The war ended with Emperor Hirohito’s “Jewel Voice” recorded radio announcement to the people of Japan on August 15, six days after the final atomic bombing run. It was over, and to the Manhattan Project the shock was deep. After this intense effort and all the frantic war research and industrial production in the United States, to have all activity stop suddenly was not exactly possible. There would have to be a short wind-down, before nuclear weapon development would rebound. The design of the plutonium implosion bomb was under constant modification and improvement, even as the Fat Man was dropping on Nagasaki, and reasons would be found to continue the work.
The model Y-1561 bomb, while successful, left much to be desired, and work was underway to increase its efficiency, as if a 20-kiloton blast was not big enough. The nuclear explosion occurred when a barely subcritical ball of plutonium metal, 3.62 inches in diameter, was crushed down to the size of a large marble by an explosive shock wave, turned inward. The nuclei of the plutonium were forced closer together than normal, and the chances of being hit with a flying neutron and fissioning were increased accordingly. The subcritical sphere became supercritical, at least three times over, and the uncontrolled chain reaction grew with devastating speed.
The little ball of plutonium was plated with 5.0 mils of nickel to prevent it from spontaneously catching fire as it was exposed to air. Around the fissile ball was assembled a “tamper” shell, 8.75 inches in diameter. Its purpose was to keep the plutonium ball together as long as possible as it was exploding to ensure that a maximum number of fissions could occur. With the fission rate doubling 90 times in a microsecond, the once-solid ball would become a superheated plasma, trying to expand from an inch in diameter to hundreds of feet in diameter as quickly as possible. Reasoning that even an atomic blast could not accelerate matter from rest instantly, the scientists decided to make the tamper shell of uranium metal, depleted of its fissile isotope. Aside from plutonium, it was the heaviest element available, and therefore it would provide the most inertial resistance to sudden expansion.
It was a touchy design. The plutonium component was built so close to criticality, the material that would immediately surround it had to be chosen carefully.[40] There was reason to believe that substituting tungsten carbide (WC) for the uranium in the tamper would up the yield by a kiloton. There was one question that could not be answered by theory: Exactly how much WC could surround the plutonium ball before the carbon atoms would reflect enough neutrons back into it to make it cross the line and go supercritical?
Improbable as it now seems, the answer to that question was to experiment standing over a plutonium bomb core with some bricks made of WC, stacking them up until the thing was on the verge of a runaway chain reaction. A plutonium ball on a workbench was not a plutonium ball crushed by an explosive shock wave, and there was no way to make it go off as a bomb, but it could be the world’s smallest, most simple nuclear fission reactor. Change its situation slightly, like by reflecting some stray, spontaneous neutrons back into it, and it could “go critical,” a condition in which it was producing exactly as many neutrons by fission as were being lost by leakage or absorption. “Supercriticality” could be slight, in which the energy-release rate increases slowly, or it could be great, depending on the degree with which it was perturbed. “Tickling the dragon” involved the skill of making an eight-story house of cards. You had to be focused, alert, and stone sober.
Haroutune “Harry” Krikor Daghlian, Jr., was born in Waterbury, Connecticut, on May 4, 1921, to Haroutune and Margaret Daghlian, immigrated from Armenia. He earned a Bachelor of Science in physics at Purdue University. In the autumn of 1943, recruiters from the Manhattan Project found him working on the cyclotron at Purdue, trying to produce 10-MeV deuterons, and by 1944 he was working in Otto Frisch’s Critical Assembly Group at the Omega Site, Technical Area 2, at Los Alamos.
The Omega Site was stuck in a canyon, out of shrapnel range of the administrative and theoretical offices, so that only the technical class would be wiped out if an experiment were to go suddenly awry. By the end of the war, Daghlian had tickled the dragon so many times, he was at that very dangerous point where experience and confidence were so extreme, there was no need to be careful. Unlike the Oak Ridge workers, as a nuclear physicist he did not have ignorance as an excuse for not being terrified of his tasks.
All day on August 21, 1945, six days after Japan gave up, Daghlian worked on the WC loading for a 6.2-kilogram Mk-2 bomb core, standing over a low steel assembly table in the 49 Room at the Omega Site.[41] There were workbenches on all four walls of the room, a desk for the SED security guard on the east wall exactly 12 feet away from the assembly table, and in the southeast corner was a special vault, made to store bomb cores isolated from each other and from any radiation source.[42] A stack of WC bricks of different sizes and shapes was piled on a rolling dolly to his left, and he would try various configurations against the ball of plutonium, always aware of the radiation counters ticking in the racks to his right. He had two fission chambers running numerical counters, each sounding a click in a loudspeaker every time a neutron hit, and a BF3 chamber indicating the neutron count rate visually with a strip-recording milliammeter.[43] An experienced lab technician could tell easily if a criticality was imminent just by hearing the ticking sound become frantic, or at least mildly excited. In a specially machined brick, a 5-millicurie Ra-Be fixed neutron source sat against the ball, providing rogue neutrons to be multiplied by the plutonium and indicate its approach to criticality.
Daghlian was using rectangular WC bricks, 2.125 by 2.125 by 4.250 inches, and he found that the ball went critical when surrounded by five layers of bricks arranged as a cube with two bricks on top. He tried stacking the bricks differently, experimenting to find the minimum amount of WC that would cause the plutonium to take off. He logged out of the room at the end of the day after returning the sphere to the vault, scheduling another experiment with the bricks for the next day.
After dinner, he wandered over to the evening science lecture at theater no. 2, but something was bothering him about his last stack of bricks. He could not get it off his mind, and when the lecture broke up at 9:10, he went back to Room 49 in the canyon, arriving at 9:30. It was against regulations to perform a criticality experiment without an assistant, and it was certainly forbidden to do it after hours, but there was something he had to try or he could not sleep that night. Lights were on in the building.
Daghlian walked into the room, stood over the assembly bench for a second, then crossed the room to the plutonium vault to recover the ball. Sitting at the desk was SED guard Private Robert J. Hemmerly, reading a newspaper. There had to be a guard on duty 24 hours a day in the room where bomb cores were. Daghlian looked nervous and apprehensive for some reason. Hemmerly said “Hi, Harry,” and returned to reading.
40
Georgia Tech physics professor Nesbit Kendrick, my faithful source of atomic bomb stories, told me about the problem of inserting the plutonium “peach-pit” into the center of an Mk-4. You had to hold it connected to the end of a T-wrench and carefully insert it through a hole that was opened up in the high explosives (HE) that surrounded it. The explosives were complex organic compounds containing hydrogen, which is a very effective neutron moderator, and as the pit passed through the hole, slow neutrons would bounce back into it and it would go supercritical on a slow period! One did not linger in the midst of the HE, lest the fission neutrons boiling off the plutonium sphere burn the skin off the hand gripping the wrench handle.
41
The information in this account comes from the classified “REPORT OF ACCIDENT OF AUGUST 21, 1945 AT OMEGA SITE.” The report was unclassified on August 28, 1979, and publicly released on January 28, 1986.
43
The “fission chamber” is an ion-chamber tube lined with uranium-235. If a neutron wanders into it, a fission will likely occur in the uranium, and this ionizing event causes the gas in the tube to conduct electricity, which is countable as one neutron interception. The BF3 chamber is not quite as sensitive. The ion chamber is filled with boron trifluoride gas, and if a slow neutron is captured by the boron, the resulting radioactive decay of the activated boron will also ionize the gas and register as an encounter with a neutron.