A replica of the Demon Core, a sphere of Plutonium originally intended to be part of the third atomic bomb dropped on Japan. Japan surrendered too early for it to be used in the war, and it stayed at the Los Alamos Laboratory where it was involved in two accidents, both of which caused the core to briefly go supercritical, killing the scientists experimenting with it days later.

In the second accident, the plutonium was placed inside two beryllium neutron reflectors. Only a screwdriver separated the reflectors; when the screwdriver slipped, Louis Slotin, the physicist conducting the experiment was exposed to 1,000 rads of radiation and a blue flash was seen across the room. He died 9 days later.

I tried to replicate the geometry of mock ups of the Demon Core as well as I could. I don't think there are any photos of the actual Demon Core on the internet, but as far as I'm aware, this is as close as you can get to 'tickling the dragon's tail" which is probably a good thing.