• ## 01 Half-thickness and fractional decayRa04TAnugget01 Activity

### A half thickness for cucumber

What the activity is for

Half-thickness is a useful and accessible introduction to constant fractional decay. Somehow adding extra slices of thickness, and seeing their effect on a count rate, is more direct than allowing extra intervals of time to pass and seeing their effect on count rate.

The choice of exact fruit or vegetable is left to you, to suit your source and counter. However, we recommend using a fruit because this is biological tissue, so making it easier to connect the effects to something that will be of importance to students.

What to prepare

• a piece of fruit
• a ruler
• a sharp knife
• chopping board
• a beta or gamma source
• a counter connected to a detector

Safety note: Students under the age of 16 years are not allowed to handle radioactive sources. Please take all reasonable precautions to ensure that the source is guarded at all times.

What happens during this activity

During this experiment, equal thicknesses of absorber are placed sequentially between the source and the detector. Start by creating these equal thicknesses of the absorber, slicing up the fruit as you discuss the extra material that will be added between the source and the detector as you add each thickness. Here's a tip: slice off the bottom of the fruit before you start, to make sure that it will stay still once in place. You'll need to practise before you start, to match the density of the fruit with the appropriate thickness of slice and the count rate and penetration offered by your source. Ideally the count rate will drop to close to zero after about seven slices are added.

Now set up the counter and the source a fixed distance apart, so that you can later add seven slices between the source and the detector. Talk through the addition of each slice in terms of the extra chance of the ionising radiation colliding with the material in that slice. Each slice adds a constant effect. So adding each slice reduces the current rate of the ionising radiation by a constant fraction. This is the important pattern. There is no completely safe thickness – each additional thickness produces a reducing return.

You can also do a similar experiment with 3 cm waves and textbooks. Each extra textbook reduces the radiation by a constant fraction.

We do suggest this emphasis on the constant – pause – fractional – pause – decay, rather than emphasising the half-thickness. We think it's better to see the half-thickness as a consequence of, and one measure of, the constant fractional decay rather than as a magic number.

• ## 02 An experimental approach to cloud chambersRa04TAnugget02 Activity

### Understanding cloud chambers

What the activity is for

This is a four-part activity that tells the story of the detection of ionising radiation using cloud chambers.

The cloud chamber is one of the very few activities that will enable students to see the effects of ionising radiation. You can set up a cloud chamber as a demonstration but it is better if students can work in groups with their own cloud chamber. Seeing the tracks being formed is hugely engaging and the subsequent discussion enables them to understand what happens during the process of ionisation.

The next demonstration shows how clouds are formed in a jar and demonstrates the need for condensation nuclei.

This is followed by a demonstration of ions forming a cloud using a high voltage to produce the ions.

Finally, the expansion cloud chamber provides a different method of producing tracks that includes a lot of the ideas already discussed.

What to prepare

Cloud chambers per group:
• Taylor diffusion cloud chambers
• lamps (12 V, 24 W) and power supply
For the teacher:
• dry ice or CO2 cylinder with attachment
• ethanol in a dropper bottle

Safety note: Cloud chambers contain weak radioactive sources and must not be used if there are any signs of flakes of radioactive paint inside the chamber. Most of the newer sources are enclosed. Make sure that there are no naked flames in the room when using ethanol. Wear eye protection and gauntlet-style leather gloves when making or handling solid carbon dioxide.

What happens during this activity

Demonstrate how to set up the cloud chamber. Start by removing the perspex lid and putting a few drops of ethanol on the black felt at the top of the chamber. Put a couple of drops on the black base of the chamber, making sure that no alcohol falls on the source. Put the perspex lid back on and turn the chamber upside-down. Remove the screw top and the foam. Wearing goggles and using appropriate gloves, put a handful of dry ice into the bottom of the chamber. You don't need very much dry ice – enough to cover the palm of your hand will work well (about a dessertspoonful). Replace the foam and screw the lid on. Turn it back the right way up. Make sure that the perspex lid is clean, particularly the underside. There are wedges that enable you to make the cloud chamber level and finally illuminate the chamber with the lamp, pointing it towards the source and slightly downwards. If you don't see tracks, try rubbing the top of the chamber with a cloth to charge it slightly. You should see faint straight tracks, like the vapour trails of aircraft.

### A discussion, starting with the vapour trails

Teacher: Sometimes a plane is so high that you can't see it but you can see two vapour trails. Why is that?

Sam: It's too high up.

Teacher: Right! But the vapour trails look a bit like clouds. How do clouds form?

Sam: Is it like steam from a kettle? The water is heated by the Sun and when it cools down it makes clouds?

Teacher: That's partly right. You are right about the water evaporating because of heat from the Sun but it takes a bit more than just cooling down. Imagine the Sun evaporating water from the sea, and that water rising and cooling down. What would make those water molecules join together to make big enough droplets to reflect sunlight so we see them as white and fluffy?

Sam: Does gravity pull them together?

### The conversation develops

Teacher: Good try, but no. First the air needs to have a lot of water molecules in it, so that we say that it is saturated. A bit like if you put loads and loads of sugar in your tea until eventually you can't get any more to dissolve. Then it needs to cool down, and then you need little tiny particles, like dust or ice. The water molecules condense on the dust or ice and make droplets, and lots of droplets make a cloud. So let's get back to planes. What might be coming out of the back of the aircraft that the water could condense on?

Sam: Exhaust fumes?

Teacher: Right idea. When the plane burns fossil fuels, one of the things produced is water vapour, so that helps to saturate the air. It's cold up there so all the conditions are right for making the water molecules condense to make droplets.

Teacher: So in our cloud chamber what did we saturate the air with? Was it water?

Sam: No, it was ethanol on the black felt.

Teacher: Good. So what was the ethanol condensing on?

Sam: The alpha particles?

Teacher: Think again. If it was condensing on each alpha particle as it was emitted, why would we see a long track? Would it not stop the alpha particle?

Sam: Something that the alpha particle is leaving behind, like the exhaust gases.

Teacher: Excellent. As the alpha particle moves through the air, the air is ionised. That means it removes electrons from air molecules leaving positively charged things called ions behind. The vapour that is cold and saturated condenses on the ions. We call them condensation nuclei – anything like dust, ice crystals or ions can do it. In fact, the ethanol works really well because an ethanol molecule is sort of oblong shaped with a positive charge at one end and a negative charge at the other. So it is attracted to anything charged. Ethanol molecules cluster around and make a droplet. Lots of droplets make a trail.

### A second step: clouds in a jar demonstration

What to prepare

• a 1 litre aspirator or large flask
• a bung to close lower outlet of aspirator
• a bung with glass tube
• a short length of rubber tubing
• a compact light source and power supply
• a large sheet of black card
• a box of matches

What happens during this activity

Start by putting a black screen behind the aspirator, and a bright light to the side. Then put a few millilitres of water into the aspirator, closing the bung and tube. Blow down the tube then pinch it with your fingers. Wait a minute for the air to cool. During this time you can talk about the similarities with the cloud chamber (saturating with water, cooling down). Turn the lights out, illuminate the aspirator and pull out the bung very quickly, allowing the air to expand. You should see a cloud forming.

Teacher: So what have I saturated the air with?

Sam: Water.

Teacher: Good. So why do we see a cloud forming when I take the bung out?

Sam: You have let some dirt in?

Teacher: Not quite. Think about the conditions for making vapour trails. The air is under pressure until I take the bung out. So what happens when I take the bung out?

Sam: The air cools?

### Engaging students in a development

Teacher: Good. When gases expand quickly they cool down. Think about the opposite. When you pump up your bike tyre the pump gets hot because you are compressing the gas. So now we have cool air saturated with water. So what happens next?

Sam: The water molecules make little droplets and we see a cloud.

Teacher: Good. If you repeat this several times you should see the clouds becoming thinner, or even not forming at all. This is a very interesting discussion point.

Teacher: Why are the clouds not forming now?

Sam: Has it run out of water?

Teacher: Not quite. We can still see some at the bottom of the jar. So it's still saturated and I keep cooling it down. What's missing?

Sam: There can't be anything for the water to condense on.

Teacher: Why not?

Sam: You have let them all out?

Teacher: Some ions have been used up making the other clouds. Some ions have sunk to the bottom of the jar. Let's see what happens when we add some more.

If you throw a lighted match into the aspirator clouds will form again.

Teacher: So why do we get clouds again now?

Sam: The match is putting smoke into the air for the water to condense on.

Teacher: Good. Not just smoke particles. The flame makes ions, just like the alpha particles did. A water molecule is a bit like ethanol – one end is positively charged and the other end negatively charged. So if we want to get water molecules to get together, ions are a good thing.

### A third step: ions start a cloud

What to prepare

• a Van de Graaff generator
• a 500 ml flask with bung and glass tube
• a glass tube drawn to a jet
• a Bunsen burner
• a tripod
• a retort stand, boss, and clamp
• a flexicam or webcam linked to a projector (optional)
• a compact light source and power supply (optional)

Safety note: Take care with the steam from the nozzle. There is a risk of scalding.

What happens during this activity

To make the effect clearly visible to the class either project a shadow onto the wall or a screen, using the compact light source at a distance of 1 m from the nozzle, or use a flexicam or webcam connected through a computer to a projector or interactive whiteboard.

Set up the nozzle a long way from the Bunsen flame (about 1.5 m) because the Bunsen makes so many ions that it will spoil the demonstration. Set up two wire electrodes to form a spark gap about 0.5 m wide and about 0.3 mm above the nozzle. This will ensure it is in the water vapour and not in the white cloud. Boil the water in the flask and observe the cloud formation as the vapour emerges from the jet. Highlight the gap between the cloud and the nozzle.

### Developing an understanding through discussion

Teacher: Look at the gap between the cloud and the nozzle. That's a bit odd. Why do you think there is a gap?

Sam: The cloud is so thin you can't see it?

Teacher: Think again. Think of the clouds in the jar. There is lots of water vapour and lots to condense on. What's missing?

Sam: Cooling down. So it has to move away and cool down before you get a cloud?

Teacher: Excellent! So here there is nothing to cool the vapour but as it moves that distance from the nozzle it cools down enough to become supersaturated. Then a cloud forms when the steam condenses on the particles in the air, just like the elements that went into our cloud chamber. We needed ethanol for the vapour, dry ice to cool it and something to condense on.

Switch the Van de Graaff generator on so that a stream of small sparks passes through the vapour jet. The cloud will be seen to intensify due to the production of ions, which act as condensation nuclei.

Teacher: Why is the cloud thicker?

Sam: The water molecules are condensing on a spark?

Teacher: Well, the spark is definitely having an effect. Let's think back to what the alpha particle was doing in the cloud chamber.

Sam: Making ions?

Teacher: Excellent. So here the spark is a result of the air conducting electricity. The few ions in the air are attracted to the positive or negative terminals and accelerate. They hit other air molecules and ionise them, and then make a sort of avalanche effect. So why does that make the cloud thicker?

Sam: There must be more particles for the water to condense on – all the ions.

Teacher: Good. That's just like an alpha particle making a trail.

### The expansion cloud chamber demonstration

What to prepare

• an EHT power supply (not an HT power supply)
• a source of alpha radiation (if it is not part of the cloud chamber)
• a pair of large forceps or pliers, if required
• a bicycle pump or other device for producing expansion
• a lamp, lens and power supply

What happens during this activity

Expansion cloud chambers are all slightly different in their operation, so you will need to refer to the manufacturer's instructions. The position of the lamp is often critical. Some have an evacuation mechanism, such as a bicycle pump, which removes the air, while others change the pressure of a water column.

### Working up an understanding of the apparatus

Teacher: Let's think about how this demonstration combines the cloud in the jar and the ions in the cloud demonstrations. Let's start with what is happening when I suddenly remove the air from the chamber. What else have I done in a different experiment that's a bit like this?

Sam: Is it like when you pulled the bung out?

Teacher: Exactly. In that case I had pressurised the gas and then let it out. Here I am removing some air with a pump. What effect does that have?

Sam: It cools the air down.

Teacher: Good. So then the air is saturated with ethanol and is cool. What happens next?

Sam: The alpha particles make ions and the ethanol condenses on the ions.

Teacher: Good. So that is just like our spark making our clouds thicker. After a while, though, we don't get tracks – they fade. In fact this cloud chamber is sometimes called a pulsed cloud chamber. It's not like the diffusion cloud chamber. So why don't the tracks last as long?

Sam: Have they run out of ions?

Teacher: Good try, but if that was the case that would happen in the diffusion cloud chambers as well. I'll give you a hint: this didn't happen in the diffusion cloud chamber because of the dry ice.

Sam: The air heats up?

Teacher: Exactly. If anything, in fact, there are too many ions. We have to keep a power supply connected to sweep away all the ions from the old tracks so we can see new ones when we expand the gas again.

• ## 03 An experimental approach to spark countersRa04TAnugget03 Activity

### Understanding spark counters

What the activity is for

Students will see a series of demonstrations, each presented as a way of consolidating the knowledge that ions are needed for current to flow or sparks to be produced. These lead to the demonstration of the spark counter and Geiger counter and ensure that they are more likely to understand how these counters work.

There are five demonstrations, leading to an understanding of the detection of the effects of the ionising effect of radiation. This sequence:

1. Leads students through a sequence that shows that charge carriers in the form of ions are needed to complete a circuit and light a lamp.
2. Uses a Van de Graaff generator to show that flames produce ions.
3. Achieves the same end but uses an EHT supply and a capacitor (and could be omitted if time was short).
4. Demonstrates a spark counter.
5. Demonstrates a Geiger counter.

What to prepare

• a lamp holder on base
• a lamp (12 V, 36 W) and power supply
• a 1000 millilitre beaker
• 2 copper electrodes
• a pair of crocodile clips and 2 connecting leads
• some distilled or de-ionised water
• a supply of table salt
• a teaspoon
• 2 retort stands, bosses and clamps

What happens during this activity

There are four connected parts in this first activity. One approach could be to do all four in quick succession and discuss the activity as a whole, or present it as a mystery to solve. Then you can go back to the beginning and discuss each part in turn, or ask more able groups to present their solutions to the mystery.

Connect up the circuit with the wires touching. The bulb will light up.

Now put the wires into the beaker. The bulb will not light up.

Now add deionised water to the beaker. The bulb will not light up.

Now add a handful of salt to the water to the beaker. The bulb will light up.

### Simple questions to get started

Teacher: So, let's start with the simple question. Why did the bulb light up?

Lydia: The circuit is complete.

Teacher: Good, but can you give me a better reason? Think back to what we learnt about electricity.

Lydia: Charges are flowing… electrons?

Teacher: OK, so did the electrons come out of the power supply when I turned it on?

Lydia: No, there are loads of them in the wires.

Teacher: Good. So why did the bulb not light with the electrodes in the air?

Lydia: There is just a gap. You didn't connect the wire. You need a complete circuit.

Teacher: Now think about when I put the water in. Does water normally conduct electricity? How do you know?

Lydia: Yes. You have pull switches in the bathroom.

Teacher: So why didn't the bulb light with that water?

Lydia: There are no electrons in the water?

Teacher: Not quite. The clue is in what I called the water. It is deionised, so it doesn't have any ions in it. An ion is an atom or molecule that has lost an electron or electrons. They are charged particles, and moving charged particles are a current. So what must the salt be doing to the water?

Lydia: Making ions, charged particles that can flow.

Teacher: Good. The salt is made up of sodium (WordSuper{Na}{+}) and chlorine (WordSuper{Cl}{-}) ions. The ions move because they are in the field made by the potential difference between the wires. Positive ions move one way and negative ions move in the opposite direction. That's a current, a flow of charge.

### Second experiment in the sequence

What to prepare

• a Van de Graaff generator
• a metal ball on metal rod on a lead connected to earth
• a means of holding the ball at a distance from the dome of the Van de Graaf generator
• a microammeter, light spot type (optional)

Safety note: In this demonstration, students will not be touching the generator. Make sure that it is fully discharged before touching it yourself. Do not add anything to it to increase the capacitance.

What happens during this activity

Bring the metal ball close enough to the dome of the Van de Graaff generator to produce sparks.

Move the ball away until it just stops sparking and leave it there. Now light a match and hold it under the gap to produce a spark. You could connect the microammeter between the ball and earth to show that charged particles are flowing.

You could ask the students to turn around and hear when you put the match under the gap. They won't hear the flame but they will hear the spark.

### Connecting the spark to ions

Teacher: Let's work out why we get sparks. What is a spark? Where have you seen them before?

Lydia: Lightning?

Teacher: Yes, lightning. What happens when lightning strikes? What is lightning?

Lydia: A lightning bolt comes down from the clouds.

Teacher: Well, what is happening is a bit like what is happening in the circuit in the first experiment. The winds inside the cloud separate out positive and negative charged particles, a bit like the charge on the dome of the Van de Graaff generator. That acts like the power pack in the first experiment. There was a gap there, and nothing happened. So nothing happens here until I bring the match close, just like nothing happened until I added the salt.

Lydia: So the flame must be doing the same thing as the salt. It must be adding electrons.

Teacher: Is it just electrons? What else did we get in the salt?

Lydia: Ions.

Teacher: Good. In fact the flame sets up a bit of a chain reaction. Positive ions attracted to the dome will hit air molecules and ionise them. They will then move and hit others, and so on. There is a current, the air heats up and the expansion makes the sound of the spark. The energy gained by the air molecules is given out as the light that you see. But when you look at the flame, can you actually see the ions being made?

Lydia: No, you can't. But you can see the spark.

Teacher: Great! So this is a way of 'seeing' ions that can't be seen directly. Let's do the experiment in a slightly different way.

### The third step

What to prepare

• a power supply (EHT, 0–5 kV, with internal safety resistor)
• 2 conducting spheres on insulated handles
• 2 retort stands and bosses
• a 0.1 microfarad EHT capacitor

Safety note: A school EHT supply is limited to a maximum current of 5 mA which is regarded as safe. For use with a spark counter, the 50 MΩ safety resistor can be left in circuit, so reducing the maximum shock current to less than 0.1 mA.

Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high-voltage conductors; use female 4 mm connectors where required.

The EHT capacitor is a special component. It has to be able to withstand a voltage of 5000 V. Do not use an ordinary capacitor in the circuit and certainly not a low-voltage electrolytic capacitor.

What happens during this activity

Set up the two conducting spheres about 1 cm apart using retort stands and bosses. Connect the capacitor across the gap. Connect one ball to the earthed negative terminal of the EHT supply. Connect the other ball to the positive terminal of the EHT. Increase the voltage until you get sparks between the gap. Decrease the voltage until it just stops sparking. Light a match and hold it underneath. You'll get sparks.

Teacher: So this really is an extension of our first experiment. We have a power supply and a gap, and nothing is happening. I add the flame and I get a spark. How is this like the experiment with the light bulb?

Lydia: The gap is like when you had the electrodes in the air or in the water with no ions.

Teacher: Good. So when I add the flame what is happening?

Teacher: Yes, and that cascade or avalanche effect is happening again. Well done.

### Finally – the spark counter

What to prepare

• a power supply EHT (0–5 kV, with the option to bypass the safety resistor)
• a spark counter
• a sealed source of radium (if available) or sealed source of americium-241
• a holder for radioactive source (e.g. forceps)

Safety note: You must have training before handling radioactive sources. The school will have a set of local rules to which you must adhere and a radiation protection supervisor who is responsible for checking that the sources are not leaking. All sources should be signed out of their store by you and a record kept of where and with which class you have used them.

What happens during this activity

Connect the positive, high-voltage terminal of the spark counter to the positive terminal of the EHT supply without the 50 MΩ safety resistor. (The spark counter's high-voltage terminal is joined to the wire that runs under the gauze.) Connect the other terminal on the spark counter to the negative terminal of the power supply and connect this terminal to earth.

Turn the voltage up until you get spontaneous discharge. This is usually at about 4500 V. Turn it down until it just stops sparking.

Start by using matches again to show that this operates on the same principle as the other two demonstrations involving sparks.

Use forceps to hold a radioactive source over the gauze. You should see and hear sparks jumping between the gauze and the high voltage wire underneath each time an alpha source is brought near to the counter. Move the source slowly away from the gauze and note the distance at which it stops causing sparks.

Teacher: So here we have a gap again and nothing happening. Now I bring the radioactive source close by and we get sparks. What is this detector detecting?

Teacher: Is it?

Lydia: No, it's showing that there are ions produced by the radiation.

Teacher: Will there be the same type of avalanche effect as before?

Lydia: Probably?

Teacher: Yes, there will. There are still two electrodes, just like before. So any ions produced will be pulled towards them and have enough energy to make more ions, and so on. Why don't we get sparks when I have moved the source away to (distance that you moved it)?

Lydia: It is too far away for the radiation to reach?

Teacher: Yes, but why? Let's work out an explanation. What is the radiation doing as it moves through the air?

Lydia: Making ions.

Teacher: Good. The radiation, in this case alpha radiation, is moving through the air. Where does the energy come from to remove electrons to make ions as it goes?

Teacher: So what happens to the energy in the kinetic store of the alpha radiation as it moves through the air?

Lydia: It has less, so if it is too far up it doesn't get there. It is stopped by the air.

### Making the invisible, visible?

Teacher: So in this experiment can you see the radiation?

Lydia: No.

Teacher: Can you see the ions?

Lydia: No.

Teacher: So this is a detector of things that we can't see. Excellent.

### The Geiger counter demonstration

What to prepare

• a scaler or other counter for the Geiger–Müller tube
• a holder for the Geiger–Müller tube
• a thin window Geiger–Müller tube
• a gamma source, as pure as possible (e.g. Co-60 with a filter to stop β, or Ra-226 with a thick and dense filter – usually lead)
• a beta source, pure (strontium 90)
• an alpha source, as pure as possible (e.g. Pu-239, or Am-241 (which emits γ as well))
• a box of matches

There are many different types of Geiger counters. Some you simply turn on. If you are using an older-style Geiger–Müller tube that plugs into a separate ratemeter or scaler, you will need to set the voltage on the scaler. Do this by following these steps.

Put a radioactive source in a holder. Fix this in a clamp on a retort stand. Put the Geiger–Müller tube in a stand. Adjust it so that it is pointing at the source, and is about 5 cm away from it.

Plug the Geiger–Müller tube into the scaler (counter) and switch on. Start the voltage at about 200 V. Make a note of the number of counts in, say, a 15 s interval. Increase the voltage in steps of 25 V until you reach the threshold when the count will reach a plateau. It will stay constant over a range of voltages. Set the voltage at a value of between 50 to 100 V above the threshold. If the clicking increases when you increase the voltage, then you have moved off the plateau. Turn the voltage back down.

What happens during this activity

It is helpful to have a diagram on the board or an animation that shows the interior of the Geiger–Müller tube. Discuss the diagram and bring out the fact that there are, again, two electrodes.

### A discussion supported by the actions

Teacher: So you may have heard of a Geiger counter. Here it is, and look, we have two electrodes again. Here the electrode in the middle is the anode, which is positive, and the outside of the tube is negative, the cathode. So let's work out how it works. Let's think of the alpha particle in the spark counter experiment. As it moves through the air it produces ions. Can you remind me what happens when you produce ions?

Lydia: It makes positives and negatives.

Teacher: Nearly. Electrons are removed from atoms or molecules. What is left is positive. So if an alpha particle gets inside the Geiger–Müller tube and does that, where will the electrons go?

Lydia: To the positive, the anode.

Teacher: Good. But they won't just wander over towards it. They will be accelerated, so what will happen when they collide with molecules of the gas inside?

Lydia: They ionise them.

Teacher: Good, and the positive ions will do the same as they move towards the cathode, and that makes the avalanche just as we talked about before. That avalanche of electrons is just a little current, which can be amplified, and that's when we hear the click. The click is just like the spark, but we are hearing it instead of seeing it. What are some of the benefits of using a Geiger counter over using the spark counter?

Lydia: You can point it at things – you don't have to bring the thing that is radioactive towards it.

Lydia: You can see it on the machine. You can find out the number of alpha particles from that.

Teacher: Excellent.

• ## 04 Questions: nature and properties of ionising radiationRa04TAnugget04 Activity

### Diagnostic questions

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 1–10}DiagnosticQuestions

Support sheet

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 and 10: Irradiation and contamination

Question

Which one of the following is emitted by some radioactive nuclei and is also classed as an electromagnetic wave?

B: Gamma radiation. 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

An ionised material differs from one that isn't ionised in that:
1. It has had electrons knocked out of its atoms.
3. It is a gas as opposed to a solid.
5. It has a shorter half-life.

A: Ionisation involves removing electrons from atoms.

Question

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?
3. Microwaves.
4. Neutrons.
5. X-rays.

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

### The question set continued

Question

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?

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

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?
3. Gamma radiation is unaffected by an electric field unlike alpha radiation.
5. Gamma radiation is unaffected by a magnetic field unlike alpha radiation.

D: 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

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.

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

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.

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 one

Question

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.

B: Ionisation involves removing electrons from atoms.

Question

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