The United States had, toward the last phase of World War II, planned out its atomic bomb attacks against Japan rather meticulously, with a list of targets that would evolve. The Target Committee listed five cities: Kyoto, Hiroshima, Yokohama, Kokura, and Niigata. First on the list was Kyoto because it was a large and virgin city. Hiroshima was an addition because of the large military base there and its geographic features, which would cause the bomb to have maximum destructive force.
By June 1945, the list was finally whittled down to four targets: Kyoto, Hiroshima, Kokura, and Niigata. So, these places were “reserved targets” to save them from conventional firebombing. However, Secretary of War Henry Stimson struck Kyoto off the list due to certain strategic and sentimental reasons, against the protests of Gen. Leslie Groves. Ultimately, President Truman agreed with Stimson, and Kyoto was spared.
Already engaged in the Potsdam Conference in July 1945, the Allies furthered their attack on Japan with firebombing. During this time, Truman would be informed about the successful Trinity test. He, along with Stimson, continued with the interest in the bomb as an instrument to end the war and give its Moscow counterpart good lessons.
The final list of targets, sent in a ciphered message from Potsdam, where Stimson was, to Groves in Washington, D.C., comprised Hiroshima, Kokura, Niigata, and Nagasaki. The directive of July 25, 1945, specified that the 20th Air Force would drop its first “special bomb” as a target chosen by the commanders on or after August 3, 1945. The bombing was to be carried out visually, with further bombs deployed as soon as they became available. This was not a directive for a single atomic bomb but an authorization to use as many as were ready.
Parallel to these wartime plans, the story of the “Demon Core” unfolded in the post-war period. On August 21, 1945, just days after Japan’s surrender, physicist Harry Daghlian managed to drop a tungsten-carbide brick onto a plutonium core, causing it to go supercritical. He quickly removed the brick, but as Daghlian received a lethal dose of radiation, he died 25 days later.
Nine months later, on May 21, 1946, Louis Slotin was demonstrating a criticality experiment with the core to a group when the screwdriver he used to separate two beryllium hemispheres slipped, allowing the hemispheres to close around the core, causing it to go critical. A blue flash and burst of radiation followed that proved fatal for Slotin within nine days.
These incidents led to the plutonium core being called the “Demon Core.” Following Slotin’s death, all hands-on experiments with criticality tests at Los Alamos ceased. The core, which had initially been slated for a third atomic bomb, was ultimately melted down and recycled into new warheads.
The tragic accident that befell Slotin drove home the point of just how much danger scientists on the Manhattan Project were exposing themselves to. Though he was perhaps among the most knowledgeable there, and quite literally one of those who had assembled the very first atomic bomb, Slotin’s name is not quite so etched in the annals of history. His sacrifice stands as a testament to just how dangerous it could be to strive for scientific and military progress.