This list includes the results of measurements made during 1972–73.

This list includes most of the samples measured from June 1972 to December 1973 and some other results as yet unpublished.

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 Vries et 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 the 14C half-life of 5568 years and the uncertainty stated as one standard deviation calculated from the statistical analyses of sample and standards counting rates.

This list reports 14C measurements completed between August 1971 and November 1973. Other projects completed in that period will be reported later. Age calculations are based on 14C half-life of 5568 yr and modern standard of 95% NBS oxalic acid, supplemented by tree rings of pre-industrial wood from a log cut in the 1850's (Tx-540; R, 1970, v 12, p 249). Deviations reported are based on counting statistics of sample, background, and modern, and are ± 1σ, except when sample count approaches either modern or background, 2σ limits are reported. Unless noted, 12C/13C measurements were not made and results are not corrected for 13C fractionation. The laboratory uses liquid scintillation counting of benzene, with Li2C2 and vanadium-activated catalyst in preparation; chemical yields average 90%. Three counters are employed: a Packard Tri-Carb Model 3002, and 2 Beckman LS-230 spectrometers obtained through a grant from the National Science Foundation.

The following list of dates is compiled from samples prepared since publication of our last date list (R, 1974, v 16, p 131–141). The counting equipment and operating procedures are the same. Ages are quoted with a 1σ counting error which includes statistical variation of the sample count as well as for background and standard, using ad 1950 as the reference year and 95% NBS oxalic acid for 14C dating as the standard. The half-life value used is 5570 years.

The following list of dates are selected from geologic and archaeologic samples measured in late 1973. The laboratory procedures and techniques are the same as indicated in R, 1974, v 16, p 402–408, where sample is synthesized to benzene and counted for 24 hours in either a Beckman LS-100-C or Packard 2311 liquid scintillation spectrometer. Ages are calculated relative to 0.95 x NBS oxalic acid using a 14C half-life of 5568 years. The quoted error is 1σ, which includes only the counting uncertainties of the background, modern standard, and sample. Sample descriptions and comments were written in collaboration with collectors and submitters.

Procedures and equipment of the laboratory have been described in previous date lists. Wood, charcoal, and peat samples are pretreated with dilute NaOH and dilute H3PO4 before conversion to the counting gas methane; marls and lake cores are treated with acid only. Very calcareous materials are treated with HCl instead of H3PO4.

The Water Resources Division of the US Geological Survey has operated a low-level tritium laboratory since the late 1950's. In 1970, 14C-measuring facilities were added to that laboratory to provide analyses of ground water and other carbonates, primarily for research and field projects of the Division.

The following list contains dates of samples measured since our previous list (R, 1973, v 15, p 435–441). As before, age calculations are based on the Libby half-life 5568 ± 30 yr, and reported in years before 1950. The modern standard is 0.950 of the activity of NBS oxalic acid.