Alex Wellerstein in The New Yorker:
The demonstration began on the afternoon of May 21, 1946, at a secret laboratory tucked into a canyon some three miles from Los Alamos, New Mexico, the birthplace of the atom bomb. Louis Slotin, a Canadian physicist, was showing his colleagues how to bring the exposed core of a nuclear weapon nearly to the point of criticality, a tricky operation known as “tickling the dragon’s tail.” The core, sitting by itself on a squat table, looked unremarkable—a hemisphere of dull metal with a nub of plutonium sticking out of its center, the whole thing warm to the touch because of its radioactivity. It had been quickly molded into shape after the bombing of Nagasaki, to be used in another attack on Japan, then reallocated when it turned out not to be needed for the war effort. At that time, Slotin was perhaps the world’s foremost expert on handling dangerous quantities of plutonium. He had helped assemble the first atomic weapon, barely a year earlier, and a contemporary photograph shows him standing beside its innards with his shirt unbuttoned and sunglasses on, cool and collected. Back then, the bomb was a handmade, artisanal product.
Slotin’s procedure was simple. He would lower a half-shell of beryllium, called the tamper, over the core, stopping just before it was snugly seated. The tamper would reflect back the neutrons that were shooting off the plutonium, jump-starting a weak and short-lived nuclear chain reaction, on which the physicists could then gather data. Slotin held the tamper in his left hand. In his right hand, he held a long screwdriver, which he planned to wedge between the two components, keeping them apart. As he began the slow and painstaking process of lowering the tamper, one of his colleagues, Raemer Schreiber, turned away to focus on other work, expecting that the experiment would be uninteresting until several more moments had passed. But suddenly he heard a sound behind him: Slotin’s screwdriver had slipped, and the tamper had dropped fully over the core. When Schreiber turned around, he saw a flash of blue light and felt a wave of heat on his face.