What it shows:
In a nuclear reactor or atom bomb, a fissile material such as 235U can capture a neutron. The resulting unstable nucleus fragments into two smaller nuclei, releasing energy and several neutrons (a typical equation is given below). Each of these neutrons can in turn cause the fission of a 235U nucleus. If there is above a critical concentration of fissile material, this chain reaction will continue unaided, and if unregulated can result in a very loud bang.
n + 235U → 236U* → 141Ba + 92Kr + 3n
How it works:
We have a 120 × 70 × 100cm high plexiglass case, onto whose base we set a 6 × 23 array of mouse traps. 1 A ping-pong ball rests on each trap. The traps represent the fissile atoms, and the balls the neutrons. When an extra ping-pong ball is dropped through a hole in the top of the case, it lands and triggers a trap. Now there are two ping-pong balls each capable of setting off a trap. Thus a chain reaction ensues; the whole explosion lasts about three seconds.
Setting it up:
If ever you envied those daring bomb disposal guys, here's a setup to test your nerve. The plexiglass case has two portholes, 20cm diameter, 10cm above the floor of the case. These are for loading the balls on the traps. It is advisable to set and lay the traps before you load them. Set each trap, tap it lightly to make sure it isn't too hair-triggered, and slide into position; all traps facing the same way. When loading the balls, rest the ball on the catch arm on the opposite side to the bait catch; if you drop the ball from a small height it will be less likely to trigger. Start loading the traps on the far side of the case, and work towards you. We found that a removable partition placed in the case splitting the sample in half allows two people to set up the traps without interfering with each other (or suffering from the other's foul-ups). It can be removed just before the last traps (keep track of the least sensitive traps for last) are laid and loaded. Our case is mounted on wheels, so it can be loaded and wheeled (carefully) into the hall when needed. Provide an initiating neutron, and keep the top hole covered until detonation.
figure 1. The loaded trap
We've got the set-up time down to 15 minutes, and like all nuclear devices, you only get one shot at this one.
The mouse trap chain reaction originated (we think) from Walt Disney's 1950s film "Our Friend the Atom."
J. Higbie (see Reference) has described a "better" mousetrap nuclear chain reaction but we prefer the old fashioned way. Setting up the last few traps while the students await the lecture to begin, dripping brow and shaking hands, certainly adds some drama. We think Enrico Fermi would agree. Reference:
J. Higbie: Am. J. Phys. 48 (1), 62 (1980)
1 Mouse traps are 10cm × 5cm in size, manufactured by Woodstream Corp., Lititz, PA or d-Con Corp., Montvale, NJ. Both are available in hardware stores.