Radioactive Popcorn

Popcorn can be used to model radioactive decay. It is a lot safer than using radioactive isotopes, as well as much tastier. What are the similarities between popcorn and radioactive isotopes?
A radioactive isotope is an unstable atom. This is often due to its size. As an atom gets larger the nucleus is no longer so tightly packed together. Its surface area gets larger which means the forces which hold it together must act over a greater distance and therefore become weaker. To become more stable the nucleus may rearrange or lose mass, this is known as radioactive decay. The original isotope, known as the “Parent” isotope, decays into the more stable “Daughter” isotope. There are three types of radioactive decay: Alpha, Beta and Gamma.

If you have a sample of radioactive material, it is impossible to know which individual isotope will decay when. Just like if you were heating popcorn, you would have no idea which kernel will pop first.  However, when we have a sample with a large amount of the radioactive isotope, it is found that half of the isotope will undergo decay in a set amount of time. This is known as it’s half-life.  Each radioactive element will have its own half-life time period.

Half-life of Radioactive sample 
Some radioactive elements have very long half-lives, which means the decay rate is very slow. This is useful for dating rocks, as they are generally very old. One example of this is Samarium- Neodymium. Samarium_147 decays though alpha decay to Neodymium and has a half-life of around 10 billion years. This is not only used for dating very old igneous and metamorphic rocks, but also meteorites.

By comparing the percentage of the Parent element to the Daughter element in a sample, we can calculate the age of the rock by drawing a decay graph.

Popcorn can be used to model radioactive decay. Check out this video to see how you can do an experiment at home to model radioactive decay
  1. Measure out 20g of popcorn – of course you can use more if you are really hungry!
  2. Pour the popcorn into a paper sandwich bag and fold the top over twice.
  3. Place the bag in the microwave with the folded edge facing upwards
  4. Set the time for about three minutes and press start.
  5. As soon as you have heard the first kernel pop, make a note of the time and continue running the microwave for 10 more seconds.
  6. Take the popcorn out and separate into kernels and popped corn. Count how many there is of each and record this in a table.
  7. Put the un-popped kernels aside. We cannot use these again for the experiment, however you can still cook them later for a snack.
  8. Repeat the investigation allowing the microwave to go for 20 then 30 and finally 40 seconds after the first pop, using the same amount of popcorn each time. Make sure you record your results in the table after each trial.

If you don’t have a microwave just use a pan with the lid on.  Make sure you spray the kernels with a little oil and give the pan a regular shake or the kernels will burn. Be careful around the hot stove and ask for parental assistance, if needed
  • For each trial, calculate the total number of corn (popped plus un-popped) and then find the percentage of un-popped kernels and the percentage popped corn of the total.

Table of results
  • Plot a graph of percentage of kernels against length of time cooked after the first pop.

Decay Rate of Popcorn 
You should end up with a graph similar to this. What you should notice is that the longer the time the less kernels there are. The line of best fit is curved and we call this a decay curve.  This is exactly what a radioactive decay curve looks like. We can use the graph to find the half-life of popcorn – in this case the percentage of kernels was halved every 10 seconds, meaning the half-life was 10 seconds.
Some sources of error:
  • Every time you use your microwave, or pan and hob, it heats up. Allow them to cool between trials.
  • The amount of popcorn used is quite small. A larger amount would give a smaller percentage uncertainty.

Now what to do with all that popcorn?
Check out these yummy popcorn recipes. 

For more activities and information on radioactive decay, have a look at our other free resources:
STEM project, Burying Nuclear Waste