The 14 lb 3.5 inch diameter sub-critical mass of plutonium (core) went critical (a self-sustaining chain reaction) when Daghlian accidentally dropped a brick on it.
On 21 May 1946 Louis Slotin fell victim to the Demon Core (and his own bravado) conducting an experiment to determine at which point the core could be made critical by the positioning of nuclear reflectors. Stolin used the unorthodox method of manipulating the reflectors with a normal flat head screwdriver.
The screwdriver slipped, causing the reflector to drop onto the core, and the core to become super-critical. It released a massive burst of neutron radiation. Stolin quickly knocked the two halves apart, stopping the chain reaction. He died nine days later from acute radiation poisoning.
It got its nickname—'Demon Core'—after this second incident.
After these two fatal incidents, the Demon Core was used in the first atomic bomb test after World War II. It performed normally and displayed the same explosive yield as the next core used in this set of two tests.
