Towards Achieving Low Background Levels in Routine Dating by Liquid Scintillation Spectrometry

Abstract

International radiocarbon intercalibration studies have revealed that radiometric laboratories using liquid scintillation (LS) spectrometry of benzene reported, on average, younger ages for near-background standards than either gas proportional counter (GPC) or accelerator mass spectrometry (AMS) laboratories. These studies suggested that the younger LS ages are probably related to the use of spectrophotometric benzene as a background standard. An analysis of successive 110-ka subfossil wood (Airedale Reef Ancient Wood: ARAW) standards shows that vacuum line memory effects occur in LS spectrometry and, consequently, must be corrected to obtain accurate 14C dates. ARAW standards, measured at monthly intervals in the Waikato laboratory, are used to provide blank corrections for both research and routine dating applications. The strong correlation between the ARAW Delta-14C data and the sample activities that preceded the standards may provide an opportunity to obtain sample-specific blank correlations. Lithium carbide synthesis is likely to prove a source of contamination. This work suggests that reproducible background levels for routine dating of less than 0.1 pMC (55 ka 14C yr) are achievable.