Types of radiation and nuclear equationsEdexcel A-Level Physics: Subtopic test

1. 1
   A student uses a Geiger-Müller (GM) tube and counter in a school laboratory. Before a source is brought near, the tube records 96 counts in 5.0 minutes. A gamma source is then placed near the tube, and the tube records 1440 counts in 2.0 minutes.

   (a)

   Why does the student record the count before the source is brought near?

   [1 mark]

   • ATo calibrate the voltage across the GM tube
   • BTo find the background count rate so that it can be subtracted from the count rate with the source
   • CTo check that the source emits only gamma radiation
   • DTo allow the GM tube to warm up

   (b)

   Which of these is NOT a source of background radiation?

   [1 mark]

   • ARadon gas from rocks
   • BCosmic rays from space
   • CPotassium-40 in food and the human body
   • DVisible light from the laboratory lamps

   (c)

   Calculate the corrected count rate from the source, in counts per minute.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An ionisation smoke detector contains a tiny sample of americium-241, ${}^{241}_{95}\text{Am}$, which is an alpha emitter. The sample is held in a small open chamber inside a plastic case, and the alpha particles ionise the air in the chamber so that a small current flows between two electrodes.

   (a)

   What is an alpha particle?

   [1 mark]

   • AA helium nucleus, made of two protons and two neutrons
   • BA fast electron from the nucleus
   • CA high-energy photon of electromagnetic radiation
   • DA hydrogen nucleus

   (b)

   Which equation correctly represents the alpha decay of americium-241?

   [1 mark]

   • A${}^{241}_{95}\text{Am} \rightarrow {}^{237}_{94}\text{Pu} + {}^{4}_{2}\text{He}$
   • B${}^{241}_{95}\text{Am} \rightarrow {}^{239}_{93}\text{Np} + {}^{4}_{2}\text{He}$
   • C${}^{241}_{95}\text{Am} \rightarrow {}^{237}_{93}\text{Np} + {}^{4}_{2}\text{He}$
   • D${}^{241}_{95}\text{Am} \rightarrow {}^{241}_{96}\text{Cm} + {}^{0}_{-1}\text{e}$

   (c)

   Explain why an alpha emitter is suitable for use in a smoke detector.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cobalt-60, ${}^{60}_{27}\text{Co}$, emits beta-minus particles and gamma radiation. It is used to sterilise sealed plastic packs of surgical equipment in hospitals. The nucleus formed by the beta-minus decay is nickel, and it is formed in an excited state.

   (a)

   Write the nuclear equation for the beta-minus decay of cobalt-60.

   [3 marks]

   (b)

   Explain why gamma radiation is emitted after the beta-minus decay, and why gamma radiation rather than alpha or beta radiation is used to sterilise the sealed packs.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student investigates how lead absorbs the gamma radiation from a caesium-137 source. A GM tube is placed at a fixed distance from the source, and lead sheets are placed between them. The background count rate is subtracted, and the corrected count rates are 640, 410, 270, 170 and 112 counts per minute for lead thicknesses of 0, 5, 10, 15 and 20 mm.

   (a)

   Describe how the student should carry out the investigation to obtain reliable results, including a safety precaution.

   [6 marks]

   (b)

   Use the data to show that the absorption of the gamma radiation is exponential, and determine the thickness of lead needed to reduce the count rate to 1% of its value with no lead. Comment on your result.

   [6 marks]

   Total for question 4: 12 marks

End of questions