In 1969, at the Nobel Symposium on “Radiocarbon Variations and Absolute Chronology” in Uppsala, a curve was presented that illustrated the functional dependence of measured conventional radiocarbon dates on true historical ages of wood samples (Suess, 1971). The curve was derived from the results of La Jolla measurements of radiocarbon in bristlecone pine wood dendrochronologically dated by and obtained from Professor C W Ferguson of the University of Arizona (Ferguson, 1968). The curve was intended to be adequate for deriving calibrated radiocarbon dates and also for allowing fairly reliable estimates of the accuracy of the absolute dates obtained in this manner. The basis for the validity of this calibration is the well known fact that, for all practical purposes, wood samples that had grown at the same time show the same radiocarbon content. However, the reverse is not always true: Wood samples showing the same radiocarbon content do not necessarily have the same age because of the windings and steps of the curve (Stuiver and Suess, 1966).

A simple procedure for the conversion of 14C NBS oxalic acid standard to benzene without direct combustion or wet oxidation is reported. Fractionation in the synthesized benzene was not detected.

The optimum operating conditions providing minimum run-time and running costs have been studied theoretically for a thermal diffusion plant to be used for the enrichment of the radiocarbon isotope from finite sample size.

The calculations are based on a simple approximate model of the enrichment process, regarding the isotope separation column as operating under quasi-stationary state conditions. The temporal variation of the isotope accumulation is given by a single exponential term. From comparison with the numerical solution of the separation tube equation, approximate models of this simple type appear hardly sufficient for analytical work but seem well suited for optimization calculations. For column operation not too close to the equilibrium state, the approximate run-times were found accurate within 0.2 d.

The approximate model has been applied to a column of the concentric type, operated on gaseous methane. Cross-section configuration and temperatures were not varied (hot and cold wall radii: 2.0 and 2.6cm, respectively; hot and cold wall temperatures: 400°C and 14°C, respectively). The column transport coefficients used were derived from measurements. Run-time was minimized by optimizing both the operating pressure and the sample collection mode for different total sample size (range studied: 24 to 100 g), mass of enriched sample (1.8, 2.4, and 3.0 g), enrichment factor (12, 15, and 20) and column length (8 to 18 m). Optimum working pressures are between 1 and 2 atm. Usually, about 90 percent of the enriched sample mass is extracted favorably from the column itself, the length of the sampling section being about 2.5 to 5 m. Typical runtimes are between 3 days and 2 weeks, and isotope yield may reach 90 percent.

Optimum operating conditions have also been calculated for other column configurations reported in literature and are compared with the experimental results.

The La Jolla Radiocarbon Laboratory has measured carbon-14 concentrations in seawater samples collected from 1957 through 1972. The dissolved inorganic carbon in seawater was extracted on board research vessels and was returned to the laboratory for processing and measurement. Both surface and sub-surface samples were collected, primarily from the Pacific Ocean, but also from the Indian Ocean. The purpose of the seawater measurements was to determine the distribution of bomb-produced radiocarbon in the surface water of the Pacific and Indian Oceans, the sub-surface penetration of bomb 14C, the change in 14C/12C ratios with depth, and thus the rate of uptake of bomb 14C by the oceans. This project was the basis of the author's doctoral dissertation (Linick, 1975).

This list includes most of the measurements made in 1975 and 1976. Results lower than Ly-1000 are old measurements obtained with counters previously described, especially in R, 1973, v 15, p 134, when the laboratory was in the basement of the Nuclear Physics Institute. At the end of 1974 all the apparatus was transferred to a specially designed building with an isolated counting room overlain by 4m non-radioactive earth.

The carbon isotope analyses reported here include all radiocarbon dates run on packrat middens in the United States and Mexico by the Arizona radiocarbon laboratory through October 1977. All samples described below report dates by CO2 (0.5 or 2.0L) counting. Age calculations are based on a 14C half-life of 5568 years, using 0.949 NBS oxalic acid as the modern value. Errors, based on counting statistics, are quoted to ± 1δ; infinite ages quoted to — 2δ.

The Radiocarbon Laboratory began operation in mid-1972, using benzene synthesis. The equipment for the laboratory was obtained under a National Science Foundation Grant*. Basic equipment consists of a benzene synthesizer (Alford Instrument Co) and a liquid scintillation counter (Picker-Nuclear 220). The equipment is essentially identical with that of Noakes and Brandau (1971). Counting efficiency and background rates are also similar; ca 70% efficiency with a 4 to 5cpm background for 5ml benzene samples. Benzene synthesis is standard with sample combustion in pure oxygen under reduced pressure. The original synthesis system has been modified to permit simultaneous processing of more than one sample.

The following list contains most of the measurements made during 1974, since our last list (R, 1973, v 15, p 241-251). All measurements were performed on a Beckman LS-200 Liquid Scintillation Spectrometer following previously published setting up (Polach, 1974), automatic cycling (Polach, 1969) and benzene synthesis (Polach and Stipp, 1966; Polach et al, 1972) procedures.

The following list contains most of the measurements made during 1977, since our last list (R, 1977, v 19, p 383-388). The laboratory procedures used were those outlined in the previous date lists (R, 1976, v 18, p 151-160; R, 1977, op cit). Dates are calculated using 14C half-life of 5568 yr and errors are reported as one-standard deviation. The collagen extraction follows Longin (1970).

The dates presented in this paper comprise results of determinations made on archaeologic and geologic material from Socialist Republic Vietnam since 1969. Radioactivity was measured twice for 48 hours with gas proportional counters of Houtermans-Oeschger type, using methane at 1000mm Hg pressure as filling gas. Influence of cosmic and local environmental radiation on the counters was reduced with 30 tons of shielding material as described earlier (R, 1970, v 12, p 400-420).

This list includes all measurements made between 1969 and 1973 that represent natural samples. Physical and chemical treatment of samples and counting technique remain as previously reported (R, 1973, v 15, p 469-478). Calculations of dates are made with the radiocarbon half-life of 5568 ± 30 years; plus or minus numbers quoted herein are the standard errors for the counting of radioactive disintegrations.

This list includes results of measurements made during 1973-1974 and 1975 in the Natural Radiocarbon Laboratory of the Centre de Recherches Radiogéologiques de Nancy (CRR).

The Radiocarbon Dating Laboratory of the Institute was established in 1974 primarily to assist in the program of research undertaken by the Institute's Department of Quaternary Palynology (Rajagopalan and Vishnu-Mittre, 1977). The measurements carried out by the Laboratory up to December 1977 are presented in this date list.

This list includes ages measured from August 1974 to May 1976 using the benzene scintillation method. All archaeologic and geologic samples except Sahara Libico series, come from Italian territory. These datings were carried out partly with the liquid scintillation counter previously described (Alessio et al, 1973) and partly with a new liquid scintillation counter (Alessio et al, 1976b). Vials, 4.5ml in volume, described in Alessio et al, 1973. Mixture consists of 3.5ml of benzene from sample and 1ml of commercial liquid scintillator NE 216. The characteristics of new counter are: background 2.90 cpm and figure of merit

In this date list we have included most of the archaeologic samples dated in this laboratory since publication of our last date list (R, 1977, v 19, p 188-228).

This list includes dates measured from July 1975 to December 1976 with CO2 proportional counters. All archaeologic and geologic samples except for Turkey and Sistan series come from Italian territory. Laboratory equipment, largely unchanged but expanded and technically improved, has been previously described (Alessio et al, 1970b, 1976).

All samples described in this date list have been measured since July 1973 to Oct 1975 using carbon dioxide filled proportional counters. Most of samples have been dated with Counter No. 1 (LI) as in our previous list (Mościcki and Zastawny, 1976). Some samples were measured with Counter No. 3 (L3), with a total volume of 1.51 and background and NBS oxalic acid standard counting rate of respectively, 3.50 or 4.00 cpm and 10.05 or 20.40 cpm when filled to 1 or 2 atm pressure (Mościcki and Zastawny, 1977).

Presented below are dates from some important archaeologic and Quaternary sites. All dates are based on τ ½ = 5568 yr; to convert the radiocarbon dates for archaeologic samples into ad/bc scale, 1950 has been used as base as per the resolution passed at the Ninth International Radiocarbon Conference, San Diego, 1976. The dates are not corrected for 13C fractionation. All the dates older than 10,000 yr have been given with 2σ errors. Due to uncertainty about the contribution of the biogenic carbon in caliche (CaCO3) samples, the dates represent apparent and not true ages.

Most dates in this list were obtained between 1972 and 1976. Treatment of samples has remained essentially the same as described previously (R, 1975, p 364).