I know what you’re thinking: “Kids today have no interest in the exponential decay of a radioactive nucleus.” And you’re not wrong—children, teenagers, and many folks in their early- to mid-twenties rarely give thought to nuclear physics. However, it’s only a matter of time before every last thing on this planet is directly or indirectly powered by nuclear energy, so they better start learning about it now.
To make it easy for their underdeveloped minds to understand—and nice and shiny so they can pay attention long enough to maybe actually learn something—we’ve cooked up a nifty little Demo Science science demo that will help you demonstrate radioactive decay and the half-life of isotopes.
A Dollar’s Worth of Priceless Knowledge
All you’ll need for this experiment is one hundred pennies, a jar or other container to put them in, and a flat surface with a ledge around it to keep pennies from rolling away. You can use the floor for this, with a barricade of books laying on their sides to contain the runaway loose change.
Then, dump out all your pennies, nice and quicklike, into your coin corral. Once that one that just keeps rolling and spinning and wobbling finally stops, remove all the pennies that landed tails up. Put these pennies in straight column at the “top” of your designated area.
Gather up the remaining pennies in your jar and dump ‘em out again. Again, remove the pennies that landed tails up and put them in another column beside the first one. Keep repeating this process until all the pennies have been moved into columns. (When you’re getting low on pennies, you may have a round or two where none land facedown. If that happens, just gather them up and do it all again.)
What Have We Learned Here?
First of all, that handling a bunch of pennies will make your hands smell funny. But, perhaps more importantly, you’ve also created a surprisingly accurate analogous demonstration of the decay of a radioactive nucleus.
You see, each penny poured from the jar to the floor has the same 50 percent chance of being removed as every single other penny. After the first penny dump, only about half the pennies remained; after the second, about one fourth of them were still in play; after the third round, you’re down to roughly one eighth of the coins; and so on and so on. A pattern like this one, where the total decrease is repeated by a fixed fraction, perfectly represents exponential decay.
Exponential decay can be used to calculate the “half-life” of radioactive materials or heaps of coins. The time it takes for half the pennies to be removed from the remaining amount—one pour-out, based on the 50 percent chance of removal per pour—is the half-life of your penny stockpile.
Every different radioactive material has a different half-life. As an additional demo, you can illustrate this principle by repeating the above process with dice, removing only those that land on one, for example. This would triple the length of the experiment, as dice have a 1/6th chance of landing one-up, as opposed to the pennies’ 50/50 heads/tails ratio.
Also, rounding up 50 dice would be a feat worth remembering in its own rite.