Questions: nature and properties of ionising radiation

What the Activity is for

The diagnostic questions can be used for two main purposes:

• To encourage students to talk and think through their understandings of the fundamental nature and properties of ionising radiation.
• To provide the teacher with formative assessment information about the students' understandings of ionising radiation.

What to Prepare

• Printed copies of the support sheets, diagnostic questions (see below)

What Happens During this Activity

It 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?

1. Infrared radiation.
2. Gamma radiation.
3. Alpha radiation.
4. Neutron radiation.
5. Ultraviolet radiation.

Question 1 answer: 2.
Infrared and ultraviolet radiations are electromagnetic waves, but they do not originate in the nucleus. Alpha and neutron radiation consist of streams of particles.

Question 2: An ionised material differs from one that isn't ionised in that:

1. It has had electrons knocked out of its atoms.
2. It contains radioactive atoms.
3. It is a gas as opposed to a solid.
4. It emits beta radiation.
5. It has a shorter half-life.

Question 2 answer: 1.
Ionisation involves removing electrons from atoms.

Question 3: A student has been given an old watch. It has radioactive paint on its dial. He puts the watch close to a radiation detector and then puts sheets of different materials between the watch and the detector. A sheet of paper makes little difference to the count rate. A sheet of lead, 1 mm thick, reduces the count rate considerably. What is the watch emitting?

1. Alpha radiation.
2. Beta radiation.
3. Microwaves.
4. Neutrons.
5. X-rays.

Question 3 answer: 2.
The watch must be emitting beta radiation, which passes easily through paper but is stopped by lead.

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?

Question 4 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 = 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?

1. Gamma radiation is absorbed more easily than alpha radiation.
2. Gamma radiation is more densely ionising than alpha radiation.
3. Gamma radiation is unaffected by an electric field unlike alpha radiation.
4. Gamma radiation is more penetrating than alpha radiation.
5. Gamma radiation is unaffected by a magnetic field unlike alpha radiation.

Question 5 answer: 4.
In this application the key feature is that the gamma radiation can pass from inside the lungs to the detector on the outside. If the source gave off alpha radiation, nothing at all would be detected on the outside of the body.

More questions

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?

1. Move the source further from the detector.
2. Place a mirror behind the beta source.
3. Put a thin sheet of metal between the source and the detector.
4. Reduce the amount of air between the source and the detector.
5. Wait for a time equal to the half-life of the source.

Question 6 answer: 4.
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?

1. No change.
2. Similar number but longer.
3. Similar number but shorter.
4. Less in number and shorter.
5. More in number and shorter.

Question 7 answer: 3.
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?

1. The beta radiation energy is trapped in the nuclei of the metal atoms.
2. The beta radiation energy is lost by knocking electrons out of metal atoms.
3. The beta radiation energy cancels out with the metal protons.
4. The beta radiation energy sticks to the metal atoms.
5. The beta radiation energy evaporates the metal atoms.

Question 8 answer: 2.
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:

1. In step 1 the apple has been contaminated.
2. In step 2 the apple will not be a source of radiation.
3. In step 2 the apple will be a radioactive source.

Which of the suggestions are correct?

1. 3 only
2. 2 only
3. 1 & 3
4. 1 only
5. 1, 2 and 3

Question 9 answer: 2.
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.