Teaching Radioactivity – A simple demonstration of alpha, beta and gamma

This is one of my all time favourite fun classroom activities. You would not normally equate teaching radioactivity to a roomful of bemused Year 10 students as fun but prepare to be amazed by this simple way to demonstrate the sizes, penetrating power and ionising properties of alpha, beta and gamma radiation.

 

Equipment Needed

One large exercise ball, 50-70cm in diameter pre-inflated

A pea shooter with peas/pellets or a potato gun with potato

A low power laser pointer, the sort used for presentations

This is best done in a large area like a sports hall or even outside weather permitting.

toy pea shooter with pellets

Safety Considerations

You may wish to give out goggles to protect eyes from the pea shooter.

The laser pointer/light beam should be directed towards peoples bodies about waist-chest high and not at faces. It would take 10 whole seconds of unblinking gaze at a typical classroom laser to damage your eyesight. There is a lot of hysteria around the use of low power laser pointers designed for presentations which is entirely unwarranted. The greatest risk of eye damage comes from the pea shooter.

 

How to do it

Choose, and I do mean choose as without fail the most disruptive student in the class will volunteer for this, three students to be the radiation.

The other students line up in 2-3 rows. If you have a class of 30 that would be 3 rows of 9 with the 3 students being radiation stood separately. To be really fancy you could even stagger the rows so students in the row behind are in the gaps of the row in front.

Now each radiation is going to irradiate this block of students who are modelling a material, e.g human skin.

The alpha (exercise ball) student should roll their particle towards the “material” and it will move fairly slowly and bounce off of the top layer of students. Let the alpha radiation have a few attempts to “ionise” the material. When “ionised” the student in the material should raise their hand. This allows everyone to see where the radiation is affecting the material.

Next the pea shooting/potato gun beta student irradiates the rows with their beta particles which are much smaller and faster, and can penetrate a bit further. Again students should raise their hand if hit by the peas and ionised. Let the student try 3-4 times to hit someone.

Finally the laser pointer student can shine their gamma rays through the material and onto the rear wall, penetrating a long way but ionising no where near as much. A straight beam from a laser pointer will skim one student at most.

It is important that the radiation is randomly directed and not aimed at the rows of students as real radiation doesn’t have a conscious intent to interact with matter. This is a limitation to the model which can be discussed at the end. I considered blindfolding the radiation students to increase the random nature of their interaction with the matter students but few teenagers are comfortable being blindfolded in front of the rest of their classmates and generally become too self conscious.

Learning Points

Alpha radiation = exercise ball – large, slow, very ionising, not penetrating

Beta radiation = peas from shooter – smaller, faster and moderately ionising and penetrating

Gamma radiation = laser beam – very fast (speed of light), very penetrating, not very ionising

The sizes and speeds of alpha, beta and gamma radiation are readily apparent from this demonstration. Alpha particles are around 8000 times heavier than an electron and consist of two protons and two neutrons bound by nuclear forces. Gamma is part of the electromagnetic spectrum and is represented as a light beam to show this.

The penetration of the radiation is modelled by the number of layers of students that can be touched by the radiation. The large exercise ball as an alpha particle will bounce around the front row but no deeper. The peas can shoot in further but only go a certain distance as they have a limited speed as a projectile. The laser/light beam will reach the opposite wall.

Ionising power is represented by the number of students being touched by each radiation hit. The slower larger exercise ball will hit at least 2 students each roll. Each hit can be seen as knocking out an electron and ionising the atom. The pea shooter will hit maybe one student each time. The laser pointer randomly aimed may hit one or two people in total.

At the end of the demonstration activity each student should complete a chart summarising the relative mass, speeds, penetrating and ionising effect of the three types of radiation. It helps to summarise the properties of each type as you go along. This can be done socratically by prompting the students with questions – how far did this “radiation” penetrate? How many atoms could it ionise? Why do you think that was? etc.

Limitations to the Model

As with all classroom models this demonstration has various limitations and it is instructive for the students to consider how this model isn’t like real atoms interacting with radiation.

The main points are that real atoms have much more empty space in them and many more electrons.

The alpha particle is deflected by repulsion of like charges not by physically hitting a large, solid atom as this model suggests.

The shell model of electrons taught to 14-16 year olds isn’t well modelled by this demonstration as we have nothing to represent the orbiting electrons. No indication is made as to the nature of the bonding between the atoms in this “material” either.

Conclusion

Students have always responded very well to this activity. It works well for less able students who find book-learning challenging as they vividly remember what happens in the demonstration and you can refer back to how “Jo was hit with the exercise ball, do you remember?” and they always do.

More able students enjoy getting out of the classroom and doing something a bit silly and are able to find the flaws in the model and extrapolate the scenario more easily than less able students who may need to be led to the learning outcomes in smaller more structured steps.

Try it out and let me know what you think.

Teaching Ideas for Sound and Waves

Here is a selection of ideas for teaching a sound waves topic to KS3 or the equivalent of 11-14 year olds. Sound and waves in general is a topic with great experimental potential. Simple activities can be used as starters to begin lessons and elicit students knowledge or as ways to demonstrate ideas. General trends such as rising pitch or volume can be shown easily. A microphone, oscilloscope or simple sound analysing software can be used to collect numerical data if needed.

The key ideas to demonstrate and emphasise are that sound is a vibration and that these vibrations need a medium to move through.

Simple Demonstrations and Activities

To show how sound travels differently through solids, liquids and gases – ask the students to work in pairs, one is to bang on the table top with their knuckles and the other to listen to the volume of the sound. The repeat with the second student listening with their ear firmly on the desk. Discuss their observations of the difference in volume. Discuss hearing sounds underwater in a pool or bath tub.

Tuning forks to demonstrate vibration – bang a tuning fork on a rubber bung and hold to the ear (to hear the sound) and then to your lip (to feel the vibration)

Large speaker connected to signal generator to show vibration – turn the speaker so the cone faces upwards, place small bits or paper or tiny polystyrene balls on top, play a note through the speaker and observe the vibrations. Ask students to predict what a higher or lower pitched note will do to the paper pieces. You can ask them to sketch what sound waves they think would represent each pitch and volume. This is a nice way of introducing amplitude and frequency as displayed in graphs.

Reeds out of plastic straws – follow the instructions here make and play reed instrument to produce a simple plastic straw reed. Need straws and scissors. Can shorten the straw by cutting pieces off to make a higher pitched note.

Rulers and change in pitch – twanging a ruler on the edge of a desk and quickly shortening the length hanging over the edge by pulling it in demonstrates nicely how more rapid vibrations produce a higher pitch.

Ripple tanks – all sorts of things can be shown with a ripple tank but at this level the demonstrations should reinforce the main topic, show how a more rapidly rotating motor on the dipper produces shorter, more tightly packed waves. Relate time and frequency, wavelength and distance to show wave velocity = distance/time = wavelength/time = wavelength x frequency. Students can measure the wavelength if a spotlight is shone down into of the tank and paper placed underneath. They can count the number of waves hitting the side in 30 seconds and find an approximate frequency.

The medium which is vibrating doesn’t move – ripple tanks can demonstrate this as the waves don’t push all the water out of the tank. It is important to compare and contrast the physical mechanism of tides and waves at sea as students are familiar with the tides bring water higher up the shore. This is of course due to an external force of gravity from the Moon and not a result of wave motion. This is a common source of misunderstanding in waves topics. Bobbing a cork up and down in the ripple tank shows this as the cork doesn’t move to the side. This principle can then be generalised to all waves including sound waves.

Standing waves in a tube – if you are feeling adventurous you can connect a clear, plastic tube (length >1m) to a microphone and cover the other end with plastic. Sprinkle some light particles (polystyrene balls, oatmeal, tissue paper etc) inside the tube and use a signal generator to produce a sound wave. Altering the frequency will cause the material inside to shift along to the nodes of the vibrations in the tube giving a rough measure of the wavelength.

Interpreting waveforms – supply students with a printout or display on the board various sound wave patterns and ask them if they can predict what sort of sounds they represent. Here is an example of the kinds of waveforms that can be used;

Examples of waveforms – what sounds would they produce?

Thought Experiments

Why do double glazed windows deaden noise? You may have to explain how they are constructed (two panes with a vacuum between).

Why can no one hear you scream in space?

What will limit how high a note a human can sing? Why do women sing higher then men?

Why did people used to listen with their ear on railway tracks?

Is it hard to breathe near a loud speaker at a concert? Does thunder produce strong winds? What does this say about the motion of air as sound travels through it?