Students read 15.1 Nuclear Weapons in How Things Work the Physics of Everyday Life and watched the following videos before class.
or if you prefer Tyler DeWitt, he has a nice video on fission.
In class I did a slide show on nuclear weapons, explaining a bit about the Gadget, Fat Man and Little Boy, the first three nuclear devices and how they worked. I had recently visited the National Museum of Nuclear Science & History in Albuquerque, NM so I showed a few slides from that trip. This is a great science museum if you’re at all interested in nuclear physics and the Manhattan Project during WWII.
The Gadget was the first atomic device, built at Los Alamos and tested at the Trinity site on July 16th, 1945. The photo below right is a model of what it looked like. It was an implosive fission device where explosive charges placed around a sphere of plutonium would cause it to collapse and reach super criticial mass. Fat Man (yellow and black bomb in the photo below left) was a similar design as the Gadget but Little Boy (longer green bomb in photo below left) was a gun type device which fired a ring of subcritical mass of Uranium onto a cylinder (another subcritical mass) of uranium. Together they reached a supercritical mass and would release enormous amounts of energy. This design was fairly straight forward and was not tested before being used at the end of WWII.
This video explains how the two designs work.
For the lab activity, I gave each student a cup of M&Ms (roughly 120 or so) and told them they were radioactive nuclei. When they poured the M&M’s out into a tray, all candies with the M showing on top had decayed and were to be disposed of (eaten). They then counted how many nuclei they had left and repeated the experiment until all the nuclei had decayed. In this activity each toss of the candy is a half-life, the time it take for half of a radioactive sample to decay. Student predicted how many candies they would have left after each toss and I heard quite a few shouts when they got it exactly right. We kept track of everyone’s data on a white board and added up the total number of nuclei (candies) that did not decay after each half life (toss) so we could graph the population of nuclei for all the candies as a function of half life. We got a beautiful decay curve. Some students graphed the data by hand and some used their iPads to make a graph.
You can also see in the data table that we lost roughly half the M&Ms with each toss. A number of students discovered at the end that the candy that was left didn’t have an M on it at all! Guess it was a stable isotope…. until they ate it. If you student has food allergies or you just don’t want to use candy, this can be done with pennies as well (I did it that way when we did chemistry), but the students are definitely more interested when it involves chocolate.
There is a movie with Paul Newman called “Fatman and Little Boy” which is about the Manhattan Project which is pretty good and I recommend my students try to see it.