The discovery of carbon-14 and CAMS

On Feb. 27, 1940, Martin Kamen and Samuel Ruben confirmed the existence of the radioisotope carbon-14 following a year of experimentation at the University of California Radiation Laboratory, Berkeley. Since their discovery and Kamen’s development of it as a tracer in biological systems, carbon-14 has been used by biochemists to understand all biochemical reactions involving carbon, as well as by countless chemists, molecular biologists and medical scientists. So too, a technique developed by William Libby in 1949, known as radiocarbon dating, has enabled scientists to date archaeological and anthropological finds as far back as 60,000 years. Kamen’s and Ruben’s discovery and use of carbon-14 as a tracer atom was revolutionary. It transformed everything from biochemistry to oceanography, and it paved the way for LLNL’s own groundbreaking uses of carbon-14 in accelerator mass spectrometry (AMS).

The potential value of carbon-14 in science had been realized even before it was first synthesized, though the production of the carbon isotope had eluded scientists for years. In the mid-1930s, it had been found that lightweight elements had radioactive isotopes with short half-lives. Carbon-14 was expected to be so short-lived that scientists feared they might not be able to detect it. There was considerable uncertainty about what reaction would produce the isotope. In 1939, Ernest O. Lawrence, founder of the Berkeley and Livermore national laboratories, asked Berkeley physicist Martin Kamen to launch an intensive campaign to determine whether or not the radioisotope could be created.

Over the next year, Kamen, along with fellow Radiation Lab colleague Samuel Ruben, worked tirelessly to synthesize the carbon isotope but met with failure. In January 1940, Kamen began a “desperation” experiment, placing a graphite target inside the Radiation Laboratory’s 37-inch cyclotron (the world’s first major particle accelerator), where, over the next month, it absorbed the full beam of deuterons throughout the evening hours. During the day, Kamen retracted the graphite from the full beams to make way for other experiments, though the target continued to absorb deuterons. The heat and physical force of the beams caused flaking of the graphite, which Kamen would periodically cement back on. Kamen’s hope was that carbon-13 atoms would absorb deuterons, emit a proton, and yield carbon-14.

On Feb. 15, 1940, Kamen terminated the bombardment of the target and headed home after several sleepless days and nights. What would follow as one of the greatest scientific discoveries was nearly derailed when a red-eyed, unkempt Kamen was picked up and interrogated by Berkeley police, who were searching for an escaped convict who had just committed several murders. Witnesses, however, cleared Kamen, who was released back to his work. Later that day, Sam Ruben detected promising radioactivity from Kamen’s sample of graphite. On Feb. 27, following repeated chemical analysis and observation of activity, Kamen and Ruben concluded that they had discovered carbon-14. To their astonishment, the half-life appeared to be about 4,000 years, which, considering the small sample size, was fairly close to the 5,730 years now assigned. When they informed Ernest Lawrence (who was due to receive the 1939 Nobel Prize in physics the next day) of their discovery, his enthusiasm was so great that both Kamen and Ruben began to doubt their results—though further experiments confirmed them.

The discovery of carbon-14 and its use as a tracer was further revolutionized with the later development of a technique called accelerator mass spectrometry (AMS). AMS is a sensitive technique for measuring concentrations of specific isotopes in very small samples: for example, the isolation of one carbon-14 isotope out of a quadrillion (a million billion) other carbon atoms. In the late 1980s, Livermore’s Jay Davis recognized the opportunity to establish a multi-user AMS capability at LLNL and sought support for a new accelerator facility from programs across the Lab.

In 1988, nearly 50 years after Kamen’s and Ruben’s breakthrough, the Lawrence Livermore National Laboratory established the Center for Accelerator Mass Spectrometry (CAMS). Accelerator mass spectrometry supports studies in environmental quality, climate change, seismology, archaeology and biomedical science, and it provides Livermore researchers with the capability to diagnose the fission products of atomic tests and monitor the spread of nuclear weapons through the detection of radioisotopes in air, water, and soil samples.

CAMS began its first full year of operations in 1990. Today, it continues to prove itself as a highly versatile research facility, performing some 25,000 measurements a year. CAMS contributes to the success of a wide range of Lab programs and collaborative external research projects. CAMS also serves as a user facility, providing analytical capabilities for researchers from around the world.

Pictured: LLNL’s Jay Davis at the CAMS tandem electrostatic Van de Graaff accelerator, 1990.