Ionising Radiation
Quantum and Nuclear

An experimental approach to spark counters

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

Understanding spark counters.

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 (Na+) and chlorine (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 the activity is for

  • 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 the activity is for

  • 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
  • some connecting leads

Safety note: A school EHT supply is limited to a maximum current of 5 milliampere 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?

Lydia: You are adding ions, like adding the salt.

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

Finally – the spark counter

What the activity is for

  • a power supply EHT (0–5 kilovolt, 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)
  • some connecting leads

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.

Sparks caused by radiations

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?

Lydia: The radiation produced by the radioactive source.

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?

Lydia: The alpha radiation.

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 the activity is for

  • 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 volt. 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.

Teacher: Good. What about working out how much radiation there is?

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

Teacher: Excellent.

Ionising Radiation
is used in analyses relating to Radioactive dating
can be analysed using the quantity Half-Life Decay Constant Activity
features in Medical Physics
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