The Maths and Science Tutor
Email: fiaraziqbal@googlemail.com
Phone: 07760257814
Total marks: 25
Time allowed: 35 minutes
[4 marks]
1. A radioactive sample contains two isotopes, X and Y. Isotope X has a half-life of 2 days, while isotope Y has a half-life of 4 days. Initially, there are 800 atoms of X and 800 atoms of Y.
a) How many atoms of each isotope will remain after 8 days?
b) Calculate the ratio of isotope X:Y at 8 days.
[3 marks]
2. A smoke detector shows a steady count rate of 300 counts per minute when functioning normally. When smoke enters, this drops to 30 counts per minute. The background radiation is 20 counts per minute.
Calculate the percentage reduction in the true count rate when smoke is present.
[3 marks]
3. After a nuclear event, the count rate from a sample is 960 counts per minute. The background count is 40 counts per minute. Calculate the time taken for the true count rate to fall to 115 counts per minute if the half-life is 20 minutes.
[3 marks]
4. A uranium nucleus undergoes the following decay sequence:
α decay → β decay → α decay → β decay
Calculate the overall change in:
a) Mass number
b) Atomic number
[3 marks]
5. The intensity of gamma radiation falls to 25% of its original value when passing through 5cm of lead.
a) What thickness of lead would reduce the intensity to 1/16 of its original value?
b) Explain your reasoning.
[2 marks]
6. A source placed 2m from a detector gives a count rate of 240 counts per minute. What is the minimum distance the source must be placed from the detector to give a count rate below 50 counts per minute?
[3 marks]
7. In nuclear power stations, neutrons are slowed down by moderators before they can cause nuclear fission. Explain why this increases the probability of nuclear fission occurring. Use ideas about nuclear forces in your answer.
[2 marks]
8. A radioactive source has an activity of 4000Bq. Its half-life is 1 hour. Calculate its activity, in Bq, after 3.5 hours.
[1 mark]
9. Explain why the count rate from a radioactive source never reaches exactly zero, even after many half-lives have passed.
[1 mark]
10. β⁻ decay involves the emission of an electron. Explain why this electron cannot be one of the orbital electrons from the atom.
End of questions