Gilbert U-238 Atomic Energy Laboratory (en.wikipedia.org)

The relevant clip from the last episode of QI.

https://www.youtube.com/watch?v=ipFIxrsYSxs#t=3m40s

If you think the link is cool, and you wish you had even one piece of it, you can build your own cloud chamber at home:

http://www.symmetrymagazine.org/article/january-2015/how-to-...