Radiocarbon measurements at LAC-UFF: Recent performance

In 2012 a single stage accelerator mass spectrometer from NEC was installed at the Radiocarbon Laboratory of Universidade Federal Fluminense (LAC-UFF), Niteroi, Brazil. Here, we present a status report of our facility. We discuss some modifications applied to our combustion protocol in an attempt to reduce our procedural blank, mostly to processed organic samples. Measurements of reference materials indicate low precision and accuracy that are partially related to beam optics through the acceleration tube. We observed that once the beam current intensity increases the measured C-13(+)/C-12(+) becomes erratic. Therefore, in order to maintain the AMS-delta C-13 values within reasonable values, so that fractionation corrections using the spectrometer C-13(+)/C-12(+) values does not affect the final C-14 results, we are forced to limit the C-12(-) beam intensity to <= 30 mu A. This requirement was confirmed during our accuracy tests, when measuring selected annual tree-rings wood samples from a Parana pine (Araucaria angustifolia) between 1927 and 1997 previously measured at the Keck Carbon Cycle AMS Facility (KCCAMS), at the University of California, Irvine (UCI). At the LAC-UFF tree-ring wood samples were processed and measured in 4 different batches during a period of about 5 months. The C-14 results were later compared to the high-precision data obtained at KCCAMS/UCI and reached a good agreement. Recently a problem associated with graphitization yield were finally identified and new measurements with secondary standards are promising. (C) 2015 Elsevier B.V. All rights reserved.