RADIOCARBON DATING BEYOND 50,000 YEARS BY LIQUID SCINTILLATION COUNTING

ABSTRACT. The maximum age that can be determined by 14C dating using the LSC method is primarily limited by the background count rate. Improved passive and active shielding have greatly reduced background contributions, but the level that remains limits the theoretical maximum determinable age to ca. 70 ka. In practice, reliable dates beyond 50 ka are rarely obtained. We studied the factors that influence the background level and its variability, and found that the standard benzene synthesis techniques for LSC result in increased variability of background levels and restricted extension of the technique back in time. Background benzene samples, obtained from synthesized background material (e.g., anthracite) and from back-burned spectrophotometric benzene, always show activity above spectrophotometric benzene levels. We attribute this to the presence of 14C in the lithium used for the generation of acetylene. The presence of 14C in the lithium requires that a constant stoichiometry be maintained between lithium and CO2 (including any excess used in the reaction), or background variability will be increased. Variation in background due to counter instability also limits the range of 14C dating. We describe a method to optimize the 14C counting window for maximum stability and the best figure of merit within the limits imposed by balance-point counting