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.
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.
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.
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.
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.