AN INDEPENDENT ASSESSMENT OF UNCERTAINTY FOR RADIOCARBON ANALYSIS WITH THE NEW GENERATION HIGH-YIELD ACCELERATOR MASS SPECTROMETERS

A T Aerts-Bijma,H A J Meijer,S W L Palstra,D Paul,M W Dee

doi:10.1017/rdc.2020.101

Abstract

ABSTRACTThe radiocarbon (14C) dating facility at the Centre for Isotope Research, University of Groningen went through a major upgrade in 2017 and this included installation of a MICADAS accelerator mass spectrometer (AMS). In the first 18 months, we performed 4000 sample and 3000 reference measurements. A careful evaluation of those measurement results is presented, to characterize the various sources of uncertainty and to ultimately assign, for every sample measurement, a realistic expanded uncertainty. This analysis was performed on the measurements of secondary references and sample duplicates in various phases of their processing steps. The final expanded uncertainty includes both the 14C measurement uncertainties and uncertainties originating from pretreatment steps. Where the 14C measurement uncertainty includes straightforward uncertainties arising from Poisson statistics, background subtraction, calibration on Oxalic Acid II and δ13C correction, the uncertainties originating from pretreatment steps are based on the spread of actual measurement results for secondary references and sample duplicates. We show that the 14C measurement uncertainty requires expansion, depending on the number of processing steps involved prior to a 14C measurement, by a maximum factor of 1.6 at our laboratory. By using these expansion (multiplication) factors, we make our reported uncertainty both more realistic and reliable.

Highlights

Ever since the first determinations of the age of archeological objects by radiocarbon (14C) in 1949 (Libby et al 1949; Arnold and Libby 1949), researchers have striven for easier and more accurate measurements
Where the 14C measurement uncertainty includes straightforward uncertainties arising from Poisson statistics, background subtraction, calibration on Oxalic Acid II and δ13C correction, the uncertainties originating from pretreatment steps are based on the spread of actual measurement results for secondary references and sample duplicates
As this study is restricted to measurements on graphite cathodes, the extra uncertainty of the graphitization step, the second contribution, is automatically incorporated in all measurements from background materials, secondary references and sample duplicates

Summary

Introduction

Ever since the first determinations of the age of archeological objects by radiocarbon (14C) in 1949 (Libby et al 1949; Arnold and Libby 1949), researchers have striven for easier and more accurate measurements. Innovations to the AMS technique have led to incremental but important improvements, mostly concerning the precision and accuracy achievable Such innovations are illustrated by the changes in equipment used for 14C measurement at the Centre for Isotope Research (CIO) of the University of Groningen. Background wood (“bgw”, Kitzbuhel I, Tirol, Austria) and background collagen (“bgc”, Latton Quary LQH 12) (Cook et al 2012) have been selected for measuring the combustion background Both materials are known to be far older than the detection limit of 14C. As the secondary references are all pure substances, they form the basis for determining and quantifying each uncertainty contribution in the whole process These secondary references are IAEA-C8, IAEA-C7 (oxalic acid, Le Clercq et al 1998), and GS-51 (Groningen Standard, cane sugar). This present study is, based on data measured with the MICADAS only

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