Through the last months of World War II, the United States began to make its final plans for its atomic attack against Japan. The Target Committee whittled the list down to five cities by early May 1945: Kyoto, Hiroshima, Yokohama, Kokura, and Niigata. Kyoto was originally the first choice; it was the largest of the four cities and remained untouched up until that time. Secretary of War Henry Stimson, for reasons both strategic and sentimental, persuaded President Truman to spare the former Japanese capital against the protests of Gen. Leslie Groves, arguing for its military significance.
By July 1945, when the Potsdam Conference had just begun, Japan was under an unrelenting attack by the Allied forces through firebombing. During this time, Truman and Stimson were briefed on the success of the Trinity test. More energetic perhaps now than he had ever been in using the bomb against Japan to conclude the war and to send a message to the Soviet Union was President Truman.
The final target list, in messages between Stimson at Potsdam and Groves in Washington, D.C., was encrypted as Hiroshima, Kokura, Niigata, and Nagasaki. This directive of July 25, 1945, authorized the 20th Air Force to deliver the first “special bomb” on the selected targets after August 3, 1945. The order was unequivocal: further bombs would be delivered as soon as they became available, with new targets to be chosen when the original ones had been destroyed.
In the aftermath of the bombings, the scientific community at Los Alamos became yet again the center of attention. Several accidents happened with the “Demon Core”. On May 21, 1946, the Canadian physicist Louis Slotin was performing a criticality experiment called “tickling the dragon’s tail.” The core was a leftover from the Nagasaki bomb and was quite near achieving criticality when the screwdriver of Slotin slipped, fully dropping the beryllium tamper over the core. A flash of blue light and a wave of heat greeted a brief, intense radiation burst.
His quick action to reverse the tamper of the core was not fast enough to save him from the lethal dose of radiation. After initial treatment, Slotin made a partial recovery from symptoms and rapidly sickened and died nine days later from acute radiation syndrome. This unfortunate incident came within a year from another similar incident with Harry Daghlian, who died in an accident when he dropped a tungsten-carbide brick on the same core back in August 1945.
These incidents brought all criticality experiments at Los Alamos to a grinding halt. The core became infamously known as the “Demon Core.” The core was to be used in an Operation Crossroads event at Bikini Atoll; however, the high levels of radioactivity, a result of Slotin’s accident, meant it would be melted down and recast into new warheads.
The stories of the atomic bombings and the Demon Core reflect just how dangerous the junction of war and scientific experimentation was at that pivotal moment in military history.