Louis Slotin was born in Winnipeg in 1910 to Yiddish-speaking Jewish refugees and entered the University of Manitoba at sixteen, later earning a doctorate in London and helping build the first cyclotron in the American Midwest to trace photosynthesis with radiocarbon. That rare blend of nuclear chemistry and radiobiology brought him to Los Alamos, where he assembled the Trinity core and became known informally as the chief armorer.

His job was criticality testing, deliberately pushing fissile cores toward a chain reaction, a task Feynman likened to tickling a dragon's tail. Slotin favored a reckless method, holding two beryllium hemispheres apart with a screwdriver blade. On May 21, 1946, the blade slipped, the shell closed, and the demon core went prompt critical in a blue flash. He flipped it off with his bare hand but had absorbed a leth ...