FROM CURRENT LITERATURE

DETERMINATION OF THE AGE OF MATERIALS OF BIOLOGICAL ORIGIN BY C$^{14}$ CONTENT

Several years ago the supposition was advanced[^1] that the Earth’s atmosphere and all carbon-containing substances that exchange their carbon with the atmosphere must contain some quantity of the radioactive isotope of carbon C$^{14}$, produced by cosmic rays. As a result of the decay of C$^{14}$, its specific activity should be lower in materials of biological origin that do not exchange carbon with the atmosphere than in living organisms, whose supply of C$^{14}$ is continuously renewed through exchange with the atmosphere, in the upper layers of which continuous formation of C$^{14}$ takes place. Special experiments confirmed this supposition[^2] and enabled the authors to express the hope that, from the degree of decrease in specific activity in “old” biological materials, it would be possible to determine the time that had elapsed since their death. Very recently,[^3] thanks to improvement and simplification of the measurement technique, this method has made it possible to determine the age of a number of materials within the range from 1370 to 4600 years, and the data obtained agree very satisfactorily with age determinations made on the basis of historical and other investigations. The proposed method is of definite interest to specialists in the most diverse fields and, in the future, with further simplification, may become a valuable tool of research.

Below, the principal features of the method under discussion are considered in somewhat greater detail.

Neutrons of cosmic rays, it is believed, are absorbed mainly in the upper layers of the Earth’s atmosphere as a result of the reaction N$^{14}(n,p)$ C$^{14}$. The C$^{14}$ thus formed is radioactive and emits soft electrons with a maximum energy $E_{\max} = 150$ kev; the half-life period is $\tau = 5700$ years. This carbon is oxidized to carbon dioxide and, together with the total mass of atmospheric carbon dioxide, participates in exchange with living organisms (chiefly as a result of photosynthesis in plants) and also with the carbonic salts dissolved in the oceans. If one assumes that such exchange takes place over a time interval small in comparison with the half-life period of C$^{14}$ (we note that the complete circulation of atmospheric carbon due to photosynthesis alone occurs in 600–800 years), then the specific activity of such carbon-containing substances that exchange carbon with the atmosphere should practically coincide with the specific activity of atmospheric carbon.

It is not difficult to estimate this activity. For this it is necessary, first, to assume that during the last 10–15 thousand years (2–3 $\tau$) the intensity of the neutron flux of cosmic rays has not changed substantially...

continued. Then the number of C¹⁴ atoms formed per unit time will be equal to the number of atoms decaying. If, furthermore, it is assumed that the absorption of cosmic-ray neutrons occurs only through the reaction N¹⁴(n, p)C¹⁴ (other possible reactions of the type N¹⁴(n, H³)C¹² and N¹⁴(n, H³)3 He take place only for energetic neutrons and are consequently less probable), then the number of C¹⁴ atoms formed and, consequently, decaying per unit time will be equal to the total flux of neutrons falling on the earth. Taking the experimentally measured value of this latter quantity and dividing it by the amount of exchangeable carbon in the atmosphere, biosphere, and oceans, known from biogeochemical investigations, one can obtain the specific activity of carbon due to the radioactivity of C¹⁴. With allowance for the uncertainty in the neutron flux and in the amount of exchangeable carbon, the corresponding value obtained is of the order of 1–10 decays per minute per 1 g of carbon.

Experiments carried out for the direct determination of the specific activity gave a value of 10.5 decays/min per gram of carbon when the counter was filled with methane of “fresh” biological origin (the methane was produced from the waste of the sewer system of a large city). Control experiments showed that the observed activity was due precisely to C¹⁴. In methane from old sources (petroleum, coal), no noticeable quantities of C¹⁴ were found. The hypothesis of the uniform distribution in the atmosphere and biosphere of radioactive carbon formed by cosmic-ray neutrons was thus confirmed.

Further experiments were made to determine age from the C¹⁴ content. The age \(T\) of an object with specific activity \(I_t\) was calculated from the formula \(T = \tau \ln (I_t/I_0)\), where \(I_0\) is the specific activity of atmospheric carbon. The activity was measured with a counter whose cathode was in the form of a grid. The carbon was distributed over the inner surface of a cylinder surrounding the counter. The area of this cylinder was 400 cm²; the working length of the counter was 20 cm. The large background of the counter (about 400 pulses/min) was reduced to 7.5 pulses/min by shielding it with a layer of iron 10 cm thick and an outer lead shield 5 cm thick. In addition, the measuring counter was surrounded by 11 anticoincidence counters.

Up to now the results of age determinations for seven samples have been published. All the samples were of wood. To measure their activity, about 30 g of the material under investigation was burned, and the carbon dioxide was reduced to elemental carbon with the aid of hot metallic magnesium. The 8 g of carbon obtained were then distributed in a uniform layer on the inner surface of the outer cylinder.

The age of the following samples was determined:

1) A piece of fir, whose mean age was estimated from the rings of the trunk as \(1372 \pm 50\) years (577 ± 50 A.D.).

2) A piece of wood from a sealed Egyptian tomb, whose age was estimated from historical data as \(2149 \pm 150\) years (\(200 \pm 150\) years B.C.).

3) A piece of wood from the floor of a palace in northwestern Syria, whose age according to historical data was considered equal to \(2624 \pm 50\) years (\(675 \pm 50\) years B.C.).

4) The inner part of sequoia wood, whose rings corresponded to the time interval from 1031 to 928 years B.C., i.e. to a mean age of \(2928 \pm 52\) years.

5) A piece of board from the funerary boat of the Egyptian king Sesostris. The age of this sample was estimated as \(3792 \pm 50\) years (\(1843 \pm 50\) years B.C.).

b) The last two samples had approximately the same age, \(4600\) years \(\pm 75\) years (\(2650 \pm 75\) years B.C.). The first sample was a piece of cypress from the tomb of Sneferu at Meidum, the second—a piece of acacia from the tomb of Zoser at Saqqara.

The results of measuring the specific activity of the carbon of these samples are shown in the figure.

Specific activity of samples of various ages.

The errors indicated in the figure are statistical. The solid curve gives the dependence of age on the measured activity and is calculated under the assumption \(\tau = 5720 \pm 47\) years and \(I_0 = 10.5\) pulses per minute per gram of carbon.

The agreement of the radioactive method of determining age with other methods, as is clear from the figure, is quite satisfactory.

These results prove the suitability of the method described for determining the age of biological materials at least up to values of 4600 years. It should be thought that this method will be applicable for periods of time up to 20,000 years (\(3\)—\(4\tau\)). At present, however, direct verification in this time range is hindered by the absence of samples of sufficiently accurately known age.

In conclusion, it should be noted that the results obtained indicate that the intensity of cosmic radiation (in any case its neutron component) has not changed substantially over the last 15–20 thousand years, since the validity of the assumption of constancy of the intensity of cosmic rays is one of the basic conditions for applicability of the proposed method.

L. Bell

References

1. W. F. Libby, Phys. Rev. 69, 671 (1946).
2. E. C. Anderson, W. F. Libby, S. Weinhouse, A. F. Reid, A. D. Kirshenbaum and A. V. Grosse, Science 105, 576 (1947); Phys. Rev. 72, 931 (1947).
3. J. R. Arnold and W. F. Libby, Science 110, 678 (1949).