National Academies Press: OpenBook
« Previous: Index of Documents Provided by the Federal Bureau of Investigation
Suggested Citation:"Appendix A: Radiocarbon Dating." National Research Council. 2011. Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters. Washington, DC: The National Academies Press. doi: 10.17226/13098.
×

Appendix A

Radiocarbon Dating

The technique of radiocarbon dating was pioneered by Libby in the early 1950s. This technique is based on the half life of 14C of approximately 5,700 years and the fact that the radioisotope is made through cosmic ray interactions in the atmosphere by the reaction of energetic neutrons on 14N to produce 14C and a proton. The 14C mixes in the atmosphere and becomes incorporated into CO2. All living things have ratios of 14C/12C that reflect the value in the atmosphere at the time and place in which they are living. Once a plant or animal dies, it stops incorporating modern CO2 and the 14C slowly decays away. The standard is to reference everything to the ratio of 14C/12C in the atmosphere in 1950. The units are in the change relative to this value as ∆14C in units or ‰ (parts per thousand.) All positive ratios are from after 1950 and all negative ratios correspond to before 1950, due to the loss of 14C because of its radioactive decay. The complication is in determining what the 14C/12C was at the time the animal or plant was living. The historical ratio is influenced by the cosmic ray flux in a given geographical region (it is stronger near the earth’s poles and sensitive to the earth’s magnetic field) and the solar activity that is a dominant source of the earth’s cosmic rays. Tree rings and corals have been used as calibrations to relate the ∆14C to actual calendar ages.

Modern humans have modified the purely cosmogenic 14C content of the atmosphere in two ways. Fossil fuel burning has introduced into the atmosphere old (dead) carbon that is so old that it has essentially no 14C left. Therefore, the “modern” 14C/12C ratio was falling until the mid-1950s, when atmospheric nuclear testing created what is now called the “bomb spike.” (see Figure A-1) The testing raised the 14C content of the atmosphere to almost twice the pre-bomb value in the Northern hemisphere at its peak around 1965. With the cessation of atmospheric testing, the exchange of the atmosphere with the ocean has gradually reduced the levels to values of ∆14C less than 100‰, down from the peak of >800 ‰. This behavior is clearly seen in the figure below, which shows a modern ∆14C curve using data from Levin and Kromer (2004). This

Suggested Citation:"Appendix A: Radiocarbon Dating." National Research Council. 2011. Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters. Washington, DC: The National Academies Press. doi: 10.17226/13098.
×

image

FIGURE A-1 Atmospheric CO2 (Northern Hemisphere).
Change in values of 14C for atmospheric CO2 since 1959. The rise from values near zero results from atmospheric testing of nuclear weapons. The decline from peak values reached in late 1963 results from the exchange of CO2 between the atmosphere and ocean. As a result of this exchange, levels of 14C in the ocean have slowly risen. The gap in the curve between 1973 and 1976 is due to a lack of atmospheric data for 1974 and 1975.
SOURCE: Courtesy of Alice Mignerey.

rapid rise and fall enables the dating of modern (younger than 1950) samples to within a few years in some cases.

Radiocarbon dating has advanced tremendously with the advent of the technique of accelerator mass spectrometry (AMS) to identify individual 14C atoms. This is a direct counting method and does not rely on detecting the radiation that is emitted when the 14C atoms decay. This improvement has led to the capability to radiocarbon date samples of less than 1 mg in mass. This new technique enabled the anthrax samples to be analyzed.

Suggested Citation:"Appendix A: Radiocarbon Dating." National Research Council. 2011. Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters. Washington, DC: The National Academies Press. doi: 10.17226/13098.
×
Page 181
Suggested Citation:"Appendix A: Radiocarbon Dating." National Research Council. 2011. Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters. Washington, DC: The National Academies Press. doi: 10.17226/13098.
×
Page 182
Next: Appendix B: The Forensics Potential of Stable Isotope Analysis »
Review of the Scientific Approaches Used During the FBI's Investigation of the 2001 Anthrax Letters Get This Book
×
Buy Paperback | $49.00 Buy Ebook | $39.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

Less than a month after the September 11, 2001 attacks, letters containing spores of anthrax bacteria (Bacillus anthracis, or B. anthracis) were sent through the U.S. mail. Between October 4 and November 20, 2001, 22 individuals developed anthrax; 5 of the cases were fatal.

During its investigation of the anthrax mailings, the FBI worked with other federal agencies to coordinate and conduct scientific analyses of the anthrax letter spore powders, environmental samples, clinical samples, and samples collected from laboratories that might have been the source of the letter-associated spores. The agency relied on external experts, including some who had developed tests to differentiate among strains of B. anthracis. In 2008, seven years into the investigation, the FBI asked the National Research Council (NRC) of the National Academy of Sciences (NAS) to conduct an independent review of the scientific approaches used during the investigation of the 2001 B. anthracis mailings.

Review of the Scientific Approaches Used During the FBI's Investigation of the Anthrax Letters evaluates the scientific foundation for the techniques used by the FBI to determine whether these techniques met appropriate standards for scientific reliability and for use in forensic validation, and whether the FBI reached appropriate scientific conclusions from its use of these techniques. This report reviews and assesses scientific evidence considered in connection with the 2001 Bacillus anthracis mailings.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!