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The dates presented in this paper comprise results of determinations made at the University Radiocarbon Dating Research Laboratory, mostly during the latter half of 1974. Radioactivity was measured with proportional counters using pure carbon dioxide at 2 atmospheres pressure as filling gas. Effects of cosmic and local environmental radiation on the counters were reduced by surrounding them completely with 1) a plastic scintillator anticoincidence shield, 7.5cm thick, viewed by 2 photomultiplier tubes operating in coincidence mode, the output pulses of which were in anticoincidence with the proportional counters signals, and 2) by a 17.5 ton lead castle.

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 against 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.

The University of Cambridge Radiocarbon Dating Laboratory has been completely rebuilt and modernized, incorporating new techniques developed here for the various stages of measurements. Sample radioactivity is measured in gas proportional counters after conversion to highly purified CO2. Four counters are mounted within a single anticoincidence shield consisting of plastic scintillation material. This is surrounded by a graded cosmic ray shield of low radioactive steel, boron loaded wax, and finally, about 16 tons of ancient low radioactive lead blocks. The electronics are all solid state devices except for the stabilized high voltage supplies.

The dates presented here comprise results obtained since our last list (R., 1973, v. 15, p. 156-164). Pure carbon dioxide prepared from the samples is used to fill copper proportional counters at two atmospheres pressure. The detectors are protected from environmental radiation by a massive lead shield inside which, and completely surrounding the proportional counters, is a high efficiency plastic scintillation anticoincidence screen (Switsur, Hall, and West, 1970). Modern sample gas is obtained from combustion of a.d. 1845 to 1855 rings of an oak tree grown near Cambridge, felled in 1950. Background samples are prepared from Welsh anthracite or Connemara limestone. The contemporary standard is compared frequently with the activity of NBS oxalic acid international standard. Age calculations are based on the 14C half-life of 5568 years and the uncertainty stated as one standard deviation calculated from the statistical analysis of sample and standard counting rates.

Measurements reported in this paper were obtained in the Cambridge Radiocarbon Dating Laboratory during 1970–71. For most determinations, pure carbon dioxide at a pressure of 2 atm was used to fill the copper proportional counters, of 0.5 L or 1.5 L volume. The counters were protected by a massive lead shield and a plastic scintillation anticoincidence screen (Switsur, Hall, and West, 1970). Modern sample gas was obtained from the 1845 to 1855 growth rings of a locally grown oak tree which was felled in 1950. Background was prepared from Welsh anthracite. The contemporary standard is compared frequently with the activity of the International NBS oxalic acid standard. Sample ages are calculated using the conventional half-life of 5568 ± 30 years and the uncertainty stated in terms of one standard deviation of the counting statistics.

The measurements presented in this paper were carried out at the University Radiocarbon Dating Research Laboratory during the second half of 1971 using carbon dioxide counting. The counting equipment used was substantially that described by Switsur, Hall, and West (1970). Oxidation of samples of sufficiently high carbon content was performed in a stainless steel high pressure combustion bomb, developed at this laboratory, otherwise by ‘wet’ oxidation using acidified permanganate solutions for lake mud and samples of low organic content.