Example Example 1: A number of lead blocks and aluminium plates are placed in front of a radioactive source. The source emits ,  and  radiation.

Example 1 mm aluminium plate 5 mm aluminium plate Lead blocks Radioactive source X Y Z GM counter GM tube

Example a) What is the function of the two lead blocks? The lead blocks provide a narrow opening through which the radiation passes. This prevents scattering of the particles.

Example b) What kind(s) of radiation can be found between X and Y? Only  and  radiation are found between X and Y because the  particles are stopped by the 1mm thick aluminium plate.

Example c) What would happen to the count rate if the detector were placed at Z? If the detector were at Z, the count rate would drop because the  particles are stopped by the 5 mm thick aluminium plate. However, we can still detect  radiation.

Example d) Explain briefly why the Geiger-Muller counter still records some counts when the radioactive source is removed. When the radioactive source is removed, the Geiger-Muller counter still records the background radiation.

Example Example 2: In a paper-making factory, the thickness of the paper produced is detected using a  source and a detector, as shown.

Example radioactive source paper feeding through rollers rollers GM tube GM counter

Example a) Explain how changes in the thickness of the paper can be detected.  particles do not have great penetrating power so any change in the thickness of the paper will affect the number of particles that reaches the counter.

Example b) Why are  and  sources not suitable for this process?  particles would be absorbed by the paper.  rays would not be stopped by the paper at all because of its high penetrating power.

Example Example 3: A Geiger-Muller tube is placed at Y, 25 cm from a radioactive source that emits ,  and  radiation. lead container GM tube radioactive source S Y 25 cm

Example a) What is the purpose of the lead container? The lead container prevents scattering of the radiation. b) What possible types of radiation may be received by the Geiger-Muller tube? The possible types of radiation received are  and  radiation.

Example c) (i) When a magnetic field is applied at right angles to the page, as shown below, the recorded count rate is reduced. Explain briefly. lead container GM tube X Y S Z radioactive source magnetic field pointing out of paper

Example magnetic field pointing out of paper radioactive source lead container GM tube S X Y Z    The count rate drops because the  and  particles are deflected by the magnetic field.

Example c) (ii) What happens to the count rate when the Geiger-Muller tube is placed at X and Z? At X, only the  radiation is recorded, so the count rate is lower. At Z, only the background radiation is recorded, which is much lower.

Example d) If a sheet of lead about 25 mm thick is placed in front of the Geiger-Muller tube at position Y, what happens to the recorded count rate? 25 mm of lead will block half of the  rays, so the recorded count rate will be halved. End