Demonstration

This demonstration shows how a circuit can be set up to detect a familiar event that produces ions. A simple circuit with a beaker of water can be made to conduct by adding a heap of salt. This is a useful step towards understanding the spark counter and the Geiger-Müller tube.

Apparatus and Materials

• Lamp holder on base
• Lamp, 12 V 36 W and power supply
• Beaker, 1000cm3
• Copper electrodes, 2
• Crocodile clips, 2
• Distilled or de-ionised water
• Table salt, supply
• Teaspoon
• Retort stands, bosses and clamps, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

Photo courtesy of Mike Vetterlein

1. Set up a series circuit consisting of the power supply, the lamp and the 2 copper electrodes separated by the width of the beaker into which they dip.

2. Switch the circuit on. You could try touching the electrodes together to show that the light comes on.
3. Add distilled water to half fill the beaker.
4. Throw a spoonful of common salt into the water. The lamp will come on.

5. Empty the beaker and wash it out including the copper electrodes. Alternatively, have a water flow through the beaker to flush away the ions.
6. Repeat (1-5) and ask the students what is going on.

Teaching Notes

• You could introduce the experiment as a mystery without any reference to Geiger counters. At the end, you could say: "You think that this is an experiment to show electrolysis. Not this time! This is a SALT COUNTER. It is a method for counting spoonfuls of salt. I throw in a spoonful of salt and the lamp lights. I clean out the beaker and refill it and throw in another spoonful of salt: the lamp lights. You count how many times the lamp lights. This corresponds with the number of spoonfuls of salt I throw in."
• The potential difference between the wires creates an electric field across the beaker of water. That field is ready to tug electric charges; positive charges (sodium ions) to the negative electrode and negative charges (chlorine ions) to the positive electrode. If there are charges there, then a current will flow. The changes are put in when the salt is added.
• Unlike the spark counter, this counter measures ions without any breakdown of the medium. The dissolved ions move sedately to their respective electrode carrying a current without a cascade effect.
• There is no amplification of ions. The only ones that move are the ones that come from the salt. With advanced students, you can make this point and suggest that, to detect ionisation from, say, a flame in the air, it will help if the number of ions is multiplied.
This links in to the demonstration

Counting matches with a Van de Graaff generator

This experiment was safety-tested in June 2007

Demonstration

The Van de Graaff generator always produces excitement for students.

Apparatus and Materials

• Microammeter, light spot type, optional

• Van de Graaff generator

Health & Safety and Technical Notes

The makers' instructions should be followed for the care and use of your Van de Graaff generator.

Read this comprehensive safety guide:

Van de Graaff generator safety

Procedure

Sparks should be shown passing between the large sphere and the smaller sphere supplied with the generator.

Teaching Notes

• As an introduction to the demonstration, you could say: "Can gases carry currents? Does air carry currents? Suppose the air carried electric currents as easily as copper, what would happen to the electric circuits that you have been working with? Then air cannot carry a current as easily as copper or it would spoil all these experiments. What would happen to cells? Or to the wall terminals at home? It looks as if air must be a non-conductor, or a very good insulator like paper, glass, cotton, wood or things like that. Yet it is impossible to make gases to carry currents."
• When the charge on the insulated dome becomes high enough, a spark will pass between it and a second dome which has been connected to earth.
• To show that it is the same kind of electricity as found in electrical circuits, connect a microammeter into the earth connection lead. Each time a spark jumps across the gap, a sharp burst of current will be indicated on the meter.
• Once the Van de Graaff generator is set up, the kit that comes with it has lots of toys to demonstrate. Doing a series of experiments with these will delight students. Look at the collection

  Van de Graaff generator

This experiment was safety-tested in July 2007

• A video showing how to use a Van de Graaff generator:

Demonstration

An EHT supply can be used to produce sparks, which can be increased if you add a capacitor in parallel with the air gap. You can use the sparks to count matches. Use this to develop the ideas behind the spark counter and, eventually, the Geiger-Müller tube.

Apparatus and Materials

• Power supply, EHT, 0–5 kV (with internal safety resistor)
• Conducting spheres, 2
• Retort stands and bosses, 2
• EHT capacitor, 0.1 μF
• Connecting leads

Health & Safety and Technical Notes

See guidance note on

Managing radioactive materials in schools

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 5,000 V. Do not use an ordinary capacitor in the circuit and certainly not a low voltage electrolytic.

CLEAPSS

can advise on a source for a suitable EHT capacitor.

Read our standard health & safety guidance

This demonstration works in conjunction with the demonstration

Counting matches with a Van de Graaff generator

It shows that the spark from an EHT supply is the same as that from a Van de Graaff generator.

To get a decent spark, you need to connect a high voltage capacitor in parallel with the air gap. This is because, for safety reasons, the EHT supply has a big internal resistance and cannot deliver a large current. On its own, the EHT will produce a small spark across a gap of a couple of millimetres. But it will not let enough charge flow quickly enough to produce a bigger spark. The capacitor solves this by storing some charge. It lets the charge go when the air breaks down. It is the capacitor’s charge that allows the breakdown to produce a decent spark.

Although the Van de Graaff produces sparks without needing a capacitor (because it has capacitance and stores its own charge), the EHT supply offers more control. You can change the voltage rather than the separation of the spheres to set the field strength so that the air is just on the verge of breaking down. This will be more useful in the long run when making a spark counter and Geiger-Müller tube.

Procedure

Setting up...

1. Set up two conducting spheres about 1 cm apart. Put their insulating handles into bosses on retort stands.
2. Connect the capacitor in parallel with the spheres.
   
3. Set the voltage on the EHT to about 5,000 volts. Move the spheres together until they start sparking.
4. Reduce the voltage until the sparking just stops.
Carrying out...
5. Hold a match under the gap between the spheres. The ions in the flame will set off a cascade of ions between the spheres – i.e. a spark.
6. Each lit match you put under the gap between the spheres should set off a spark.
Optional...
7. With advanced students you can show that, without the capacitor, the EHT will produce only very weak sparks across a small gap.

Teaching Notes

• Explain that the apparatus is behaving like a match counter. Although this is a roundabout way of counting matches, it is doing so because of the invisible ionisation that is happening in the gap, i.e. it is detecting something that can’t be seen.
• You could ask them to turn away from the apparatus and see if they can hear when you put a match under the gap. They can’t hear the match flame but they can hear the effect it produces – a spark in the gap. This is a useful step towards building a radiation detector. Although the radiation is invisible, the effect it produces can be made visible.
• The high voltage produces a strong electric field between the two spheres as positive and negative charges build up on the spheres and across the plates of the capacitor so that there are charges waiting to be driven across the gap. A few ions in the air in the gap will start a spark.
• Point out that the sparks are the same as those produced by the Van de Graaff generator. Explain that the EHT supply gives more control than the Van de Graaff generator when setting the field strength. And that control will be useful later.
• With advanced students, discuss the need for the capacitor (see technical note above). When it is charged to 4,500 V, a 0.1 μF capacitor stores about 1 J. This is about the same as the energy stored elastically in the spring of a mousetrap. The snap of the mousetrap is about the same as the snap of the spark from the charged capacitor.

This experiment was safety-tested in March 2006

Demonstration

This ‘match counter’ is a useful step towards understanding a Geiger-Müller tube.

Apparatus and Materials

• Microammeter, light spot type, optional

• Van de Graaff generator

Health & Safety and Technical Notes

Read this comprehensive safety guide...

Van de Graaff generator safety note

Use the same apparatus and set up as for the demonstration:

Showing that a spark can pass through air

Procedure

1. Set up the Van de Graaff generator and switch it on. Bring the small sphere up to the dome of the Van de Graaff so that sparks are jumping between them.

2. Move the spheres apart until they just stop sparking. Keep the generator running.
3. Light a match and hold it under the gap between the spheres. This should produce some ions which will set off a cascade of ions – i.e. a spark.
4. Each lit match you put under the gap between the spheres should set off a spark.

Teaching Notes

• Explain that the apparatus is behaving like a match counter - although this is a roundabout way of counting matches. It is doing so because of the invisible ionisation that is happening in the gap, i.e. it is detecting something that can’t be seen.
• You could ask your students to turn away from the apparatus and see if they can hear when you put a match under the gap. They can’t hear the match flame but they can hear the effect it produces – a spark in the gap. This is a useful step towards building a radiation detector: the match is inaudible, but the effect it produces can be made audible. Similarly, although ionising radiations are invisible, the effect they produce can be made visible.
• You can do this demonstration in conjunction with

  Counting matches with an EHT supply

  It is useful to start with the Van de Graaff generator (as described here} because students will have seen it sparking before.

This experiment was safety-tested in February 2006

• A video showing how to use a Van de Graaff generator:

Demonstration

The spark counter is a highly visible (and audible) way of showing and counting ionisation of the air caused by alpha radiation (or a match). It is a useful step towards understanding the Geiger-Müller tube.

Apparatus and Materials

• Power supply, EHT, 0–5 kV (with option to bypass safety resistor)
• Spark counter
• Sealed source of radium, 5 μC (if available) or sealed source of americium-241, 5 μC
• Holder for radioactive source (e.g. forceps)
• Connecting leads

Health & Safety and Technical Notes

See guidance note on...

Managing radioactive materials in schools

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 the 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.

Read our standard health & safety guidance

The spark counter is a special piece of apparatus (see image above). It consists of a metal gauze with a wire running underneath. Philip Harris calls it a Spark Discharge Apparatus.

• Any kink or bend in the wire in the counter is liable to cause a spark discharge at that point. If that happens the wire should be replaced.
• A continuous spark (which will very soon damage the wire) shows the voltage is too high.
• The spark counter should be dust free. Dust around the stretched wire can usually be blown away.
• The gauze on top is connected to the earth on the EHT supply as a safety precaution.
• Radium is a source of alpha, beta and gamma radiation. Beta and gamma radiations do not cause enough ionisation of the air to start a spark.

Procedure

Setting up

1. 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.)
2. Connect the other terminal on the spark counter to the negative terminal of the power supply and connect this terminal to earth.
3. Turn the voltage up slowly until it is just below the point of spontaneous discharge. This is usually at about 4,500 V.

Carrying out

1. 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.
2. Move the source slowly away from the gauze and note the distance at which it stops causing sparks.

Teaching Notes

• Draw attention to the random nature of the sparks and hence of the radiation. By counting sparks you are counting the number of alpha particles emitted.
• You should find that the range of the alpha particles is about 5 cm.
• You could mention that this is alpha radiation, the most ionising of the three main types of radiation.
• The sparks are similar to those produced by a Van de Graaff generator. The alpha particles ionise the air forming positive and negative ions. When these ions recombine to form neutral atoms then blue light is emitted. The noise of the spark is due to warming the air in the narrow region of the avalanche current, which produces a sound wave just like in a lightning strike.
• A thin sheet of tissue paper or gold foil held between the spark counter and the source will show a reduced range for the alpha particles or even prevent them getting to the counter.
• A version of this apparatus can be seen in the CERN visitor centre (if you happen to be passing). It detects cosmic rays and makes them visible using a 3D array of wire meshes with high voltages between them. The paths of rays can be seen by the trail of sparks that they leave as they ionise the air between the wire meshes.
• This type of 3D array of high voltage meshes is the principle used to detect the paths of particles produced in the collision experiments at CERN.
• Before you use the spark counter to show ionisations from an alpha source, you could use the spark counter to count matches (as in

  Counting matches with an EHT supply

This experiment was safety-tested in June 2007

Demonstration

This is a general introduction to the Geiger-Müller (G-M) tube describing and explaining the basic principles of its operation.

Apparatus and Materials

• Scaler
• Holder for GM tube
• Thin window GM tube
• Gamma source, as pure as possible e.g. Co-60 with a filter to stop β, or Ra-226 with a thick filter
• Beta source, pure (Strontium 90)
• Holder for radioactive sources
• Gamma GM tube if available
• Box of matches

Health & Safety and Technical Notes

See guidance notes:

• Managing radioactive materials in schools

• Read our standard health and safety guidance

Geiger-Müller tubes are set up to operate at a voltage within their ‘plateau’. In self-contained systems, this is set automatically.

The voltage across a Geiger-Müller tube is generally kept low enough so as not to produce a roaring spark when an energetic particle enters it.

Geiger-Müller tubes are very delicate, especially if they are designed to measure alpha particles. The thin, mica window allows alpha particles to enter the chamber. It needs a protective cover to prevent it from being accidentally damaged by being touched. A good alpha detecting Geiger-Müller tube will also count photons. If you light a match in front of it, a few ultra violet photons will be detected.

Procedure

Setting up

This will depend on the type of Geiger-Müller tube you are using. If you have a self-contained system, then simply get it ready and switch it 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.

1. Put a radioactive source in a holder. Fix this in a clamp on a retort stand.
2. 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.

Photograph courtesy of Mike Vetterlein
3. Plug the Geiger-Müller tube into the scaler (counter) and switch on.
4. Start the voltage at about 200 volts. Make a note of the number of counts in, say, a 15 second interval.
5. Increase the voltage in steps of 25 volts.
6. You will find that the counts vary with voltage and then reach a plateau. A graph would look like this (you do not need to plot the graph):

7. After the threshold voltage, 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.
8. If the clicking increases when you increase the voltage, then you have moved off the plateau. Turn the voltage back down.
9. Put the source back in a safe place until you carry out the demonstration.

Carrying out the demonstration

1. Switch on the Geiger-Müller tube counting system.
2. Highlight the fact that there is a background count.
3. Bring a radioactive source up to the Geiger-Müller tube and draw attention to the increase in counts.
4. You could measure the background count and the count with the source nearby. Do this over a period of 30 seconds. Draw attention to the difference.

Teaching Notes

• Discuss what is happening in the Geiger-Müller tube. Point out that it is more sensitive and more stable than the spark counter.
• Draw attention to the differences in count between the Geiger-Müller tube and the spark counter. The Geiger-Müller tube detects all of the ionisation events that take place inside it. Every one is registered.
• Discuss the Geiger-Müller tube as a development of the spark counter. You could say: "The wire of the spark counter is placed inside a metal shield, which acts as the other electrode, and the high voltage supply is incorporated in the scaler The scaler does the counting of the pulses of charge delivered by the electron avalanches to the central wire. The tube is filled with a suitable gas mixture to make sure that each spark does not last too long. When the spark is quenched, the tube and the scaler are ready to count a ‘bullet’ from another radioactive atom’s ‘explosion’. As you can see and hear, the tube can react very quickly to each avalanche."
• The Geiger-Müller tube works on the same principle as the spark counter: an ionisation between two high voltage electrodes produces a pulse of current (an avalanche of charge) between the electrodes. The differences are that the Geiger-Müller tube is sealed, it contains a low pressure gas (usually argon with a little bromine), and it is usually part of a circuit with a scaler counter.
• The scaler counter records and counts each pulse of charge.
• The actual phenomena inside a tube are much more complicated than the simple story of ionisation producing an avalanche of electrons. Inside the tube ultra violet photons probably play an important part, as well as colliding electrons and ions, and the detailed picture is extremely complex.
• An ionising particle will produce a pulse of charge of almost constant size. The size of the pulse does not vary with the energy or amount of ionisation produced by the ionising particle.
• The number of pulses represents the number of ionising particles coming into the tube.
• Geiger-Müller tubes do not distinguish between one kind of particle and another, or between a more energetic particle and a less energetic one, provided the particle enters the tube and does not pass right through (as most gamma rays do).

This experiment was safety-tested in August 2007

Class practical

This is a simple toy radiation counter that enables students to see the bright flashes of light, scintillations, produced by alpha particles when they strike a fluorescent screen.

Apparatus and Materials

• Spinthariscopes with 0.02 μCi sources, (740Bq)
• Spinthariscopes with 0.5μCi sources, (18500Bq)
• United Nuclear, an American supplier, markets a Super spinthariscope designed so that the distance from the source to screen can be varied, eliminating the need for two different source activities.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Modern spinthariscopes do not use radium nitrate or bromide. They are more likely to contain a small speck of thorium ore. Moreover, the source is not removable.

Crookes spinthariscope: A = ZnS screen; B = radium nitrate source; C = eyepiece lens.

Procedure

1. The eye must be light-adapted to see the scintillations, so it is not possible to use spinthariscopes in daylight. Use a blacked-out room with a very limited amount of artificial light. It is then relatively easy for the pupils to see scintillations.
2. Pass the spinthariscopes around the room so that each of the students can see the scintillations for themselves.
3. Two strengths are recommended: one gives a shower, the other makes it easier to see the random nature of the process. The students should see both if possible.

Teaching Notes

• The spinthariscope was the first radiation detector and was the forerunner of scintillation counters. A modern scintillation counter employs a photomultiplier tube as a light sensitive component in an electronic circuit to count the flashes in the fluorescent material.
• Geiger and Marsden used a type of spinthariscope to count the number of alpha particles being deflected into each angle in the gold foil experiment. They had to sit in a darkened room counting these flashes of light; no wonder Rutherford encouraged Geiger to develop the Geiger tube!
• The spinthariscope was developed by William Crookes in 1903 after an accidental discovery. He was looking at the fluorescent glow produced by radium bromide on a zinc sulphide screen. He spilt some of the radium salt onto the fluorescent screen and, given its huge expense at the time, started to pick up all the specks of dust. He needed a magnifying glass to locate each speck. He then noticed the individual flashes, or scintillations, produced around each speck on the screen. Each flash corresponded to a single alpha particle. This was the first radiation detector. Crookes made it into a self-contained scientific toy to impress his lecture audiences.
Thanks to Paul Dolk, Haarlam, Netherlands, for suggesting the super spinthariscope.

This experiment was safety-tested in March 2006