Yes, what they did there was to drop increasing amounts of U235 through a ring made of U235, so that for a split second there would be critical mass, with the reaction just starting. The rate of neutron emission was measured and gave the information about the propogation rate of the reaction. The reaction was stopped a split second later, when the U235 piece had dropped through the ring. Once the piece got stuck, one scientist pulled it away and was lethaly irradiated by the intense neutron and gamma radiation. He died a few weeks later of radiation disease, but with his action he saved the rest of the crew. BTW, though there was an intense fission reaction going on, there was no danger of a full yield nuclear explosion, because both the core as well as the ring of U235 were not contained. The whole assembly would have been blown appart before the total amount could react.
Jan
