Neutron Emission Model

Fission model of neutron emission using magnets.


Magnets can be used to model the emission of neutrons during a nuclear fission event.


Three donut magnets

Three pill magnets that just fit into the holes of the donut magnets.

To Do and Notice

Make a pile of three donut magnets on a nonmagnetic tabletop.

Stack two pill magnets on the top of the donut magnets.
(Doing this aligns the poles in the correct orientation.)

Two pill magnets rest on the upper face of the donut magnets.
Slide them over and down into thhe hole.

Slide the two pills down into the hole in the donut magnets until the top of the stack of two pill magnets is just below the rim of the donut magnets.

Slide the pill magnets into the hole iin the donut magnets uuntil they are just below the rim.

Hold the third pill magnet about 10 cm, or one fist width, above the center of the stack of donut magnets. See the top image.

Drop the pill magnet and watch what happens.

The two pill magnets plus the magnet that you drop leap out of the pile.

So What?

One pill magnet colides with the magnet array and three pill magnets leap out.

This models what happens in nuclear fission when one neutron hits a uranium nucleus and three neutrons come out.

Two major problems with this model are that the pile of donut magnets should fission into two piles during this process and that the neutrons should come oout heading in diifferent directions not all joined together.

What's Going On?

Assume that the magnets are oriented with their north face up.

The pill magnets will also have their north face up.

As the pill magnets are inserted into the hole they are repelled by the donut magnets.
You can feel the repulsion.

The repulsive force on the pill magnets is shown by an arrow.

However, when the top of the pill magnets are flush with the top of the donut magnets the repulsion force of the north poles cancels and the force goes to zero.

Drop a pill magnet onto the pile and the south pole will orient down. It will fall onto the two pills and extend the north pole of the pill stack out of the hole.

When a third pill magnet falls onto the stack the north pole of the stack is once again above the surface of the donut magnets and the entire stack is repelled.

The entire stack will then be ejected.


The trick of making the stack of magnets jump out of the hole was shown to me by master magician and author of the Mathematical Games column in Scientific American magazine, Martin Gardner.

Scientific Explorations with Paul Doherty

© 2005

27 July 2005