The dates presented here were calculated from measurements made at the University Radiocarbon Dating Research Laboratory mostly during 1973–74. The radioactivity of the samples was counted in gas proportional detectors, either silica lined (de Vrieset al, 1959) or of pure copper, filled to 2 atmospheres pressure with pure carbon dioxide. The detectors were fixed in, and completely surrounded by, a plastic scintillator anticoincidence shield mounted inside a massive lead shield to protect

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... environmental radiation (Switsur, Hall and West, 1970). Modern sample gas was obtained from the combustion of ad 1845 to 1855 rings of an oak tree grown near Cambridge and felled in 1950. Background samples were prepared from Welsh anthracite. The contemporary sample was frequently compared with the activity of the NBS oxalic acid international standard. Age calculations were based on the14C half-life of 5568 years and the uncertainty stated as one standard deviation calculated from the statistical analyses of sample and standards counting rates.