In the fall of 1939, it was generally believed that no long-lived radioactive isotope of carbon existed. In fact, Harold Urey, who had discovered the stable hydrogen isotope deuterium, and who was the foremost proponent of the use of stable isotopes in biological research, was promoting his research program partly on the basis that no long-lived radioactive isotopes existed for any of the elements of primary biological importance - hydrogen, carbon, nitrogen, and oxygen. Tritium,
The superscript in these symbols designates the atomic mass of each atom. The nuclei of hydrogen, carbon, nitrogen, and oxygen contain 1, 6, 7, and 8 protons. Each of these positively-charged protons is accompanied by a negatively-charged electron moving in a cloud throughout the atom. Most of the chemical characteristics of each atom are determined by these charged particles.
The stabilities of the atomic nuclei are, however, determined by both the protons and the neutrons of which they are composed. (This is now understood in far more detail than will be mentioned here.) Each neutron has a mass approximately the same as each proton, but the neutrons are not charged and are not, therefore, associated with the overall chemical properties.
11
C, which has 6 protons and 5 neutrons, has about the same chemical properties as 12C, which has 6 protons and 6 neutrons. The two can be used interchangeably in chemical systems - except that 11C is radioactive and spontaneously self-destructs with half of the atoms disappearing every 21 minutes. 12C is completely stable. The lack of a sixth neutron confers a fundamental instability on the 11C nucleus.Since most of the atoms in naturally occurring substances are stable, scientists are able to label specific atoms in molecules by introducing radioactive atoms into the molecular positions of interest. Those atoms can then be followed along through chemical processes by measuring the radiation that is emitted when they self-destruct. Each unstable atom has a constant probability of death depending upon its nuclear structure, so a steady amount of radiation is produced, which gradually decreases as the atoms disappear.
This is illustrated by Figures 1 and 2 from
Isotopic Tracers in Biology, Third Edition by Martin D. Kamen, Academic Press, New York (1957). Figure 1 shows the molecular structure of cholesterol, where the "m''s and "C''s are carbon atoms. By labeling the carbons of acetate, CH3CO2- - the first carbon being the "methyl'' (m) carbon and the second being the "carboxyl'' (C) carbon, biochemists were able to show that cholesterol is synthesized in animals from acetate and to determine which cholesterol carbons originate as acetate methyl carbons and which as acetate carboxyl carbons. Through many tracer experiments of this sort, a complete understanding of the biochemistry of the production of cholesterol was obtained. This has been of great importance to biochemistry and to medicine.After a complex molecule has been radioactively labeled, there is often a substantial amount of organic chemistry required for the complete interpretation of the experiment. The molecule must be degraded atom by atom through procedures that keep track of where each atom was located in the intact molecule. Figure 2 summarizes some of this degradative chemistry in the case of radioactive carbon tracer experiments on the biosynthesis of heme - the red, oxygen-carrying molecules within the protein hemoglobin in blood. Heme is shown in the figure with its central iron atom removed and its carbon atoms numbered for reference through the degradative procedure.
Without the knowledge gained by radioactive carbon tracer experiments of this sort, modern biochemistry would have been delayed at least 50 years. Only recently have mass spectrometers capable of meeting the demands of this research by quickly and reliably measuring the amounts of stable isotopes at high sensitivity come into existence. These machines are still not available to many biochemists.
In 1939, however, these experiments were not possible. The short half-life of
11C made it unsuitable for most of the needed experiments. Kamen and Reuben were trying to use 11C in experiments to understand the first biochemical steps in photosynthesis. Kamen had initiated some experiments to find a longer-lived isotope, but could not obtain enough cyclotron time and resources for this work.
The Berkeley cyclotrons had been built under the direction of E. O. Lawrence to accelerate atomic particles to energies necessary for nuclear reactions. Kamen was a member of the team of scientists who kept these particle accelerators in operation and improved and modified them for various purposes. Of special importance was the obtaining of continued financial support of the research, for which Lawrence needed to demonstrate practical uses for the new isotopic materials. This requirement and the competition with Urey ultimately caused Lawrence to give Kamen the cyclotron priority and resources he needed. Six months later, the results changed the history of biochemistry.
The exciting history of this whole era is vividly described in Martin Kamen's autobiography,
Radiant Science, Dark Politics, A Memoir of the Nuclear Age, published by University of California Press (1985), which is in print and available from U. C. Press at 1-800-822-6657. See also "Reflections on the First Half-Century of Long-Lived Radioactive Carbon (14C)'' by Martin D. Kamen, Proceedings of the Ameri-can Philosophical Society 138, pp 48-60 (1994).The most efficient procedure for the synthesis of
14C turned out to be bombardment of nitrogen, 14N, with slow neutrons. Even with this, the amounts of 14C that could be made with the Berkeley cyclotrons were quite small - although they did provide material for the first 14C tracer experiments.In his autobiography, Kamen tells of the unusual way in which he learned that this limitation on the amounts of
14C would cease to exist.Although the existence of the atomic piles, the first built at Chicago under the direction of Enrico Fermi and the second at Oak Ridge, was vaguely known about by many scientists within the Manhattan Project, the operating parameters of these machines were very closely guarded secrets. It happened that arrangements were made for the production of a small amount of
24Na in the pile at Oak Ridge for Kamen's use in work he had been assigned in the isotope purification effort there. He was officially told that 5 grams of sodium chloride would be put into a"machine'' for several hours. This apparently involved a misunderstanding of his requirements or of the capabilities of the pile, but not knowing the parameters, Kamen could only take what he was given.
That turned out to be a huge lead container which he opened to reveal a sample that, to his astonishment, emitted a purple glow. Shocked by the realization that the pile neutron flux must be millions of times greater than that achievable in the cyclotron at Berkeley and realizing the revolution this would make possible in the synthesis of radioisotopes including
14C, Kamen excitedly told E. O. Lawrence the news in the presence of Lawrence's Army guard.The result was a security investigation to try to find a nonexistent person who had given Kamen this secret information. The security people knew too little physics to understand that Kamen had been able to calculate it himself from the information at hand.
Fifty years later Kamen, the man who first synthesized carbon 14, is being honored in the name of Fermi, who built the first atomic pile.
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Vol. 23, No. 5
Newsletter: Access to Energy Newsletter Archive Volume: Issues Issue/No.: Vol. 23, No. 5 Date: January 01, 1996 01:45 PM (For actual publication date see newsletter.) Title: A Double Honor
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