The historical perspective on the development of radiocarbon dating is well outlined in Taylor's book "Radiocarbon Dating: Libby and his team intially tested the radiocarbon method on samples from prehistoric Egypt. They chose samples whose age could be independently determined.
A sample of acacia wood from the tomb of the pharoah Zoser or Djoser; 3rd Dynasty, ca. The results they obtained indicated this was the case. Other analyses were conducted on samples of known age wood dendrochronologically aged.
The tests suggested that the half-life they had measured was accurate, and, quite reasonably, suggested further that atmospheric radiocarbon concentration had remained constant throughout the recent past. In , Arnold and Libby published their paper "Age determinations by radiocarbon content: Checks with samples of known age" in the journal Science.
In this paper they presented the first results of the C14 method, including the "Curve of Knowns" in which radiocarbon dates were compared with the known age historical dates see figure 1. All of the points fitted within statistical range. Within a few years, other laboratories had been built. By the early 's there were 8, and by the end of the decade there were more than The "Curve of Knowns" after Libby and Arnold The Egyptian King's name is given next to the date obtained.
The theoretical curve was constructed using the half-life of years. The activity ratio relates to the carbon 14 activity ratio between the ancient samples and the modern activity.
Each result was within the statistical range of the true historic date of each sample. In the s, further measurements on Mediterranean samples, in particular those from Egypt whose age was known through other means, pointed to radiocarbon dates which were younger than expected. The debate regarding this is outlined extensively in Renfrew Briefly, opinion was divided between those who thought the radiocarbon dates were correct ie, that radiocarbon years equated more or less to solar or calendar years and those who felt they were flawed and the historical data was more accurate.
In addition to long term fluctuations, smaller 'wiggles' were identified by the Dutch scholar Hessel de Vries This suggested there were temporal fluctuations in C14 concentration which would neccessitate the calibration of radiocarbon dates to other historically aged material.
This enables radiocarbon dates to be calibrated to solar or calendar dates. Later measurements of the Libby half-life indicated the figure was ca. This is known as the Cambridge half-life. To convert a "Libby" age to an age using the Cambridge half-life, one must multiply by 1.
The major developments in the radiocarbon method up to the present day involve improvements in measurement techniques and research into the dating of different materials.
Briefly, the initial solid carbon method developed by Libby and his collaborators was replaced with the Gas counting method in the 's. Liquid scintillation counting , utilising benzene, acetylene, ethanol, methanol etc, was developed at about the same time.
Today the vast majority of radiocarbon laboratories utilise these two methods of radiocarbon dating. Of major recent interest is the development of the Accelerator Mass Spectrometry method of direct C14 isotope counting.
The crucial advantage of the AMS method is that milligram sized samples are required for dating. Of great public interest has been the AMS dating of carbonacous material from prehistoric rock art sites, the Shroud of Turin and the Dead Sea Scrolls in the last few years.
The calibration research undertaken primarily at the Belfast and Seattle labs required that high levels of precision be obtained which has now resulted in the extensive calibration data now available.
The development of small sample capabilities for LSC and Gas labs has likewise been an important development - samples as small as mg are able to be dated to moderate precision on minigas counters Kromer, with similar sample sizes needed using minivial technology in Liquid Scintillation Counting.
The radiocarbon dating method remains arguably the most dependable and widely applied dating technique for the late Pleistocene and Holocene periods.