Edexcel iGCSE Physics (4PH1) 7.1 Units Exam Style Question Paper 2B - New Syllabus
Question
Protactinium is an element with several different radioactive isotopes.
(a) Protactinium-234 has a half-life of \(6.7\,\mathrm{hours}\). A sample of protactinium-234 has an initial activity of \(800\) units.
(i) Give a suitable unit for activity.
(ii) On the axes below, sketch a graph for the decay of the sample of protactinium-234 during its first three half-lives.
(iii) When protactinium-234 undergoes beta \((\beta^-)\) decay it becomes uranium-234. The incomplete nuclear equation shows this process.
Complete the nuclear equation to show the beta decay of protactinium-234. Write your answers in the dashed boxes.
(b) A student suggests an experiment to determine the type of radiation emitted by a different isotope of protactinium, protactinium-231. This is the suggested method.
Step 1 connect a suitable radiation detector to a radiation counter
Step 2 place a source of protactinium-231 at a fixed distance of \(3\,\mathrm{cm}\) from the radiation detector
Step 3 record the count of detected radiation for a time of one minute
Step 4 place a sheet of paper between the source and detector
Step 5 record the count of detected radiation for a time of one minute
Step 6 repeat Steps 4 and 5 using a sheet of aluminium and then a sheet of lead instead of the sheet of paper
The table shows the results of the investigation when it is done by a teacher.
(i) Which of these is the dependent variable in the investigation?
A count measured by the detector
B distance between source and detector
C material between source and detector
D time the count is measured
(ii) The student’s method does not allow for background radiation. Describe how the student’s method should be modified to allow for background radiation.
(iii) Describe how the student’s method could be modified to improve the reliability of the results.
(iv) Evaluate the data from the experiment to conclude the type of radiation emitted by protactinium-231.
Syllabus Topic Codes (Edexcel International GCSE Physics 4PH1):
• 7.11: Radioactive Decay and Activity — part (a)(ii)
• 7.12–7.13: Half-Life and Half-Life Calculations — part (a)(ii)
• 7.7: Nuclear Changes During Radioactive Emission — part (a)(iii)
• 7.8: Balancing Nuclear Equations — part (a)(iii)
• 7.6: Core Practical: Radiation Penetration — parts (b)(ii)–(iv)
• 7.9–7.10: Detection of Ionising Radiation and Background Radiation — parts (b)(i)–(iii)
• 7.4–7.5: Types and Properties of Alpha, Beta, and Gamma Radiation — part (b)(iv)
▶️ Answer/Explanation
(a)(i) Unit of activity [1 mark]
A suitable unit for activity is the becquerel, \(\mathrm{Bq}\).
(a)(ii) Radioactive decay graph [3 marks]
The initial activity is \(800\) units, so the graph should start at:
\((0,800)\)
After one half-life of \(6.7\,\mathrm{h}\), the activity is halved:
\(A=\dfrac{800}{2}=400\)
So the graph should pass through:
\((6.7,400)\)
The subsequent values are \(200\) units after \(13.4\,\mathrm{h}\) and \(100\) units after \(20.1\,\mathrm{h}\).
The decay curve should be smooth and decrease with decreasing steepness.
(a)(iii) Beta decay equation [2 marks]
In beta-minus decay, the mass number remains unchanged, while the atomic number increases by \(1\).
Protactinium has atomic number \(91\), so uranium has atomic number \(92\).
Therefore:
\(\boxed{{}^{234}_{91}\mathrm{Pa}\rightarrow{}^{234}_{92}\mathrm{U}+{}^{0}_{-1}\beta}\)
(b)(i) Dependent variable [1 mark]
The dependent variable is the quantity being measured in response to changing the material placed between the source and detector.
Therefore, the correct answer is A, count measured by the detector.
(b)(ii) Background radiation [3 marks]
- Remove the radioactive source from the experimental area.
- Measure the background radiation count for one minute using the same detector.
- Subtract the background count from each experimental result.
This gives a corrected count due to the radioactive source rather than the background radiation.
(b)(iii) Improving reliability [2 marks]
Repeat the count measurements for each material and calculate a mean value. Repeats help reduce the effect of random variation in radioactive count readings.
(b)(iv) Identifying the radiation [3 marks]
The count decreases significantly when the sheet of paper is placed between the source and detector.
There is no additional significant decrease in the count when aluminium and lead are used instead.
Alpha radiation is stopped by paper, so the results are consistent with the source emitting alpha radiation.
Therefore: \(\boxed{\text{protactinium-231 emits alpha radiation}}\)