6. Harry K. Daghlian
Harry Krikor Daghlian was a member of the Manhattan Project. He researched for ways to reduce the mass needed by a plutonium core to reach criticality. In an experiment, he stacked a number of 4.4kg brick-shaped tungsten carbide in an incremental fashion around the core.
As Daghlian about to place the final brick, sensors alerted him that the last brick would trigger super-critical state. Unfortunately, the brick fell accidentally onto the assembly and a power excursion occurred. Daghlian was exposed to about 500 rem of neutron radiation and died three weeks later.