What the activity is for
The diagnostic questions can be used for two main purposes:
What the activity is for
What happens during this activityIt would be a good idea to get the students to work in pairs on these questions, encouraging each pair to talk through their ideas with each other. Collect responses from all of the pairs and discuss in a whole-class plenary. Alternatively, the questions might be set for homework prior to the lesson, so that you have time to read through the responses. The questions review the basic features of ionising radiation: Question 1: Nature of the radiation Questions 2–8: Absorption, penetration and ionisation Question 9: Irradiation and contamination Question 1: Which one of the following is emitted by some radioactive nuclei and is also classed as an electromagnetic wave?
The question set continued
Question 4: A radioactive beta source is placed at the top of a glass tank full of water and a radiation detector is placed at the bottom. A plug is removed from the bottom of the glass box and the water drained out. If the count rate is continually recorded during this process, which sketch graph below best represents the count rate against time?
Answer C: Initially, the beta radiation must travel through the full tank of water, so there will be a high level of absorption and the initial count rate at t is 0, will be low. Graphs A and C are consistent with this initial condition. As the water drains out of the tank there is progressively less liquid to absorb the beta radiation, and so the count rate must increase. Since the radioactive source is emitting only beta radiation, it is to be expected that as the water level gradually falls, the count rate gradually increases (as in Graph C).
Question 5: Radioactive xenon–133 is a gas used to check for blockages inside the lungs. It is put in the lungs and a radiation detector outside the body takes readings. Which statement best describes why it is important in this situation that the source gives off gamma and not alpha radiation?
Question 6: The drawing shows a source of beta radiation about 20 cm from a radiation detector and electronic counter. What is the best action to take to increase a 10 s count on the electronic counter?
Answer D: The best action to take is to reduce the amount of air between the source and the detector. This will lower the rate of ionisation as the beta radiation travels to the detector and so increases the 10 s count.
Question 7: Ionisation paths are caused by alpha radiation passing through air. If a source producing alpha radiation at the same rate but with less energy replaces the original, what description will best describe the new tracks?
Answer C: The number of tracks is a measure of the activity of the source. Each alpha particle emitted from the source produces a track, so the more active the source, the more tracks are produced. If alpha radiation is produced at the same rate there will be no change in the number of tracks. The length of the track is a measure of the energy of the emitted alpha particle. The longer the track, the greater the initial energy of the emitted alpha. So, taking these two factors together, alpha radiation produced at the same rate but with less energy must produce a similar number of shorter tracks.
The final questions
Question 8: Which description best describes what happens inside a sheet of metal when it stops beta radiation?
Answer B: Ionisation involves removing electrons from atoms.
Question 9: In step 1 an apple is exposed to radiation from a radioactive source. In step 2 the source is then removed to leave the apple on its own. Some students are talking about this and make the following comments:
Answer B: In step 1, the apple has been irradiated, but there is no contamination: in other words, no radioactive material from the source ends up on the apple. Furthermore, irradiation cannot lead to the apple becoming a source of radiation.