In a liquid scintillation counter, the beta particle excites the emission of light from a complex organic molecule or "scintillant." Because only about 13.5 decays per minute occur in one gram of modern carbon, it was necessary to use fairly large samples of several grams of carbon.
It was recognized that direct measurement of the number of C atoms in the sample would greatly enhance the sensitivity, and several unsuccessful attempts were made in this direction using conventional mass spectrometry.
Accurate dating also had to wait for a good calibration of the radiocarbon time-scale in the 1960s, using an absolute chronology based on tree rings.
The radiocarbon time-scale has now been calibrated with tree rings to more than 10000 years before present, and beyond that using a coral chronology (Stuiver, et al., 1993).
For historical reasons, uncalibrated radiocarbon measurements are often referred to a half-life of 5568 years.