IGCSE · 5 · practical
Nuclear physics · Question 10
Nuclear physics · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- discriminating
- Marks
- 8
- Topics
- 1
- Answer
- Complete
A student determines the half-life of a radioactive source using a Geiger–Müller tube and a counter. Before bringing out the source, she records the count over 10 minutes with no source present and obtains 240 counts. She then places the source at a fixed distance from the tube and records the count rate every 20 minutes.
| time / minutes | 0 | 20 | 40 | 60 | 80 |
|---|---|---|---|---|---|
| measured count rate / counts per minute | 404 | 214 | 119 | 72 | 48 |
- (a)
Determine Determine the background count rate, and explain why it must be measured.
2 marks - (b)
Determine Correct the readings for background and hence determine the half-life of the source.
4 marks - (c)
Suggest The student's corrected values do not halve over exactly equal intervals. Suggest why, and suggest one improvement to the experiment.
2 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Determine Determine the background count rate, and explain why it must be measured.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
background count rate = 240 / 10 = 24 counts per minute
- 2
radiation from cosmic rays, rocks, and other natural sources is detected as well as the source, so it must be subtracted from every reading to obtain the count rate due to the source alone
(b)
Determine Correct the readings for background and hence determine the half-life of the source.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
subtracts 24 from every reading: corrected rates 380, 190, 95, 48, 24 counts per minute
- 2
identifies that the corrected rate halves from 380 to 190 between 0 and 20 minutes
- 3
checks a second interval — 190 falls to 95 over the next 20 minutes, and 95 to 48 over the next
- 4
half-life = 20 minutes
(c)
Suggest The student's corrected values do not halve over exactly equal intervals. Suggest why, and suggest one improvement to the experiment.
Turn the task into a measurable method: name the independent, dependent and controlled quantities; describe apparatus and repeats; then explain how the evidence will be processed and how uncertainty or safety will be managed.
- 1
radioactive decay is a random process, so the count in any fixed interval fluctuates
- 2
improvement: count over a longer interval at each time, take repeat readings, or plot all the corrected values on a graph and take the half-life from a smooth curve
Private on this device