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  • Writer's pictureDale DeBakcsy

Leona Marshall Libby and the American Atomic Bomb.

Leona Woods (1919-1986) was only 23 years old, fresh from wrapping up her PhD work in spectroscopy in the basement of the University of Chicago's Eckhart Hall, when she was hand-picked by Herbert Anderson to become the youngest member, and only woman, on a top-secret project that was to forever alter human destiny. It was 1942, and just three years earlier, Lise Meitner, Robert Frisch, and Otto Hahn had published their discovery of nuclear fission, a process that theoretically could produce a chain reaction that released energies heretofore unimaginable, and it would become Woods's task for the next three years to help ensure that it was the United States which tapped those destructive energies before Nazi Germany or Soviet Russia.

Woods's life before the atomic bomb project is thinly documented, but speaks to the exceptional ability that made her stand out even in rooms containing the likes of Enrico Fermi, Edward Teller, or Robert Oppenheimer, all of whom she worked closely with at one point or another. Born in Illinois in 1919, she graduated high school at the age of 14 and received her Bachelor's degree from the University of Chicago by age 18, which put her on pace to receive her PhD at an age when most people have just about wrapped up their Master's. After being told by James Franck that he would be her doctoral advisor, but that she should, as a woman attempting a career in physics, expect to starve, she switched advisors to future Nobel winner Robert Mulliken, who gave her carte blanche to study whatever she wanted in the field of spectroscopy.

While conducting that research, she developed an expertise in vacuum technology that was desperately needed by the team gathering around Enrico Fermi to attempt the world's first nuclear chain reaction. Theory said that, if neutrons could be slowed down, they would be absorbed by Uranium isotopes in a way that caused those isotopes to split, releasing a massive amount of energy but also new neutrons which could themselves go on to impact with other Uranium atoms in a potentially ferocious chain reaction. But at the time nobody knew how many neutrons were released for each fission event, and there was only the slimmest of hard data on how different materials slowed and absorbed neutrons. The success of the project depended on accurate measurements of neutron speed and absorbance in various substances, and those measurements in turn relied on Geiger counters that employed vacuum technology.

Woods was brought in to construct boron trifluoride neutron counters to be situated in the massive stacks of graphite and uranium that formed the first nuclear reactor pile, CP-1. When neutrons come in contact with the boron isotopes in enriched boron trifluoride, there is an emission of highly energetic ionized particles which can be detected and used as a measure of neutron flow through a substance. Successful in this critical assignment, Woods was then tasked with designing the electrical circuitry for a "neutron chopper" of Fermi's creation. This was a rotating drum of alternating cadmium and aluminum layers. Cadmium strongly absorbs neutrons, and aluminum doesn't, so when the drum spins it is alternately transparent and opaque to neutrons that come in contact with it. If you synchronize the timing of the neutron chopper with one of Woods's boron trifluoride neutron counters placed several feet above it, you can then back-calculate the speed of the neutrons being emitted, an important value to know as uranium reacts very differently with slow versus fast neutrons.

Woods had no prior electrical engineering experience, but Fermi trusted her mind not only to learn what was needed, but to use that new knowledge to design and test circuitry for a new and exquisitely fine tuned piece of instrumentation crucial to the entirety of the Manhattan Project. Woods succeeded, aided in the task by John Marshall, a handsome fellow researcher whom she would go on to marry in July of 1943, and have a child with in 1944. The newlywed couple told Fermi of the pregnancy, and they all agreed to keep it a secret, as the chief of operations for the second nuclear pile (CP-2) would not have allowed a pregnant woman to continue working in the reactor building.

Hiding the pregnancy turned out to be a relatively simple affair, as it turned out. The nuclear reactors were built primarily from large slabs of graphite which coated everything in layers of black soot, so that massive baggy overalls were the practical dress code for most of the scientists, and Woods (now Marshall) always amplified the baggy effect with her pockets stuffed full of pliers, micrometers, slide rules, and notebooks, so that nobody marked the pregnancy into its ninth month. She had her child, a son named Peter, and went right back to work, pushing through the punishing hours born of desperation that characterized every scientist on the project.

As far as the Manhattan scientists were concerned, they were in a race against Germany for nothing less than the salvation of humanity from nuclear annihilation at the hands of fascism. Germans and Austrians had made the initial discovery of fission, and in spite of a number of high profile emigrations, there was still a core of brilliant scientists left behind in Germany whom everybody on the US project believed were more than capable of carrying out the design of a nuclear reactor and eventual nuclear bomb. As it happened, that was not the case - the Germans did not put the time into measuring neutron absorption by graphite in the way the American team did, and as a result convinced themselves that a nuclear reactor could only be built out of deuterium, which was a case of barking up the wrong molecular tree that left the Americans in sole charge of developing nuclear technology.

Marshall knew none of this, however, as in 1944 she and her husband went to the Hanford Site in the state of Washington to oversee the construction of the reactors that would provide plutonium for the bomb project. In the process of building the first reactors, it was noticed that natural uranium contains far more U-238, which does not react with neutrons in a way that triggers a chain reaction, than U-235, which does. This caused much consternation, as separating isotopes is a much trickier business than separating elements, until somebody realized that U-238, in absorbing neutrons, became Plutonium-239, which is itself useful in nuclear reactions. The Hanford site, then, was dedicated to the process of using nuclear piles to produce plutonium.

To put the scale of effort employed at Hanford in perspective, whereas a modern nuclear reactor can take a decade to be constructed and pressed into service, Hanford was built within a year, pushed out its first irradiated uranium slugs within three months, and shipped its first plutonium to Los Alamos two months after that, an achievement that rested on the coordination of the University of Chicago contingent's expertise with the precision work of the DuPont engineers.

The first atomic bomb test occurred on July 16 of 1945, followed in less than a month by the attacks on Hiroshima and Nagasaki, the second of which employed the plutonium created at Hanford. At the time the attacks were explained as necessary to avoid a land invasion that would cost the lives of American soldiers, and to the end of her days Woods held to that justification. We've learned much about the deeper, more sinister, motivations of the decision to drop the atomic bomb in the interim (for more about which, James Carroll's history of the Pentagon is a good starting point), but it seems clear that Woods sincerely believed in the public reasons given by the government for their decisions, without a trace of after-the-fact self-recrimination.

After the war Woods worked at a number of high profile labs on issues of high energy physics, radiation, engineering, stellar spectroscopy, and environmental studies, applying her abilities to a broad selection of problems that interested her rather than doubling down on her position at the center of nuclear weapons research. She separated from John Marshall in 1954, divorced him in 1966, and remarried soon after to Willard Libby, with whom she worked on applications of radioactivity to environmental issues. She reasoned that isotope decay could be used in the study of tree rings to provide a history of climate change, and those tests went on to establish new ways of studying man's impact on the planet.

As with the beginning of her life, so with the end - we must rely on scraps and fragments of information. Leona Marshall Libby (as she wrote her name after 1966) wrote hundreds of articles, but when she chose to write about biographical matters, overwhelmingly chose to document the figures she knew and worked with rather than discuss her own life. Thus, in her memoir The Uranium People (1979) we have detailed descriptions of Fermi and Szilard and Teller and Oppenheimer, a mass of information about post-war nuclear policy, but of herself only the barest outlines. She does, however, speak to us one more time directly, in an interview given the year of her death to historian S.L. Sanger. The voice is thin as she speaks with effort through a persistent cough of people and things that happened a near half century earlier, but when it comes to herself her voice becomes firm and she brooks no nonsense - she sees herself as a scientist who did what needed to be done and was respected for the breadth of her accomplishments and the depth of her work ethic, and whose time can't be wasted with wide-eyed optimism or teary-eyed regret. These are thoughts and opinions from another, harder, time, and who is to say that when we head for the next era of global mass insanity, we won't need a Leona Marshall Libby again, ready to put her head down and calculate and invent as the chasm of humanity's capacity for self-violence yawns open behind her.

FURTHER READING: The Uranium People is a great book, full of priceless insights into the earliest days of experimentation on uranium and plutonium, but as a source of information on Woods it is pretty sparse, in addition to which it is hard to find, and expensive when you do. The interview above gives a good flavor of the steel-willed pragmatist that shines through the pages of that book, so it's a good starting place. There are also descriptions of Libby in the book written by Laura Fermi (wife of Enrico Fermi and eventually a nuclear power critic), Atoms in the Family.

This piece was originally published as the 140th column in the Women in Science Series.


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