Most of the population have some concept, if not detailed knowledge, of “radiocarbon dating”. The procedure depends on the fact that the radioisotope carbon-14 (half-life 5730 years) is constantly being formed in the atmosphere, mainly at altitudes of 9-15 km, by cosmic rays which transform stable nitrogen-14 into carbon-14 (C14).
The C14 produced forms carbon dioxide and then mixes in the atmosphere with regular carbon (C12) dioxide in a ratio of about 1.2 parts in a trillion (1 followed by 12 zeros) and is absorbed by growing plants during photosynthesis. When a plant dies, carbon is no longer absorbed and the C14 concentration in the material decays following the standard decay formula. By measuring the ratio of C14 left in a sample of a plant or any material incorporating the plant one can determine when the plant died.
This technique was first demonstrated by Willard Libby at the University of Chicago in 1949. It assumes that the rate of production of C14 remains constant over many millennia. One way of crosschecking this assumption is to look at tree rings in ancient dead trees (100s or 1000s of years old) and compare the age thus determined with the one that the radiocarbon dating technique predicts.
It has been found that the assumption of constant production rate of C14 is not completely valid and that several factors have influenced the rate of production over time. Among the more important are the changes in the earth’s magnetic field, which in turn influences the rate at which cosmic rays interact with the atmosphere. A second important factor is the variation in the flux of cosmic rays from our sun and the universe in general. Recent research has shown that enormous solar bursts (Miyake events) that have been traced back at least 13,000 years have occurred and greatly enhanced the production of C14 for short periods. In recent times, the atmospheric tests of nuclear weapons have also increased the C14 production rate. The burning of ancient carbon in industry over the last century and volcanic eruptions have added C12 to the atmosphere with very little C14 and have skewed the ratio the opposite direction. New calibration curves have been constructed using tree-ring and other dating data. They show the need for corrections by up to 10% for dates in the 40,000 to 50,000 year range.
Two techniques have been used in carbon dating. The earliest, which required gram quantities of material, measured directly the decay of C14. For smaller samples a newer, Accelerator Mass Spectrometer (AMS) technique, which accurately measures the relative numbers of C12 and C14 atoms after passing through a particle accelerator, has been used. Several Chalk River alumni have played prominent roles in developing the latter technique.
Harry Gove (University of Rochester) and Ted Litherland (University of Toronto), both prominent nuclear physicists at Chalk River in the 1950s, played a lead role at Rochester and John McKay, long-time member of the TASCC facility, played a role in a group at McMaster University led by Erle Nelson. The Rochester group gained world-wide recognition when they established that the Shroud of Turin, long claimed to have been a cloak over the body of Jesus Christ, was in fact only produced during the Middle Ages.
Over the past several decades an entirely new cadre of radioisotopes, using the AMS technique, have been used to determine the ages of materials. Uranium 235 (half-life 700 million years) and uranium 238 (half-life 4.5 billion years) have been studied in ancient rocks. Their decay chains lead to the formation of stable lead-207 and 206, respectively, which is captured within the rocks. By determining the lead/uranium ratios, the age of the rock formations can be determined. The decay of potassium-40 (half-life 1.3 billion years) into argon-40, which is again retained in the rocks, has similarly been used for aging volcanic outflows. Other isotopes include the use of chlorine-36 (half-life 301,000 years) to determine the age of ancient, deep, underground water and the decay of rubidium-87 (half-life 49 billion years) into strontium-87. This later combination was used to determine the age of lunar rocks.
Most of the artifacts at the Canadian Nuclear Heritage Museum are less than 90 years old and none of the above techniques are applicable to dating them – we rely instead on the word-of-mouth technique. We invite all visitors to offer their recollections so that we may improve our accuracy. To arrange a visit contact us at info@nuclearheritage.com
Facial section of Shroud of Turin