Hot Carbon

Carbon-14 and a Revolution in Science

John F. Marra

• Publisher: Columbia University Press
• Year: 2019
• Topic: History
• eBook ISBN:
  9780231546782
• Subtopic: Science History
• Index: History

There are few fields of science that carbon-14 has not touched. A radioactive isotope of carbon, it stands out for its unusually long half-life. Best known for its application to estimating the age of artifacts—carbon dating—carbon-14 helped reveal new chronologies of human civilization and geological time. Everything containing carbon, the basis of all life, could be placed in time according to the clock of radioactive decay, with research applications ranging from archeology to oceanography to climatology.

In Hot Carbon, John F. Marra tells the untold story of this scientific revolution. He weaves together the workings of the many disciplines that employ carbon-14 with gripping tales of the individuals who pioneered its possibilities. He describes the concrete applications of carbon-14 to the study of all the stuff of life on earth, from climate science's understanding of change over time to his own work on oceanic photosynthesis with microscopic phytoplankton. Marra's engaging narrative encompasses nuclear testing, the peopling of the Americas, elephant poaching, and the flax plants used for the linen in the Shroud of Turin. Combining colorful narrative prose with accessible explanations of fundamental science, Hot Carbon is a thought-provoking exploration of how the power of carbon-14 informs our relationship to the past.

1

DISCOVERY

The police officers in the patrol car eyed the figure hunched over in the rain, his gait a little wobbly, and pulled up beside him. He certainly looked suspicious. Rumpled, red-eyed, and with a three-day growth of beard, he was, in a word, a mess. The Berkeley police were on the lookout for an escaped convict who had committed a multiple murder the previous evening; maybe now they had their culprit. They shoved him into the patrol car—he was in no condition to resist—and drove to the station. The suspect was put in front of a survivor of the crime. The two individuals—one distraught, the other tired and disheveled—stared blankly at each other. Clearly, neither knew who he was looking at. After some further questioning, the police let their suspect go. In the early morning hours of February 27, 1940, Martin Kamen could finally go home to bed.

Kamen had been working three nights straight inside the Berkeley Radiation Laboratory, bombarding graphite targets with deuterons, atomic particles made up of a proton and a neutron, produced by the Berkeley cyclotron. Kamen’s work was relegated to nights because of the daytime needs for cyclotron time for producing established isotopes, phosphorus-32 and iron-59, used in cancer therapies. His research was more of a fishing expedition, important but lower priority. He had been at it for more than a month, pasting back pieces of irradiated graphite that had been blasted off the target during the previous night’s bombardment and, in so doing, exposing himself to even more radiation. Kamen, frustrated, discouraged, and not a little desperate, stayed up those three nights for a final push.

The third night, torrential rain pelted the windows from a violent storm that would flood low-lying areas of Berkeley and Oakland. There were lightning strikes outside. High-energy cannonades burst from the cyclotron. Screams and moans were broadcast from the recording of a Gallic tragedy played by a French drama class that had convened that night on the mezzanine above the cyclotron control desk. The scene was set. The beam of deuterons from the cyclotron targeted a piece of graphite with the hope of slowly revealing a new form, or “isotope,” of carbon. It was not exactly the “It’s alive!” moment when Boris Karloff, as Frankenstein’s creature, raised his arm, but maybe just as exciting, if delayed. Later that night, Kamen collected the pieces of blasted-off graphite, which looked like black gravel, into a small bottle and left the sample on Sam Ruben’s desk.

Finally making it home after his run-in with the police, Kamen slept until midafternoon. That gloomy night and morning of February 27, 1940, began a revolution in physiology, biochemistry, archeology, geology, biomedicine, oceanography, paleoclimatology, and anthropology, as well as nuclear chemistry. Carbon-14, perhaps the most important isotope to life on Earth, was “born.”¹

“Isotope” comes from the Greek, meaning “same” (isos) “place” (topos). Isotopes of an element occupy the same square on the periodic table of elements, but have an extra proton or neutron or two, or may be missing one, giving them a slightly heavier or lighter atomic mass. The periodic table of the elements is perhaps the singular achievement of nineteenth-century chemistry, ordering the elements by their atomic characteristics and chemical behavior. Each element is made up of atoms; the atoms are made up of protons and neutrons in a nucleus, surrounded by electrons. The number of protons determines each element’s atomic number, and the protons plus neutrons give the element its atomic mass. The elements, so carefully displayed, one to each square on the periodic table, are, in fact, families, with different isotopes of an element crowding into the square. Chemically, they are nearly indistinguishable. But identifying isotopes of an element has allowed tremendous advances in the basic sciences.

Some 98.99 percent of all the carbon in the universe has an atomic weight slightly in excess of 12. The carbon atom has six protons and six neutrons in its nucleus. Orbiting electrons account for the slight excess weight. This is the most common isotope of carbon, carbon-12, or in the symbology of nuclear chemistry, ¹²C. An isotope with an extra neutron, designated ¹³C, makes up most of the rest of the carbon in the universe, about 1 percent of the total. Carbon-13 is a stable isotope; it has been around on Earth as long as carbon-12 has, something like 4.6 billion years. The other isotope, carbon-14, much rarer than its siblings, occurs once in a trillion carbon atoms and is radioactive. It decays over time, thereby changing to become carbon’s next-door neighbor in the periodic table, nitrogen, with a mass number of 14—seven protons and an equal number of neutrons. As you might guess, ¹⁴N, or nitrogen-14, plays an important role in the carbon-14 story.

Radioactivity was discovered by accident in the 1880s, by Henri Becquerel, as energy emanating from a special type of mineral that gives off its own light, a glow called fluorescence (see chapter 3). Later, Marie Curie, a contemporary of Becquerel, named this energy “radioactivity.” We now know that radioactivity occurs naturally, especially for elements at the high-numbered end of the periodic table and particularly those heavier than bismuth, with an atomic number of 83. Radioactivity is energy; it is the energetic decay of elements that changes them to other, more stable forms. And like the parents and siblings of human families, each isotope’s radioactivity behaves differently—for example, how fast it decays, what part of the atom splits off, and how much energy the isotope liberates in the process. For our purposes, the most important of these behaviors is how fast the isotope decays. In the early 1900s, Ernest Rutherford, then at McGill University in Montreal, came up with the idea of the “half-life”—the time it takes for half the quantity of an isotope to disappear through radioactive decay.

Many people have a bank account of some kind, often an interest-bearing account. The interest is the return banks give you for the use of your money. Over time, without doing anything, your funds increase according to that interest rate. The half-life works more like a spendthrift who withdraws 50 percent of his funds at regular intervals. If your account has a balance of $1,000, withdrawing half at the end of the first month leaves $500. After another month, withdrawing 50 percent of $500 leaves $250, and so on. For those with a more mathematical mind, a half-life means expressing values in logarithms to the base 2, the logarithm to the base 2 being the exponent of 2. The more familiar logarithms, especially to those of us who remember slide rules, are base 10: 10¹ = 10, 10² = 100, and log(100) = 2. Logarithms to the base 2, or “log2,” allow one to compute things that double, quadruple, or conversely, divide in half, like the half-life.

For example, a doubling of something, like bacterial cells means log₂(2) = 1, or one doubling. When we divide something in half, or log₂(0.5) = −1, that is one half-life. Halving again, or log₂(0.25) = −2, is two half-lives. It turns out that half-lives can range from seconds to millions or even billions of years, depending on the isotope. Given the age of the universe, some of the isotopes originally created after the Big Bang (some 13 billion years ago) have simply decayed out of existence. Another isotope of carbon, carbon-11 illustrates this point. Carbon-11 has five protons and six neutrons, a very unstable mix. Carbon-11 is not “comfortable in its skin” and has a half-life of 22 minutes. It lives for only a few hours before decaying away to boron, carbon’s other neighbor on the periodic table.

Martin Kamen (figure 1.1) was born in Toronto in 1913. His parents were Lithuanian and Byelorussian émigrés. When Kamen was still a young boy, they moved to Chicago to be near relatives. His father set up a photography business, his mother went into real estate, and soon they were leading a comfortable life in 1920s Chicago. Kamen had a remarkable intellect, skipping grades and finishing high school early. He was a child prodigy on the violin, but switched to the viola in his teens. With the Wall Street crash in 1929, his parents lost much of their middle-class wealth, and they urged him to forego music for something more practical. Two blocks from his home was the University of Chicago, and he attended, majoring in chemistry. His music turned to jazz, and he found he could earn extra money playing in Chicago’s speakeasies. After completing his bachelor’s degree, he stayed at the University of Chicago for graduate school and in 1936 received his Ph.D. in chemistry, on proton-neutron interactions. He was 27 years old.

FIGURE 1.1

Martin Kamen at the 60-inch cyclotron, September 10, 1939.

Photograph by Donald Cooksey.

With the understanding of radioactivity, isotopes, and atomic structure, nuclear chemistry was quickly taking over from classical chemistry. Elements that had been fixed on the periodic table could now change, and some elements could be created in the lab—notions that would have been heretical only years before. Kamen’s dissertation research upended theories about the nature of atomic particles, but the low energies in the accelerators available to him meant that his results would be difficult to verify for another 10 years. Still, becoming proficient in the use of the cloud chamber helped his later research.

The cloud chamber, invented in 1911 by Charles Wilson, had been the prominent mode of investigation into atomic particles from the 1920s to the 1950s. The sealed chamber’s internal atmosphere is saturated water vapor. Atomic particles are shot into the chamber and ionize the water vapor, forming condensation nuclei, the “cloud” that reveals the track of the atomic particle as a fine mist. The characteristics of the track are used to identify the particle. An alpha particle—a nucleus of a helium atom stripped of its electrons—makes a broad, straight track. An electron, or beta particle, makes a thin one, and is perturbed by other collisions. Applying an electric field can send differently charged particles in different directions. Later, at Berkeley, Kamen identified tracks in a cloud chamber filled with water vapor and nitrogen gas as resulting from carbon-14.

Although he opted for the cloud chamber, Kamen also pursued another kind: chamber music. Science became his profession, but he carried his viola everywhere. Playing chamber music with other musicians and friends added up to his perfect evening. Later he counted the famous violinist Isaac Stern as a close friend. Kamen was gregarious, very open, and in love with life—a socializer and an extrovert.

In 1935, he endured the tragedy of losing his mother in a car accident. Also, although successful there, Kamen never adapted to the rigid academic environment at Chicago. So, between personal misfortune and professional strains, Kamen looked to move elsewhere. Right after getting his Ph.D. in 1936, and on the advice of his thesis adviser, Kamen left Chicago to visit E. O. Lawrence’s Berkeley Radiation Laboratory at the University of California. Lawrence (called E.O.L. by his staff), the lab’s director, was gathering together nuclear physicists to work with his new and powerful atomic particle source, the cyclotron. Hoping for a position there, Kamen arrived in the Bay Area with enough money to last him for about six months. Even with precarious finances, compared to the weather in Chicago, Kamen felt as though he had entered heaven. He used his time well, making friends, playing music, and meeting Esther Hudson, who would become his wife. Kamen’s sociability, easy camaraderie, interest in politics, relish for science, and outspokenness worked against him later on. Late into his six-month respite, Kamen wrangled what today we would call an unpaid intern position at the Berkeley Radiation Laboratory. Figuring out the cause of an error in an experimental result got him onto the payroll.

Lawrence (figure 1.2) and one of his associates had done experiments on platinum bombarded with deuterons from the cyclotron. They put stacked foils of platinum in the path of the deuteron beam and measured the loss of energy through the stack. Instead of a smooth, simple decline passing through each platinum foil, there were discontinuities, or bumps, in the energy curve. Lawrence considered the discontinuous energy loss to be a major result of cyclotron research. J. Robert Oppenheimer was enlisted to devise a theory to explain the bumps, and he did so, making a presentation to a packed audience that included a visiting Niels Bohr, the Danish physicist and 1922 Nobel Prize winner who was responsible for the current model of the atom. Bohr questioned the results, which contradicted some of his own unpublished experiments. He pointed out that Oppenheimer’s theory, while elegant, might be useless.

FIGURE 1.2

E. O. Lawrence at his desk.

Photograph from the Lawrence Berkeley Lab Nobelists, www.lbl.gov/nobelists/1939-ernest-orlando-lawrence/.

To have their work criticized by the famous Niels Bohr shocked Lawrence, especially as it came after another recent experience with erroneous experimental outcomes. He was in line for a Nobel himself for developing the cyclotron, and another misstep might cost him the prize. Lawrence desperately wanted to clear the matter up, and he called on one of his faculty members, Ed McMillan, to create a team to investigate the result. Kamen was chosen for his radiochemical background, and McMillan, presciently believing chemistry important, also chose a young graduate student from Berkeley’s chemistry department, Sam Ruben.

Samuel Ruben (figure 1.3) received a Ph.D. from the University of California in chemistry, the same field as Kamen, but emphasizing biological processes. Andrew Benson, whom we will meet in chapter 6, described Ruben as gentle and a quintessential experimentalist. Kamen credits Ruben with “almost single-handedly” spurring interest in the use of isotopes of elements to trace chemical and biological processes during the years leading up to the discovery of carbon-14. He could take an idea—whether or not the idea was originally his, and whether or not he was familiar with the subject—and quickly design the decisive experimental test.

FIGURE 1.3

Sam Ruben.

Photograph courtesy of the Seaborg Archive, Lawrence Berkeley National Laboratory Image Library.

To validate or invalidate Lawrence’s bumpy curve of energy loss, the team—McMillan, Kamen, and Ruben—repeated the earlier experiments, and tested the pretreatments of the platinum foil for removal of trace contaminants. Solving the problem meant exhaustive work—exhaustive physically because of the long hours in the lab, and exhaustive scientifically to ensure that all possibilities were covered. Their labors were rewarded with a solution, but not one Lawrence might have liked. Despite all precautions, one contaminant had not been removed during a purification step for the platinum, but had actually been added in: laboratory dust. The dust had been baked into the foils when the foils were flamed. The bumpy energy curve was an artifact.

The effort to test Lawrence’s earlier experimental results had three important outcomes. The first w...

Table of contents

• Cover 
• Title Page
• Copyright
• Contents 
• Preface
• Acknowledgments
• Prologue: Aboard the Research Vessel Endeavor, South of Iceland, May 1991
• 1. Discovery
• 2. Discovery’s Wake
• 3. The “Invisible Phenomenon”
• 4. Dating
• 5. Photosynthesis
• 6. Calvin’s Cycle
• 7. Scintillations and Accelerations
• 8. The Shroud of Turin and Other Relics
• 9. Ocean Circulation
• 10. Carbon-14 in the Ocean
• 11. Ocean Fertility
• 12. Resolution: Plankton Rate Processes in Oligotrophic Oceans
• 13. Carbon-14 and Climate
• Epilogue
• Appendix 1. List of Nobel Prize Winners Mentioned
• Appendix 2. The Periodic Table of Elements
• Notes
• References
• Index