Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How will you:

• deal with perceptions of risk
• draw out children's understandings of radioactive
• connect ionising radiations with other radiations

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How will you:

• separate ionisation from contamination
• separate penetrating power from activity
• separate damage done from activity
• connect ionising radiations with other radiations
• explain half-life, connecting this to randomness events, and to safety

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Building up an understanding of something that is not visible

Teaching and learning about ionising radiation takes us into one of those physics contexts where the phenomenon being studied, in this case the radiation, is not directly visible. The challenge for the learner (and for the teacher) is therefore to build up a picture of the similarities and differences between the types of radiation in terms of their properties (what they do) and origins (where they come from). Here we'll consider teaching and learning challenges relating to the properties of ionising radiation before moving on to the origins/sources of ionising radiation in episode 05.

A further challenge for teaching and learning in this area of physics follows from the fact that ideas about ionising radiation and radioactivity are quite commonplace in everyday situations. Thus students are likely to have heard or read about:

• Debates relating to nuclear power, particularly in light of the Fukushima Power Station disaster.
• The terrible damage from nuclear weapons.
• The use of radioactive materials in medical contexts.

Such existing knowledge is good because it provides a starting point for subsequent teaching and learning to build upon. At the same time, the teacher needs to be aware that these common-sense ideas are often at odds with the physics point of view and therefore need to be challenged directly if a deep understanding is to be the outcome:

Teacher: So today we're going to start a new topic: radioactivity. Who can tell me anything about radioactivity?

Jade: It's dangerous. Look at that Japanese accident!

Teacher: Well, in some situations it is, in some it isn't. We need to find out!

Students' understandings of aspects of radioactivity

The following questions were given to 144 school students aged 15–16 years and drawn from four schools. The students had all previously studied the topic of radioactivity.

The first question focuses on a familiar school demonstration with a radioactive source and Geiger–Müller tube, which students are likely to have seen in their lessons and involves radiation travelling a short distance. The second question focuses on a familiar example of contamination, over large distances, from the Chernobyl accident. The questions were developed together to probe students' explanations of effects at large and small distances. Both questions were answered by all of the students.

Students' views about why the count rate falls with distance

In a demonstration experiment, a teacher places a Geiger counter near a radioactive source. The counter counts quite rapidly. The teacher then moves the Geiger counter away from the source. As she does this, the count rate gradually goes down. By the time she has moved the counter right across the room, the Geiger counter has almost stopped recording any counts. Explain why the Geiger counter counts less and less as it is moved away from the source.

About 5 % of students gave complete responses using correct scientific ideas, while about 40 % of students gave incomplete responses using correct ideas.

About 40 % of students gave responses including incorrect scientific ideas.

The correct scientific ideas were as follows:

• Emitted radiation diverges from source.
• Radiation causes ionisation of the air.
• Intensity (count rate) decreases with distance.

The incorrect ideas included:

• The source emits radioactive material into the atmosphere (see Challenge: Sorting out words and ideas.
• Radioactive particles reach the tube (see Challenge: Are alpha particles radioactive?).
• Radioactivity is emitted (see Challenge: Sorting out words and ideas).
• Radioactive source becomes weaker.

Students' views about radioactive fallout

In April 1986, a serious accident occurred at the nuclear power station at Chernobyl in Russia. A week later, radiation detectors (Geiger counters) in Britain recorded higher than usual levels of radiation. Britain is more than 1000 miles from Chernobyl! Explain what reached the Geiger counters in Britain to make them record extra counts.

About 12 % of students gave complete responses using correct scientific ideas, while about 24 % of students gave incomplete responses using correct ideas.

About 50 % of students gave responses including incorrect scientific ideas.

The correct scientific ideas used were:

• Radioactive material reached the UK.
• Radioactive material was transferred by the wind.
• Radioactive material emits radiation.

The incorrect ideas included:

• Radiation was carried by the wind (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).
• Radiation polluted the atmosphere in UK (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).
• Gamma radiation reached the UK (see Challenges: Sorting out words and ideas and The difference between radiation contamination and irradiation?).

Thinking about the teaching

The numbers of students giving complete correct responses to these two questions is strikingly low at 5 % and 12 %, respectively. Viewed the other way round, 40 % and 50 % of students gave responses including incorrect scientific ideas. What are the messages for teaching and learning? The key point is that the incorrect responses all map onto the learning challenges identified in this episode. An optimistic point of view is that if these challenges are addressed directly through teaching then there is a greater chance of more positive learning outcomes.

Radioactivity, radioactive material and radiation

Wrong Track: Radioactivity was carried across the sea by the wind from the damaged nuclear reactor.

Wrong Track: Radiation was carried across the sea by the wind from the reactor.

Right Lines: Radioactive material/dust was carried across the sea by the wind from the damaged reactor.

Teaching the underlying meanings

Thinking about the learning

It is quite common for students to use the three terms radioactivity, radioactive material and radiation interchangeably.

Thinking about the teaching

In addressing this teaching and learning challenge it's not just a matter of learning new words. The underlying meanings of the terms need to be developed:

• Radioactivity is the general name for the phenomenon. There are parallels here with use of the word electricity (See the SPT: Electric circuits topic). Just as electricity does not flow around a circuit, radioactivity is not given out by a radioactive material. In fact (as with electricity), once the term Radioactivity has been used as a title for this topic, students will not really need to use it again. Some teachers go so far as to ban its further use.
• The radioactive material is the bulk substance, such as isotopes of uranium, which emits radiation.
• The radiations are often emitted with distinctive properties: alpha, beta and gamma radiations are three that do have such distinctive properties.

What's the difference between radiation, contamination and irradiation?

Wrong Track: Radiation was given out from the ruined reactor at Chernobyl, travelled through the air and got into the grass in North Wales and on the Lakeland hills in Cumbria, so affecting the sheep.

Right Lines: When the explosion occurred at Chernobyl, the resulting fire sent a huge plume of radioactive dust or fallout into the atmosphere over an extensive area. The plume drifted over large parts of the Western Soviet Union and much of Northern Europe, including North Wales and the upland hills of Cumbria. This radioactive dust settled and was ingested by the sheep as they nibbled away at the grass. In this way, accumulations of radioactive material built up in the bodies of the sheep, creating a danger for anybody later eating meat from those sheep.

Teaching about radiation contamination and irradiation

Thinking about the learning

This teaching and learning challenge is related to the previous one. If students are not clear about the difference between radiation and radioactive material, confusion between the processes of radiation contamination and irradiation is likely to follow.

Thinking about the teaching

The difference between radiation contamination and irradiation can be made clear by drawing upon examples such as the Chernobyl accident.

Teacher: The problem with the sheep in Cumbria and North Wales came about due to contamination by radioactive dust carried by the wind at high altitudes. In other words, the radioactive dust, or fallout, fell on the grass, which was then eaten by the sheep. On the other hand, irradiation was a terrible danger for all of those people on the site at Chernobyl who were exposed directly to radiation from the radioactive fuels in the damaged reactor core. The basic point to understand here is that radiation from radioactive materials on the Chernobyl site could not possibly have travelled all the way to the UK. It was the radioactive dust that did the travelling and that contaminated the hillsides.

Lots of people believe that if an object such as an apple is irradiated then it becomes radioactive

Wrong Track: I don't know much about irradiation of food, but it doesn't seem natural to me and I think that radiation would be left in the food. So I would avoid irradiated foodstuffs.

Right Lines: Careful irradiation of foodstuffs kills any bacteria on the food and this helps to preserve it for longer. The food itself can't become radioactive.

Teaching and learning about irradiation

Thinking about the learning

The idea of irradiated objects necessarily becoming radioactive is quite common in everyday thinking but is not correct. Many believe that objects like syringes and dressings that are sterilised using radiation will become radioactive (and so emit radiation). Similar ideas are found regarding the irradiation of food. For example, some people are nervous that the radiation that is used to kill any bugs around a box of apples, might actually make the apples radioactive. This kind of thinking involves seeing the radiation as somehow becoming stored inside the absorber (I think that radiation would be left inside the food) such that it can be re-released later.

However, please be aware that irradiation with neutrons can result in nuclear changes, perhaps producing unstable nuclei. That's what the careful is all about in the right lines quote – don't choose a radiation that can trigger nuclear changes.

Thinking about the teaching

This teaching and learning challenge takes us back once again to stressing the difference between irradiation and contamination.

Teacher: Cobalt–60 is commonly used in food irradiation. This source emits gamma radiation, which acts to kill any bacteria, viruses or insects on the food. The gamma radiation also irradiates the food and although there may be small chemical changes, it can't make the food radioactive. It's not as though any cobalt–60 is left on the food. This is not a case of contamination.

Alpha particles, beta particles and gamma radiation

It is quite common for teachers and students to talk about alpha, beta and gamma radiation. In fact, on first discovery (around 1900), all three were referred to as rays before their actual identity was established. Now we know that only gamma is a form of electromagnetic radiation, while alpha and beta radiation consist of streams of fast-moving particles.

The collective term radiation conceals the differing nature of the radiations. But as they all have the same effects, that's perhaps not a bad thing at this stage. Later on, in post-16 studies, it might be more appropriate to focus on the differences, when you can explain these more explicitly.

Is it correct to say that alpha particles are radioactive?

Wrong Track: The radioactive material gives out alpha particles, which are radioactive and therefore can do harm to human tissue, causing cancers.

Right Lines: The source gives out alpha particles. Each alpha particle consists of two protons and two neutrons. The alpha particles are potentially dangerous as they can disturb the molecular composition of human tissue, increasing the risk of cancer.

Alpha particles are not radioactive

Thinking about the learning

Some students attribute the property of being radioactive to alpha or beta particles.

Thinking about the teaching

The important point to stress here is that alpha particles are not in themselves radioactive. Each alpha particle consists of two protons and two neutrons and that's it. The particles do not have some mysterious, additional property of being radioactive.

Teacher Tip: The alpha particles are the radiation: nothing more, nothing less!

Does the rate of counting of a Geiger–Müller tube indicate how dangerous a source is?

Wrong Track: If the Geiger–Müller tube tube clicks or counts at a high rate, this must mean that there is a very dangerous radioactive source close by.

Right Lines: The rate of clicking or counting of the Geiger–Müller tube measures the activity of the source and is not a direct indicator of how dangerous it is.

You can't tell how dangerous a source is just from the activity

Thinking about the learning

When observing demonstrations with radioactive sources and a Geiger–Müller tube, students are likely to associate a high rate of counting with a high level of danger from a source.

Helen: Wow! Look at the counter. It's flying round! I wouldn't like to get too close to that!

Thinking about the teaching

The rate of counting of the Geiger–Müller tube is not a direct measure of the potential danger of a source. For example, it could be that the high rate of counting is due to a high-activity beta source that, in fact, has a less harmful effect at close range on human tissue than an alpha source of similar activity. Thus if both sources were swallowed or breathed in, the highly ionising alpha source would be much more dangerous. The beta source is releasing electrons at a high rate but these have a smaller damaging effect on human tissue. So, you can't tell simply by the rate of clicking of a Geiger–Müller tube how dangerous a radioactive source is.

Is gamma more dangerous because it has greater penetrating power?

Wrong Track: There are three types of radiation. Gamma is the most dangerous because it can even go through a sheet of lead. Alpha can be stopped by a sheet of paper so it is least dangerous.

Right Lines: The danger of a source to human tissue is not necessarily linked directly to its penetrating power.

You can't tell how dangerous a source is just from the penetrating power

Thinking about the learning

Students often assume that gamma radiation is more dangerous than alpha and beta because it has a greater penetrating power.

Thinking about the teaching

Students tend spontaneously to associate the greatest penetrating power with the greatest danger, but in fact alpha radiation, which has the smallest penetrating power, can in some cases be the most dangerous. For example, if the alpha-emitting radioactive material is taken into the body through the mouth or lungs, highly-ionising alpha radiation is emitted directly into human tissue with critical consequences.

Do alpha particles cause more ionisations of the air than beta particles?

Wrong Track: Alpha radiation is more ionising than beta radiation.

Right Lines: If both alpha and beta particles start with the same energy, then they will each undergo the same number of ionising collisions, shifting the same amount of energy per collision with air molecules before coming to a stop.

Rate of ionisation and total ionisation

Thinking about the learning

It is quite common to see statements along the lines of alpha particles are more ionising than beta, but this is not actually the case if both kinds of particle start with the same energy.

Thinking about the teaching

The point to stress here is that the alpha radiation causes ionisation over a shorter path, creating a higher density of ionising collisions than beta radiation.

The greater the rate of ionisation, the shorter the path length

Wrong Track: If beta particles travel farther in air, their rate of ionisation of air molecules must be greater than for alpha particles.

Right Lines: If beta particles travel farther in air, their rate of ionisation of air molecules must be less than for alpha particles. The longer path in air is linked with a lower rate of ionisation.

Why is the rate of ionisation of air greater for alpha particles?

Thinking about the learning

The thinking that causes difficulty here follows on directly from the previous challenge. Relationships of the form more of this results in more of that are common in physics. For example, a larger battery potential difference drives a bigger electric current around a fixed circuit. In this case, however, there is an inverse relationship between path length and rate of ionisation.

Thinking about the teaching

The key point to address here is why the rate of ionisation is greater for alpha than for beta particles. In other words, why is it that with alpha particles the initial energy of the particle is shifted within a relatively short path? The idea to get across here is that the alpha particles, in comparison with beta particles, are massive and carry double the charge. This means that they are much more likely to interact with air molecules as they pass through air causing ionisation.

The ionisation process is often described in terms of the alpha particle knocking an electron out of the atom, which gives rise to an image of some kind of physical collision. In reality, the collision is electrostatic in nature as the positively charged alpha particle removes the negatively charged electron through a process of electrostatic attraction.

There's a good chance you could improve your teaching if you were to:

Try these

• dealing with the properties of ionising radiations before exploring their sources
• linking the behaviour of nuclear radiations to the behaviour of radiations from other sources
• exploiting the photon model to make the link between ionising and non-ionising radiations
• using the introduction of detectors as an opportunity to reinforce the process of ionising
• relating half-thickness and half-life
• comparing risks from ionising radiations to risks from other sources

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• treating ionising radiation as a completely isolated phenomenon
• getting bogged down in the different measures of ionising radiations
• not challenging gut-reaction reponses

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.