Tuesday, February 12, 2013

Radioactive Decay of Candium

Last week, I posted on G+ a brief preview about an in-class learning activity that I do in my geochemistry course, which I refer to as the "Radioactive Decay of Candium".  The idea is to use a student-centered activity in class that is enjoyable & interesting in order to learn about how radioactive decay works.  I didn't originate the idea, rather I've taken the main idea from an activity on the SERC geoscience education website & modified it a bit.

The activity begins with a very short discussion about how radioactive decay works, but really I want to get them going quickly, so we talk about the fact that each m&m has a 50/50 chance of landing m-side up or m-side down, a lot like flipping a coin.  So I give them each a couple hundred pieces, a bag, and a couple of pieces of clean, white paper, and a handout.  Their job is to count the pieces, place them in the bag, shake them up, pour them out, remove those showing m-side up, and count the ones that remain.

Fig 1. Science in progress!

Those that remain are placed back in the bag & the process is repeated.  Each time, they record their results on the board.  After they reach zero m&ms, I give them a second handful of pieces, they count those & then add them to their first pile and do it all over again with a larger sample.  At this point, they might have ~300 pieces.  This time, however, those that "decay" each turn might get eaten.  After all of the groups (I usually have them do this in pairs) have finished, everyone records all of the data.  We then walk through the graphs they have to create with the numbers, now working on their own.  So here's a graph of all the trial runs, including the "class total", which is just a sum of all pieces on each step.
Fig. 2.  Decay curves of Candium for all experimental runs.  

This obviously shows the number of pieces that remain on each turn after they shake them out & separate out the "decayed atoms".  Then I have them calculate the percent of the total number of "atoms" that have remained on each turn, which looks like the figure below.
Fig. 3.  Percentage of total atoms that remain on each turn for all experimental runs.
I like the comparison of the two graphs, in that it shows that no matter how many atoms you start with, the decay in each case is the same percentage.  It's a fun activity that helps students really connect to the idea of radioactive decay, and a tasty one too!

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