Administration Building
Los Alamos, New Mexico
1:00 PM, July 17, 1944
“Now let me get this straight,” Groves bellowed. “The plutonium ‘bullet’, which is traveling at 3000 feet per second, will ‘MELT’ before it can fully enter the plutonium ring? Come on, fellows, it only has to travel, what, 3-iches or so and its traveling at 36,000 inches per second and you say it will melt before it can move that short distance. You must admit that sounds crazy. Why, it would only take about a thousandth of a second to travel that far!”
The General was gathered in the conference room of the Administration building on the Hill with the top ‘brains’ of the Manhattan project, including Fermi, Bethe, Compton, Oppenheimer, Segrè, and Rabi. They had invited him to the Lab to break the news to him in person about the impact of the Pu-240 isotope on the project’s direction. He wasn’t taking it too well.
Fermi, busy working his slide rule while Groves ranted, looked up from it and said, “You are correct about the time, General, or at least very close. It would actually take 83 microseconds to travel that far, about one ten-thousandth of a second. But the next question you need to ask yourself is how many neutrons would the Pu-240 kick off during this journey, bearing in mind that once a critical mass is present it only takes a few, say 5 to 10 neutrons to initiate the chain reaction.”
“Okay,” Groves replied gruffly, “I’ll bite. How many neutrons will it kick out?”
Emilio Segrè, who had measured the spontaneous fission rate of Pu-240 and the other fissile isotopes, responded, “Pu-240 will produce about a million neutrons per second per kilogram. So in 83 microseconds a kilogram of it would produce 83 neutrons. If the concentration of Pu-240 is say 1 percent, and it is probably higher, and if the critical mass is say 10 kilograms, this means that there would be 0.1 kilograms of Pu 240 present, so the answer, General, is about 8 neutrons.”
Groves had stared at Segrè throughout this discourse. The room fell silent awaiting his response. Finally he said, “Eight cotton-pickin’ neutrons, is that all? Oppie, are you going to tell me that with all those millions of neutrons that you say are going to be released when this thing goes bang, our whole damn project has to totally change direction because of eight measly, cotton-pickin’ neutrons?”
“That’s a real pisser, isn’t it General,” Oppie answered in the Generals own vernacular. “Yes, sir, those eight damn neutrons will bring our whole show to a halt if we don’t do something about them.”
Groves just sat there, shaking his head as he looked around the room. “Look at all you smart guys,” he said. “I can’t believe it. All these brains and you can’t figure out what to do about eight crummy neutrons.”
Will felt it was time for him to intercede and defend his brainy peers. “Actually, sir, we have figured what to do about it. If we drive those masses together with an implosion device we can assemble the critical mass in perhaps a tenth of the time. So that means only 1 neutron would be emitted. We will have in effect removed 7 of the 8 neutrons you find so objectionable.”
It took great discipline for the assembled scientists to keep from sneering or snickering at this grand over-simplification, but they took their cue from Oppie, who said, “Will’s right, General. That’s why we have invited you out here today. We want to start an all-out assault on the implosion method. We have a long way to go to perfect it, and we want your blessing for this redirection of our effort.”
“Can you make it work?” Groves asked, ever the pragmatist.
“We don’t honestly know,” Oppie answered.
“What are the odds?” Groves, growing impatient, persisted.
“I would bet dollars to donuts that we can make it work in time for the arrival of sufficient quantities of plutonium,” Oppie responded, cleverly redirecting Groves thinking toward the schedule.
Groves responded, “That means you would be ready by next spring or early summer at the latest.”
“Yes,” Oppie responded simply.
“What, specifically, does this ‘all-out’ effort entail?” Groves asked.
“I want to double the size of the theoretical directorate,” Oppie said. “I want to bring over some additional help from the UK. I want to reorganize the directorate into a ‘gadget’ design division and an explosives division and put Parsons in charge of ‘gadget’ design and Kistiakowsky in charge of explosives.” This last, Will knew, would set Groves off.
“That’s nuts,” Groves said, fulfilling Will’s expectation. “Parsons is an explosives guy.”
“I’ve talked to him, General” Oppie said. “He is okay with the move. He agrees that he has run out of ideas on how to make implosion work.”
“What about Kistiakowsky?” Groves asked.
“He is raring to go,” Oppie replied. “Luis Alavarez will assist him, as well as John von Neumann. It should be a real strong team with fresh ideas.”
“Go for it,” Groves said, bringing to Will’s mind a similarly simple imperative that Groves directed in Oak Ridge last November. The group emitted a brief, collective sigh of relief.
Groves turned to another topic. “What about the uranium gadget, why doesn’t it have a similar problem?” he asked.
Segrè picked this one up too. “Do you recall, General,” he asked, “that I said a kilogram of Pu-240 spontaneously emits about a million neutrons a second? Well a kilogram of U-235 spontaneously emits only one neutron in a hundred seconds. So it is just not a problem.”
“In fact,” Bethe chimed in, “this brings about some good news. You may recall that initially we thought we would need a 15-ft long gun to assemble the uranium gadget at a speed of 3000 ft per second. We now believe we can do this with only a nine-ft. long gun tube and at a much slower 1000 ft per second. This means much less explosive driver is needed, which correspondingly reduces the barrel pressure and allows us to reduce the wall thickness of the gun barrel. The upshot is that we can now significantly reduce the overall weight of the weapon.”
“Terrific,” Groves said. “So we are going to have one ‘fat’ gadget and one ‘skinny’ one, kinda like me and Oppie.”
Everybody laughed. “Fat Man and Little Boy,” Will noted. “Sounds like the gadgets have now been christened.”
“Don’t go getting religious on us, Townsend,” Groves warned. |
