The core looked like an enormous gray pearl resting inside a shiny beryllium shell. Slotin used a screwdriver to lower the top half of that shell—and then, at about 3:20 in the afternoon on May 21, 1946, the screwdriver slipped, the shell shut, the core went supercritical, and a blue flash filled the room. Slotin immediately threw the top half of the tamper onto the floor, halting the chain reaction. But it was too late: he’d absorbed a lethal dose of radiation. And he, more than anyone else in the room, knew it.
