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Leona Libby’s Atoms and Isotopes

On December 2, 1942 at 3:25pm the world changed. That was when a team of about 48 scientists led by physicist Enrico Fermi was able to achieve the world’s first sustained nuclear chain reaction. Also important on that fateful day was that of the 48 scientists there, all of them were men except for one woman, 23-year-old Leona Harriet Woods. In a field dominated by men she was a key member of the team that built what came to be known as the Chicago Pile, a wood, graphite, and uranium structure that demonstrated that humans could manipulate atoms, the tiny particles that everything in the universe is made of. The construction and operation of the Chicago Pile set the standard for future nuclear reactors.

Other women had made notable contributions to the events that led to the world’s first self-sustaining chain reaction. Marie Sklodowska Curie, for example, had done groundbreaking work in both physics and chemistry and had won two Nobel Prizes. Lisa Meitner and Otto Hahn created the first nuclear fission in 1939. Although Meitner was nominated for a Nobel Prize in both physics and chemistry, unfortunately, only Hahn got the Nobel Prize for the discovery. In spite of Curie’s early recognition, it would not be until 1963 that another Nobel Prize was awarded to a woman in physics.

Leona Woods was born in LaGrange, Illinois in November 1919. She was a precocious child who graduated from high school at age 14 and earned a Bachelor’s Degree in chemistry at age 19. By 23 she had earned a Ph.D. in chemistry from the University of Chicago. In her life she made significant contributions in a number of scientific fields.

Through scientists she knew at Chicago, she became involved with the top-secret Manhattan Project set up to build an atomic bomb before the Nazis. Leona joined the scientists in Enrico Fermi’s Metallurgical Laboratory and became an important part of the team. Fermi’s group took on the task of creating a sustained nuclear reaction that involved having neutrons shoot from one uranium atom to another.

When a neutron hits a uranium nucleus it splits the nucleus and dislodges other neutrons which then fire out to hit another uranium nucleus. The process continues at nearly the speed of light from atom to atom until the process is slowed by a control rod that absorbs the neutrons so they don’t keep firing from one uranium nucleus to another. In an atomic bomb, the process is allowed to run uncontrolled creating a large explosion. In a nuclear power plant, control rods absorb some of the neutrons to keep the reaction going at a steady but safe pace. The whole concept is based the on idea of mass – a few kilograms of uranium or plutonium — turning into an enormous amount of energy: Energy=Mass times the speed of light squared.

A major contribution Leona made to the experiment was to create a boron trifluoride counter which, similar to a Geiger counter, detected the strength of the neutron activity as the neutrons shot from one uranium atom to another. During the atomic chain reaction experiment, her measurements let Fermi and the others know the level of the radiation being emitted. After the chain reaction in the Chicago Pile had run for 28 minutes, a high level of radioactivity was reached, and the experiment was stopped by pushing into the reaction area a cadmium control rod that absorbed the neutrons to stop their flow through the uranium nuclei. The experiment had been a great success. The groundwork had been established for both the atomic bomb and as a heat source to produce steam to generate electricity.

Following the successful chain reaction at Chicago, Woods was sent to Hanford, Washington to work on producing plutonium, a highly radioactive element first synthetically produced at UC Berkeley in December 1940. Other scientists went to Oak Ridge, Tennessee to work on methods to enrich uranium to make it more radioactive. Both plutonium and uranium would be turned into bombs that were dropped on Japan. A uranium bomb was dropped on Hiroshima on August 6, 1945, and a plutonium bomb was dropped on Nagasaki on August 9. The thousands of people involved in the Manhattan Project had succeeded in accomplishing their goal of stopping and winning World War II.

With her work done at Hanford, Leona returned to Chicago to do research at the University of Chicago Institute of Nuclear Studies headed up by Fermi. She had married fellow scientist John Marshall and by 1949 had two children. Leona and John remained in Chicago the next few years, but when Enrico Fermi died in 1954, John took a position at UCLA and Leona stayed in Chicago until 1957 when she went to the Institute for Advanced Studies at Princeton University.

A year later she went to Brookhaven National Laboratory on Long Island. In 1962 she was made a professor at New York University where she spent the next three years before taking a professorship in physics, astronomy, and cosmology at the University of Colorado. As part of her work in cosmology, she used her analytical skills to study the spectroscopy of quasi-stellar objects or quasars to determine their structure by examining their electromagnetic spectra.

In 1966 Leona and John Marshall divorced. The next year she married physicist Willard Libby and joined him at UCLA where she became a professor of nuclear engineering and environmental studies. In 1960, Willard had won a Nobel Prize for working out how the isotope Carbon 14 could be used to determine the age of organic material up to about 40,000 years old. Most carbon atoms have 6 protons and 6 neutrons, but carbon 14 has 2 extra neutrons making it unstable and radioactive. It decays to nitrogen with a half-life of 5730 years. Carbon 14 dating soon became an indispensable tool for archaeologists for dating bones, wood, teeth and other organic artifacts.

While working with Libby, Woods also became interested in dating things in the past, except instead of dating objects, she worked out a system of dating past climate events by using isotopes of oxygen along with tree dendrochronology, the study of aging a tree by counting its growth rings. One of her research projects involved determining periods of warm and cold weather and the effects on tree growth as well as how weather patterns affected early human migration patterns.

An isotope is an atom with a different number of neutrons than its most common stable version. For example, about 98% of all oxygen atoms have 8 protons and 8 neutrons giving them an atomic mass of 16. About 2% of them have two extra neutrons giving them an atomic mass of 18 and making them a heavier isotope than the most common oxygen atom. Since water is made of hydrogen and oxygen, the extra weight of an oxygen atom with 10 neutrons affects the weight of a water molecule and its evaporation rate from lakes and oceans.

Evaporation draws up O16 in nearly all weather. More of the heavier O18, however, is drawn up in warmer weather when evaporation is stronger than in colder weather. Then as the moisture-laden clouds pass over land, they drop precipitation. In warm weather, since there are more O18 atoms in the rain, more them will show up in tree rings in ratio to the number of O16 atoms. From this data, it can be determined when the Earth is warmer and when it is cooler.

Leona called this type of dendrochronology tree thermometry, a type of paleoclimatology using wood from both recently cut trees and fossilized trees in which she was able to draw conclusions about temperature patterns at different times in Earth’s history. Her system is used today to study Earth’s current climate as compared to hundreds or thousands of years ago.

We know that Earth has been through a number of warm and cold ages and they all left their remnants. Oil, for example, was created at a time when the Earth was warm and swampy and plant and animal life was abundant, especially zooplankton and phytoplankton in the oceans. Dead plants and animals settled to the bottom of the ocean, lakes, or swamps and in time were covered by layers of sediment that often became deep over time.

We seldom stop to think about it, but all of the energy wrapped up in the once living creatures that decayed into oil, gas, and coal came from the sun through photosynthesis. The energy was bound up in the photosynthetic plants that chemically converted sunlight, water and atmospheric carbon dioxide into glucose. The energy was then passed on to the animals that ate the plants resulting in oil being over 83% carbon. Coal and natural gas also have high percentages of carbon.

Leona, through her work with tree rings and other climate markers, was able to tell which periods in Earth’s history were conducive to such things as oil production and which were too cold. During ice ages and other cold periods, plants and animals were not abundant and very little fossil fuel was being made. Today when we burn oil, gas or coal, we are releasing back into the atmosphere the carbon taken in by the photosynthetic plants millions of years ago. The problem is that we are putting the carbon and heat back into the into the atmosphere at a much faster rate than it can safely absorb them and this creates the greenhouse effect we are currently experiencing. We certainly need carbon dioxide in the atmosphere to fuel the growth of plants, but too much of it can hold in the sun’s heat and keep it from dispersing out from the Earth’s surface.

The same age markers have been used to determine migration patterns of early hominins from Australopithecus to Homo sapiens as they moved around in Africa and into the rest of the habitable world seeking hospitable climates. One example is that hominins who once lived in the Olduvai region of Tanzania, migrated north to the Omo River basin in today’s Ethiopia as the climate dried and many of their food animals migrated out of the area. Climate change was the cause of the migration.

Various other migrations took place in the next few thousand years as climate conditions changed from rainy to dry or warm to cold. One of the earliest migrations was when Homo erectus began leaving Africa about 1.8 million years ago going as far as Asia. A later hominin species, Neanderthals, began populating Eurasia about 150,000 years ago and even though they were tough enough to last through an ice age, their fossil record ceased around 40,000 years ago.

Then about 100,000 years ago Homo sapiens, the latest Homo species, began migrating out of Africa into many of the same areas where Neanderthals were already living. There is strong evidence that the two species lived side by side for several years before the Neanderthals either died out or interbred with Homo sapiens. Homo sapiens outlasted Neanderthals, but traces of Neanderthal DNA are found in some people today.

Without any warning about weather or climate fluctuations, these resilient early hominins survived through both warming trends and ice ages. Perhaps the fact that there was so much open land available for them helped them survive. If one area became inhospitable, the people could pack up and move to a place where there was more water or warmer temperatures.

In the crowded world of today, however, aside from a few “snowbirds” who have the luxury of going toward the equator in winter and away from it in summer, most of us must cope with the weather where we are. But thanks to great strides in the science of meteorology in the last few years, we can know what is coming and put on our rain hats or scamper into our storm shelters. It is the many hardworking weather scientists today who give us the comfort of at least knowing on a day-to-day basis whether it will be sunny or rainy so we can get ready for it.

The long-term climate history of Earth that Leona with her knowledge of atoms and isotopes, tree rings and carbon 14 dating was able to delve into has gained a lot of momentum in the last few years as we have come to see signs that our global climate is going through a change. The trend currently, at least in some areas of the world, seems to be toward extremes: colder winters and warmer summers, dry conditions that could be conducive to forest fires, or heavy rains that lead to floods.

Some say these changes are the normal patterns Earth has been going through for thousands of years. A number of people agree with that assessment but say that our heavy use of fossil fuels has probably exacerbated the problem by rapidly putting large amounts of carbon from ancient plants and animals into the atmosphere.

Unfortunately, the problem has become an economic as well as a political issue. Some nations whose economies depend on industries powered by fossil fuels are struggling to limit their carbon emissions. A few people whose political agenda does not recognize the importance of science, say that burning fossil fuels in cars, trucks, coal-fired power generators, and industry is not a serious problem. In nations that allow freedom of thought, each person can weigh the evidence and draw her or his own conclusions on the issue.

Due to the rotation of the Earth and its effect on wind and weather patterns, predicting the weather both daily and long-term can be challenging. Two organizations that carefully study the meteorological systems people encounter in the United States and beyond, are the National Oceanic and Atmospheric Administration or NOAA and the National Aeronautics and Space Administration or NASA. Founded in 1970, NOAA studies Earth’s climate from the depths of the oceans to gathering data from satellites orbiting the Earth.

NASA, involved in numerous space related programs, recently launched the Plankton, Aerosol, Cloud and Ocean Ecosystem or PACE satellite to study closely such things as how plankton health in the oceans affects climate. These two organizations keep the United States and the rest of the world informed about weather events from daily temperature ranges to the paths of destructive hurricanes and tornadoes.

These agencies, along with many others around the world use the science developed by Leona Woods Libby and other scientists to help us understand how our weather works and to alert us of impending weather events. It is extremely important to understand the formation and potential impact of such things as hurricanes, tornadoes, and severe storms so we can take precautions. Modern life could not function as well as it does without accurate weather information. We have come to depend on it from planning family picnics to launching rocket ships.  

Leona Harriet Woods Marshall Libby was a brilliant woman did important things to benefit the world. She played an important role in helping stop a devastating war. Then she turned her talents to helping us understand the weather patterns that have shaped our Earth from ice ages to the hot swampy conditions that fostered the growth of the plants and animals that became the fossil fuel that runs our global civilization. And her work on past climate patterns helps explain why our ancestors left Africa and how they came to settle various parts of the world.

Besides being an innovative scientist, Leona was also a prolific writer who published over 200 scientific papers and at least three books. One of her books, Uranium People, published in 1979 told the story of early atomic research that led up to her work on the atomic reactor at Chicago. Past Climates is a highly technical yet very readable account that takes the reader on a voyage from the history of the discovery of isotopes and how they are used in climate science, to how climate events have influenced human migrations for thousands of years. It is a book about us and our common species heritage. Her last book, The Life Work of Nobel Laureat Willard Frank Libby, written after the death of Willard Libby in 1980, paid tribute to her husband’s many scientific achievements.

Leona Libby is one of those unsung people who we do not hear about often, yet who had a lot to do with shaping how we live today.

Ted McCormack

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