yield.
By May 31 enough plutonium had arrived at Los Alamos from Han-ford to begin critical-mass experiments. Seth Neddermeyer's shell-configured core had been abandoned even though thin-walled shells give the highest compressions in implosion. Designing out their hydrodynamic instabilities required calculations too dificult to accomplish by hand. Berkeley theoretician Robert Christy designed a more conservative solid core, two mated hemispheres totaling less than one critical mass that implosion would squeeze to at least double their previous density, shortening the distance that fission neutrons would have to travel between nuclei and rendering the mass supercritical. Frisch's group confirmed the core configuration experimentally on June 24. For the high-density form of Pu the critical mass within a heavy tamper is eleven pounds; even with a nut-sized central hollow to encapsulate an initiator the Trinity core cannot have been larger than a small orange.
Delivery of full-sized molds for the implosion lens segments paced the test; they began arriving in quantity only in June, and on June 30 the committee responsible for deciding the test date moved it back to July 16 at the earliest. Kistiakowsky's group worked night and day at S-Site to make enough lenses. “Most troublesome were the air cavities in the interior of the large castings,” he recalled after the war, “which we detected by x-ray inspection techniques but could not repair. More rejects than acceptable castings were usually our unfortunate lot.”
Groves met with Oppenheimer and Parsons on June 27 to lay plans for shipping the first atomic bombs to the Pacific. They agreed to send the Little Boy U235 projectile by water and the several U235 target pieces later by air; the shipping program acquired the code name Bronx because of that New York borough's adjacency to Manhattan. The metallurgists at Los Alamos cast one target piece before the end of June and the U235 bullet on July 3. The next day, Independence Day, the Combined Policy Committee met in Washington and the British officially gave their approval, as the Quebec Agreement provided, for the use of atomic bombs on Japan.
Truman had agreed to meet with Stalin and Churchill in the Berlin suburb of Potsdam sometime during the summer; he told Stimson on June 6 that he had succeeded in postponing the conference until July 15 “on purpose,” Stimson wrote in his diary, “to give us more time.” Though Truman and Byrnes had not yet decided to tell Stalin about the atomic bomb, a successful test would change the Pacific equation; they might not need a Soviet invasion of Manchuria to challenge the Japanese and might therefore have to trade away less in Europe. To make sure the President had news of the test at Potsdam, Groves decided during the first week in July to fix the test date at July 16, subject to the vagaries of the weather. He had learned late in June of the possibility of dangerous radioactive fallout over populated areas of New Mexico — “What are you,” he berated the Los Alamos physician who gave him the news, “some kind of Hearst propagandist?” — or he would not have waited even on the weather.
So the shot was set for sometime in mid-July, in the heat of the desert summer when the temperature on the Jornada often burned above 100° late in the day. Oppenheimer wired Arthur Compton and Ernest Lawrence: ANY TIME AFTER THE 15TH WOULD BE A GOOD TIME FOR OUR FISHING TRIP. BECAUSE WE ARE NOT CERTAIN OF THE WEATHER WE MAY BE DELAYED SEVERAL DAYS.
The senior men arranged a betting pool with a one-dollar entry fee, wagering on the explosive yield. Edward Teller optimistically picked 45,000 tons TNT equivalent. Hans Bethe picked 8,000 tons, Kistiakowsky 1,400. Oppenheimer chose a modest 300 tons. Norman Ramsey took a cynical zero. When 1.1. Rabi arrived a few days before the test the only bet left was for 18,000 tons; whether or not he believed that might be the Trinity yield, he bought it.
As of July 9 Kistiakowsky did not yet have enough quality lens castings on hand to assemble a complete charge. Oppenheimer further compounded his troubles by insisting on firing a Chinese copy of the gadget a few days before the Trinity shot to test its high-explosive design at full scale with a nonnssionable core. Each unit would require ninety-six blocks of explosive. Kistiakowsky resorted to heroic measures:
In some desperation, I got hold of a dental drill and, not wishing to ask others to do an untried job, spent most of one night, the week before the Trinity test, drilling holes in some faulty castings so as to reach the air cavities indicated on our x-ray inspection films. That done, I filled the cavities by pouring molten explosive slurry into them, and thus made the castings acceptable. Overnight, enough castings were added to our stores by my labors to make more than two spheres.
“You don't worry about it,” he adds fatalistically. “I mean, if fifty pounds of explosives goes in your lap, you won't know it.”
Navy Lieutenant Commander Norris E. Bradbury, a brisk, energetic Berkeley physics Ph.D., took charge of assembling the high explosives. On Wednesday, July 11, he met with Kistiakowsky to sort the charges according to their quality. “The castings were personally inspected by Kistiakowsky and Bradbury for chipped corners, cracks, and other imperfections,” writes Bainbridge. “… Only first-quality castings which were not chipped or which could be easily repaired were used for the Trinity assembly. The remainder of the castings were diverted for the Creutz charge” — so named for Edward Creutz, the physicist who was running the Chinese copy test. The castings were waxy, mottled, brown with varnish. They weighed in total, for each device, about 5,000 pounds.
Everyone felt the pressure of the approaching test. It took its toll. “That last week in many ways dragged,” Elsie McMillan remembers; “in many ways it flew on wings. It was hard to behave normally. It was hard not to think. It was hard not to let off steam. We also found it hard not to overindulge in all the natural activities of life.” In a letter to Eleanor Roosevelt in 1950 Oppenheimer recalled an odd group delusion:
Very shortly before the test of the first atomic bomb, people at Los Alamos were naturally in a state of some tension. I remember one morning when almost the whole project was out of doors staring at a bright object in the sky through glasses, binoculars and whatever else they could find; and nearby Kirtland Field reported to us that they had no interceptors which had enabled them to come within range of the object. Our director of personnel was an astronomer and a man of some human wisdom; and he finally came to my office and asked whether we would stop trying to shoot down Venus. I tell this story only to indicate that even a group of scientists is not proof against the errors of suggestion and hysteria.
By then the two small plutonium hemispheres had been cast, and plated against corrosion and to absorb alpha particles with nickel, which made the assembly, as metallurgist Cyril Smith would write, “beautiful to gaze upon.” But “an unscheduled change began to be evident three or four days before the scheduled date.” Plating solution trapped beneath the plating on the flat faces of the hemispheres began to blister the nickel, spoiling the fit. “For a time,” says Smith, “postponement of the whole event was threatened.” Completely filing off the blisters would expose the plutonium. The metallurgists salvaged the castings by grinding only partway through the blisters and smoothing the bumpy fit with sheets of gold foil. The core of the first atomic bomb would go to its glory dressed in improvised offerings of nickel and gold.
A tropical air mass moved north over Trinity on July 10, just as the test meteorologist, Caltech-trained Jack M. Hubbard, thirty-nine years old, had predicted. Hubbard had resisted the July 16 date, a Monday, since he first heard of it; he expected bad weather that weekend. The Gulf air suspended salt crystals that diffused a slight haze. On July 12, worrying about Potsdam, Groves confirmed the test for the morning of July 16. Bainbridge passed the word to Hubbard. “Right in the middle of a period of thunderstorms,” the meteorologist stormed to his journal, “what son-of-a-bitch could have done this?” Groves had been awarded such scurrilous genealogy before.
The general's decision started Norris Bradbury and his crews of Special Engineering Detachment GI's — SED's, the science-trained recruits were called — assembling the Trinity and Creutz high-explosive charges at two separate canyon sites near Los Alamos mesa that Thursday. They debated filling the small air spaces between the castings with grease. Kistia-kowsky decided against such filler, writes Bainbridge, “on the basis that the castings assembled were much better than any previously made and that the air spaces left by the spacer materials were insignificant.” The charges, each of which had been X-rayed one last time and numbered, were papered into snugness instead with facial tissue and Scotch tape. The simplified and improved casing of the unit to be tested, which was designated model 1561, differed from the earlier 1222 casing of bolted pentagons; it featured an equatorial band of five segments machined from dural castings to which were bolted large upper and lower domed polar caps. When the explosives that lined the lower hemisphere had been papered into place Bradbury's SED's winched down the heavy tamper sphere of natural uranium, which filled the cavity like the pit in an avocado. The tamper was missing a cylindrical plug; the resulting hole would receive the core assembly. The explosive blocks that formed the upper shell followed next.
