallen to a level that is too low to obtain recordable counts over a large enough range of distances to conduct a meaningful experiment