http://www.todayifoundout.com/index.php/2016/02/the-demon-core/
Using the bricks, Daghlian built walls, about ten inches on a side and ten inches high, around the plutonium. He then took a brick and slowly positioned it—he was simply holding it in his hand—over the opening at the top of the structure, right over the core. The geiger counter clicked wildly. Enough neutrons were now being reflected back into the core that it was headed toward a supercritical state.
Daghlian went to jerk the brick away…and dropped it.
The brick landed right on top of the ball of plutonium. The plutonium was now effectively surrounded by neutron reflecting material, and it went supercritical immediately. There was a blue flash—an effect of the sudden release of radiation—and the geiger counter was screaming. Daghlian grabbed the dropped brick in a panic…and dropped it again. He tried to overturn the table he was working on—but it was too heavy. He finally just started taking the bricks away from around the plutonium, one by one. The chain reaction finally stopped, and the geiger counter quieted down. Roughly one minute had passed. It was one minute too much for Harry Daghlian. He had been exposed to a massive amount of radiation. Within hours he started feeling nauseated, the first sign of radiation sickness. He checked himself into a hospital. After a few days his hands, which had received the brunt of the radiation, began to blister due to radiation burns. He deteriorated steadily after that, and, on September 15, twenty-five days after the accident, Harry Daghlian died.
Re-enactment of the Slotin incident:
Using the bricks, Daghlian built walls, about ten inches on a side and ten inches high, around the plutonium. He then took a brick and slowly positioned it—he was simply holding it in his hand—over the opening at the top of the structure, right over the core. The geiger counter clicked wildly. Enough neutrons were now being reflected back into the core that it was headed toward a supercritical state.
Daghlian went to jerk the brick away…and dropped it.
The brick landed right on top of the ball of plutonium. The plutonium was now effectively surrounded by neutron reflecting material, and it went supercritical immediately. There was a blue flash—an effect of the sudden release of radiation—and the geiger counter was screaming. Daghlian grabbed the dropped brick in a panic…and dropped it again. He tried to overturn the table he was working on—but it was too heavy. He finally just started taking the bricks away from around the plutonium, one by one. The chain reaction finally stopped, and the geiger counter quieted down. Roughly one minute had passed. It was one minute too much for Harry Daghlian. He had been exposed to a massive amount of radiation. Within hours he started feeling nauseated, the first sign of radiation sickness. He checked himself into a hospital. After a few days his hands, which had received the brunt of the radiation, began to blister due to radiation burns. He deteriorated steadily after that, and, on September 15, twenty-five days after the accident, Harry Daghlian died.
Re-enactment of the Slotin incident: