In this section, Professor Michael Short shares how he used 1,000 bananas to engage students in a real-world context.
A few years ago a friend of mine, Marco Pellegrini, was asked to give a TEDx Talk at Tokyo Tech, and he wanted a way to relate radiation to real quantities. And I said, “Well, why don't you use the banana-equivalent dose, the dose of radioactive potassium you get from eating one banana, which everyone will agree is a perfectly safe level?" Then you can compare the radiation dose from other normal and abnormal activities to how many bananas would you have eaten. So someone could say, oh, I can imagine eating 1,000 bananas. Well, that's like living in a brick building for a year, because brick's fairly radioactive. Or it's like taking two flights across the country, or something like that. When you make data relatable, it's a lot easier to understand.
We all talk about the banana-equivalent dose, but a good scientist doesn't believe what you tell them. They should be able to go and repeat that calculation.
— Michael Short
Marco's TEDx talk went really well. And I got to thinking, we should actually measure this, because we all talk about the banana-equivalent dose, but a good scientist doesn't believe what you tell them. They should be able to go and repeat that calculation.
You can't do it with one banana and the sort of cheap spectrometers we have available. You have to boost your statistics—more bananas for a larger dose. I decided I would buy 1,000 bananas, which turned out to be seven full boxes. To measure the radiation, you first need to convert the raw bananas to ash, to create a concentrated sample. At first I wanted to just give the students the ashes and save them the time. So I called all the crematoriums in the Boston area. And they all said “No,” except one. But when the manager found out that the guy had said “Yes,” he said, “Absolutely not.”
So my plan to give them the ashes failed. I said, “All right, everyone's going to take 50 bananas home, peel them, burn them at a low heat,” so the ashes would contain the potassium we want to concentrate.
So they all burned them. We crushed them into chunks and powder, and we counted them in the spectrometer at the nuclear reactor lab. And you could see some very clear potassium-40 peaks. Then you could integrate the area underneath and get the total radioactivity of that mass of bananas.
The students loved it. Some of them said it was the highlight of their year.