As I understand it, this was just a demonstration of the technique. The actual experiment called for metal shims between the cores, which I assume were of know

As I understand it, this was just a demonstration of the technique. The actual experiment called for metal shims between the cores, which I assume were of known thicknesses. Also, from the American Physical Society's writeup: "Nine days after the accident, Slotin died after “a total disintegration of bodily functions.” Ironically, he had become disillusioned with the postwar atomic tests, and one reason for that ill-fated experiment was to train a replacement, so Slotin could resume his prewar research in biophysics and radiobiology at the University of Chicago. He never got that chance. In response to his death, Los Alamos ceased all hands-on experiments with critical assemblies, using remote controlled machines to protect the operators. As for the killer “demon core,” it was used in one of the first postwar atomic bomb tests at Bikini Atoll, a mere five weeks after Slotin’s death. The test went off without a hitch." https://www.aps.org/publications/apsnews/201405/physicshistory.cfm https://www.aps.org/publications/apsnews/201405/physicshisto...