Leona Libby, the Pioneering Nuclear Physicist Who Helped Build the Atomic Bomb

‘Oppenheimer’ was one story of many to come out of the Manhattan Project. Leona Libby's is another.

Leona Libby with colleagues in 1946.
Leona Libby with colleagues in 1946. / Corbis Historical/Getty Images

The world was at war and on the brink of nuclear destruction. Leona Marshall Libby had a more pressing concern: How to disguise her pregnancy.

In her early twenties, Libby was part of an elite team assembled by Italian physicist Enrico Fermi. Their work would inform the eventual assembly of the nation’s first atomic bomb, the deployment of which would bring a hastened end to World War II. For Libby, the responsibility was compounded by the fact that she was fresh out of college. Of the 41 members of Fermi’s team, she was also the only woman scientist; and now she was pregnant, a condition that in the 1940s often meant being involuntarily removed from employment.

Not wishing to be distanced from the most important work of her life, Libby buried her increasingly distended stomach in bulky clothes, keeping it a secret from almost everyone. Each day, she went in looking for breakthroughs that would spare lives in war, sometimes at the risk of her own. And at the end of it all, she would wonder if helping to create the most formidable weapon of mass destruction yet conceived was a noble act—or one that that humanity would come to regret.

Atomic Adventures

Leona Marshall Libby was born Leona Woods in Proviso, Illinois, in 1919, the daughter of Weightstill Woods, a lawyer, and Mary Holderness Woods, a teacher and potter. Academia seemed a likely path: She graduated high school at the age of 14 and had earned a Bachelor of Arts in chemistry at the University of Chicago at age 19.

As Libby worked on her doctoral thesis and pursued a joint fellowship in physics and chemistry, Pearl Harbor was attacked. The assault forced the United States to make a formal entry into World War II by declaring war on Japan on December 8, 1941. With that came a concurrent need to hasten U.S. progress in the secret nuclear arms race with Germany. America’s best and brightest, including J. Robert Oppenheimer, were recruited for the Manhattan Project to research and ultimately build an atomic weapon. Officials hoped that the mere existence of such a weapon would deter and ultimately defeat the Axis Powers.

Leona Libby is pictured
Leona Libby. / Argonne National Laboratory, Wikimedia Commons // Public Domain

The project was not centralized in any one location but across many institutions and groups. Enrico Fermi, an Italian physicist, was associated with Libby’s own University of Chicago. His mission: to create a self-sustaining nuclear chain reaction, a key building block in the development of the bomb.

By this time, Libby had earned her doctorate, and her credentials made her an easy pick, though her gender and age (23) didn’t make her an obvious one. As with many professions in this era, men often looked askance at women as peers. And women were certainly in the minority: They made up roughly 11 percent of Oppenheimer's Los Alamos workforce, and about half of them were scientists.

Physicist Anne McKusick, who worked at the Oak Ridge, Tennessee, laboratory (which enriched uranium for the Los Alamos crew), recalled, “When I got to Oak Ridge, it was perhaps not surprising that there were no girls who were physicists … It was just that women weren’t thought to be capable of learning the subject, or thought that it was strictly a man’s field at that time.”

Libby had no such thoughts. In fact, she appeared brimming with confidence. Fermi once asked how she felt about a book he had authored about thermodynamics. It’s a good baby book, she said, as though he had written a decent beginner’s guide on the subject.

Libby joined the University of Chicago team at the metallurgical lab in August 1942 for the purpose of developing a boron trifluoride counter, which could detect neutrons vital to the chain reaction. In addition to being complex, the work had the added stress of being potentially hazardous. Once, after being exposed to gamma rays, Libby was informed her white blood cell count had dropped by half. Physicians warned her further exposure could potentially affect her fertility. It was salient advice: Libby was newly married to John Marshall, a fellow chemist and Manhattan Project recruit, and they were planning to start a family.

Because Fermi and his team didn’t have explicit knowledge of how much progress German nuclear scientists had made with their weapon—as far as Fermi knew, it was possible they had already developed a human-generated nuclear chain reaction—they worked at incredible speed. In the fall of 1942, they slowly built up what would be known as Chicago Pile 1 (CP-1), a towering, 20-foot-tall construction that was assembled from graphite blocks and uranium. If done properly, the construct should be able to release a controlled amount of energy, with uranium atoms split by neutrons. That would result in more neutrons, which would in turn split more of the atoms. Scaled up, the energy would result in a bomb of previously unimaginable destructive power. Day in and day out, Libby hefted graphite blocks, getting the sooty dust all over her face and hands. It was the graphite that would keep things controlled.

Chicago Pile I: The World's First Nuclear Reactor
Chicago Pile I: The World's First Nuclear Reactor / Historical/GettyImages

For the CP-1 experiment, Fermi, Libby, and the team didn’t want to immolate themselves: They merely wanted to prove such a self-sustaining chain reaction was possible. Finally, on December 2, 1942, almost one year to the day after the attack on Pearl Harbor, the scientists gathered on an old squash court at the University of Chicago and observed their achievement: a tower made of roughly 40,000 blocks of graphite and uranium, each the size of a large loaf of bread. The uranium used had low amounts of fissionable isotopes, which meant, in theory, the experiment could demonstrate the reaction without causing a massive explosion.

In theory.

One by one, the control rods plated with cadmium to control the reaction were removed. As Libby and others watched and took readings over the next few hours, CP-1 began reacting. Nearby, a few men stood ready with a neutron-absorbing material in the event things went awry. Then CP-1 went critical, creating the first human self-sustaining nuclear chain reaction. Though it released only about a half-watt of energy—perhaps enough to fuel a flashlight—it was lighting the fuse that would ultimately transform nuclear capabilities.

“It was a day that had been building up for many months,” she later recalled. “It was exciting—not a happy birthday kind of excitement, but the quiet excitement of a physics experiment.”

Later, a bottle of Chianti was passed around to celebrate. Libby, like all team members, wound up signing the bottle. She was the only woman to witness an event that would lead directly to the end of the war.


Following the team’s success in Chicago, Libby moved on to Hanford, Washington, where the Hanford Engineer Works was producing plutonium for the Los Alamos operations (a chain reaction large enough to fuel an atomic bomb needed a lot of plutonium). Again, Libby was the only woman physicist. She was a “reactor babysitter,” watching for signs of trouble as part of a 24-hour shift. She took days; her husband, John, took nights. In a concession to her status as the lone woman, the facility’s supervisors installed a ladies’ bathroom.

It was in Hanford that Libby became pregnant. Not wishing to pause her work, she kept her pregnancy from her supervisor and from her colleagues right up until she left to go into labor.

After a week’s maternity leave following the birth of her son, Peter, she returned to work in Hanford and to the plutonium. In 1944, the reactor suddenly lost power, which Libby and colleague John Wheeler were able to diagnose as being the result of xenon poisoning, a byproduct that polluted the process. Once identified, Hanford scientists were able to resolve the issue.

Their preliminary work made the bomb possible: The Trinity test, conducted on July 15, 1945, demonstrated the atomic bomb was viable. It was also proof the work of Libby and the other project scientists had been successful in making a radical advancement in weaponry in a compressed period of time.

Libby was not in New Mexico for the test of the bomb, though Fermi was. Nor did Libby work directly with Oppenheimer during the development of the bomb. The two knew each other, however. In 1979’s The Uranium People, Libby’s account of the work, she related a story in which a parlor game was being played at a party and each person was asked who else they’d like to be for a day. Oppenheimer’s answer was Fermi. (Libby fancied being Hollywood star Greta Garbo.)

The world soon learned the full impact of the Manhattan Project’s creation. The bomb was used to devastating effect over Hiroshima, killing over 80,000 people immediately and subjecting tens of thousands more to the insidious effects of radiation. The bomb deployed over Nagasaki caused another 40,000 casualties, prompting Japan's surrender within days and effectively ending the war. The plutonium consolidated in Hanford was used in the bomb that detonated in Nagasaki.

People are pictured with newspapers
The Manhattan Project helped bring an end to the war. / Galerie Bilderwelt/GettyImages

Although Libby didn’t seek the spotlight for her contributions, she was still singled out. Mademoiselle, a popular women’s magazine of the era, anointed her one of their Women of the Year in 1946. A decade later, The Chicago Tribune declared her “one of the nation’s half-dozen top-flight woman physicists."

Her stature in the field led to a series of high-profile positions at numerous institutions, including fellowships at the University of Chicago and Princeton (where she worked with Oppenheimer), professor at New York University, and visiting professor of environmental studies and engineering at UCLA. She and John Marshall divorced; Libby became Leona Libby after remarrying in 1967.

Throughout her life, she was a champion of nuclear power and its potential to benefit humanity. Some decades later, she was asked about the ramifications of her work and whether it weighed on her mind as it had other members of the Manhattan Project, some of whom had unsuccessfully petitioned for the U.S. to threaten adversaries with the bomb's existence rather than actually deploy it. The interview, which was conducted shortly before her death in 1986, provided an unequivocal response.

“It was pretty clear the war would continue, with half a million of our fighting men dead[,] not to say how many Japanese,” she said. “I have no regrets. I think we did right, and we couldn’t have done it differently. Yeah, I know it has been suggested the second bomb, Nagasaki, was not necessary. The guys who cry on shoulders. When you are in a war, to the death, I don’t think you stand around and ask, ‘Is it right?’”

Additional Sources: The Uranium People

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