Collected by Alfred B. Garrett Ohio State University
Columbus
II
The Rorh of Genius, 13
Carbon-14 Dating: Willard F. Libby
Often interesting and extremclv simifirant appliaittions develop from fundemekd resezrtrch. i'hG is the case of the method of dating geological time periods and archaeological objects with the long-lived radioisotope of carbon, carbon1 4 ~hdf-life enrai ~ .. "-dd h e r I n n d i v e d irot,onea. W. F. -~~~~~~~ -~~ - 57617 .. . v,.-~.,, -. ...~.~ Libby, the American scientist who developed this process in the lsto 1940'8, describes his work a s f o l 1 o ~ s . l . ~ ,
~~~~
"The radiocarbon dating technique was discovered as a result of a general interest in the possible effects of cosmic radiation on the earth and its atmosphere. It became apparent, as a result of thinking about the possible effects of high energy protons which, of course, constitute the main part of the cosmic rays, that in the bombardment of oxygen and nitrogen nuclei that "C should be the most abundant product in the conversion of any neutrons produced in the initial high energy collisions because-.neutrons released in the atmosphere will . . . always form I4C: in the main.a ~
"The realization of this fact, and learning of the abservations by Dr. Korff of New York University of neutrons as cosmic ray secondary products, led to radiocarbon dating via tho following chain9'
"Radiocarbon atoms introduced into the atmosphere make radioactive carbon dioxide. "Radioactive carbon dioxide thus produced in the atmosphere will mix with normal carbon dioxide making the mixture radioactive. "Since plants live off carhon dioxide and depend on it for carbon, this makes all plants on earth radioactive. "Since animals live off plants, this nukes all animals radioactive. "Since the turnover time for the biosphere is short compared to the half-life of "C (5760 years), the specific radioactivity of all living matter should be the same and should be proportional to the ratio of the neutron production rate per unit area of the earth's surface to the total amount of carbon in equilibrium with atmospheric carbon dioxide per unit area of surface. In calculating this ratio, one must take account of the ocean. I t turns out that the ocean contains the major part of the carhon exchanging with the atmosphere and fortunately the ocean carbon content can be estimated quite accurately. Although the carbon content of the biosphere is difficult to estimate, the total of the biosphere plus the carbonate and bicarbonate in t.he sea can be estimated quite accurately. "If the cosmic-ray intensity has remained constant
' Person$
Communication. LIBBY,W. F., Science, Val. 133, March 3, pp. 621-9 (1061). 'LIBBY,W. F., Phys. Review, 69, 671 (1046). E. C., AND LIBBY,W. F., Phys. Reu., 7 2 , 931 ANDERSON, (1947).
76
/
Journal o f Chemicol Education
-
with t,ime. anv fluctuations occurrine in ;1 < ~,neelectine ~ ~ ~ ~ ~ ~ time short to the mean life of 14c, the specific radioactivity of ancient living matter should have been the same as for living matter today. "At the time of death, the assimilation of radiocarbon stops abruptly because the only cont,act living organisms have with the atmosphere is through the life process. Exchange reactions between organic molecules and atmospheric carbon dioxide are too slow to he important and rotting and decay processes lead to reduced molecular weights, so that all one has to do is to select material in which the smaller molecules formed in the decay process can be washed out to be sure that primeval carbon is being measured. In this way, the specific radioactivity of ancient material gives its age a t the rate of 50% loss of act,ivity every 5760 vears ela~sedsince death. " "sinceAthe rate of 14C decav is unaffected by .aw known physical conditions we could expect, t,hat t,his would give an absolute time or age. Of course, we know that differences between species, small inconstancies in the average cosmic ray flux and errors in measurement can all add up to give error in the final dates. But, in principle, the method should produce absolute dates if the assumptions given are correct). "The method can be applied to samples of k n o m age back through the dawn of history, 5,000 years ago. "Out of these thoughts, we proceeded to look for radiocarbon in nature and after having found it, developed a method for measurement which would be satisfactory and then proceeded to check against samples of known age back to about 5,000 years and applied to prehistoric materials. The r e s u k seem to be satis factory and it does appear that the technique is sound for the last 5,000 years, and probably as far hack as radiocarhon can be measured-about 40,000 or 50,000 years." "The research on radiocarbon dating x i s carried out in several stages. In the first place, my collaborator, E. C. Anderson and I had to determine whether the living mat,erial actually had the radioactivity ex. pected. At that time we had no measurement techniques sufficiently sensitive to detect the radioactivit,ies involved directly because these levels are quite low. Later we developed methods for making the measurement but a t that time me did not have them, so we used the method of concentrating the heavy isotope of carbon. An apparatus for this purpose had been built by and was being used by A. V. Grosse of Temple University, then of the Houdry Process Corporation a t Marcus Hook, Pennsylvania. Grosse mas concentrating the carbon-13 isotope for medical tracer purposes and kindly agreed to try to concentrate some biological methane for the test so crucial to our research. We had to use biological, as contrasted with ~~
~
~
~
~
petroleum, methane, for we had a t this point arrived a t a distinction between living and dead organic chemicals. We had both "dead" methane and "living" methane in the sense that methane from oil wells in which the oil has been long buried would be expected to be entirely free from radiocarhon while the methane made from the disintegration of living organic matter should contain radiocarhon with an activity of 14 disintegrations per minute per gram of c a r b ~ n . The ~
task was to take this living methane and concentrate it in the isotope separation column to see whether the heavily enriched product mas radioactive. Happily for our research, it mas found to be so, and to about the expected degree. The material used mas methane gas from the sewage disposal plant of the city of Baltimore."?
' ANDERSON,1C. C., AND LIBBY,W. F., Science, 105,576 (1947).
Volume 40, Number
2, February 1963
/
77
