Field of Science

Born on this day: Physicist Emilio Segrè, who changed the direction of the Manhattan Project

The road to mutually assured destruction unintentionally started in 1944, at the end of a mountain trail with flowers and snakes, with a man called The Basilisk. But he didn't know it then. In the fall of 1944, the sprawling, top secret atomic bomb project at Los Alamos faced a huge crisis. Ever since the laboratory had been set up in 1943, it had been working on two kinds of bombs. One would work on uranium which was being separated using cyclotrons at Oak Ridge, Tennessee. The other would work on plutonium which was being created in nuclear reactors in Hanford, Washington. The basic design for nuclear weapons had been worked out in 1942 and was simple in principle: slam together two lumps of uranium and plutonium using a "gun" so that they come together to create a critical mass, and the ensuing exponential chain reaction would guarantee a tremendous explosion. In 1944 it seemed like that design was well underway for both uranium and plutonium. 

Then in the fall of 1944 came news that this design would be useless for plutonium. Millions of 1944 dollars and the resources of thousands of workers and scientists were at stake. The laboratory's director, Robert Oppenheimer, felt so embattled that he briefly considered resigning. The man who delivered this bad news to Oppenheimer and the others was Emilio Segrè.

Emilio Segrè who was born today in 1905 was a member of the small band of brilliant physicists led by Enrico Fermi in Rome in the 1930s. Each member of the team had a nickname. Fermi was "The Pope" because of his infallible knowledge of physics. Segrè's sharp tongue and quick mind made others name him "The Basilisk". Segrè was part of all of Fermi's key experiments, including the ones leading up to his Nobel Prize. Segrè himself won a Nobel Prize later for his discovery of the antiproton. But in 1944, he must have been seen as the bearer of bad news. This news, however, galvanized the lab and forced it to explore a novel idea that became the basis of nuclear arsenals around the world.

At Los Alamos Segrè was assigned the task of measuring the neutron properties of uranium and plutonium and their byproducts. Because his work involved large quantities of potentially dangerous neutrons, he was put in a cabin several miles removed from the main laboratory. The cabin was at the end of a trail populated by rattlesnakes and beautiful flowers. What Segrè discovered in that cabin was that the plutonium bomb would fizzle out prematurely if it were assembled by the gun technique. Premature explosion had been considered by the Los Alamos scientists and was not thought to be a serious issue. The probability of premature explosion depended on the rate of neutrons spontaneously generated by the fissile mass. Left to themselves, uranium and plutonium have a small but fixed background rate of spontaneous neutron emission. During critical assembly, a high rate of neutron emission meant that the fission would start occurring before the critical mass was reached, and the result would at best be a small, inefficient explosion. It was thus essentially a contest between this rate and the speed with which the two pieces were brought together that would decide the fate of the bomb.

Physicists were not ignorant of the rate of spontaneous fission in uranium and plutonium and in fact had measured it to reassure themselves that it would not matter. However, they had reached this conclusion based on experiments on small quantities of uranium-238 and plutonium-239 produced in cyclotrons built by Ernest Lawrence's team at Berkeley. However cyclotrons cannot be used to make plutonium in any measurable quantity. By the time the Manhattan project picked up steam, Pu-239 was being produced in gram quantities in the nuclear reactors at Hanford.

What the physicists had not realized was the role that a rogue isotope of plutonium would play in thwarting their plans for a gun type bomb. It turns out that the spontaneous fission depends on the precise isotope under consideration. Plutonium-239 is produced by bombarding uranium-238 by neutrons. However there is another isotope of plutonium that is produced in the process: Pu-240. Pu-240 has a much greater spontaneous fission rate than Pu-239. The net rate depends on the ratio of the two isotopes.

The crucial discovery which Segrè made was that reactor-produced plutonium had a much higher percentage of Pu-240 than cyclotron-produced plutonium and therefore a much higher spontaneous fission rate. All the spontaneous fission rate measurements done by the physicists on cyclotron-produced plutonium were therefore useless for reactor-produced plutonium. The high spontaneous fission rate in reactor-produced plutonium would doom a gun type plutonium weapon to a premature fizzle. Chemical separation of the Pu-240 from the Pu-239 was also out of the question: chemically separating uranium 235 (the fissile isotope of uranium) from the more abundant uranium 238 was already a mammoth undertaking that strained the country's resources. Separating Pu-240 from Pu-239 would be almost impossible.

Segrè's bad news plunged the laboratory in a big crisis. Fortunately there was a physicist named Seth Neddermeyer who had proposed a backup alternative for assembling a plutonium bomb. This alternative was implosion and consisted of rapidly squeezing a sphere of plutonium inwards to criticality. Implosion would be much faster than a gun type assembly and the spontaneous fission rate would not be a problem. The laboratory was rapidly reorganized by Oppenheimer and Leslie Groves to make implosion a high priority.

Implosion was still a risky endeavor, so the first atomic bomb test that the world saw on July 16, 1945 was of the implosion bomb. Less than a month later, Nagasaki was destroyed by the same kind of bomb. And ten years later the United States and other countries were building terrible hydrogen bombs in which plutonium implosion was an essential mechanism. Since then thousands of hydrogen bombs have been added to the world's nuclear arsenals, placing humanity at the risk of instant obliteration. What Segre thought about this is lost to history.

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