Count rates, representing the rate of 14C decay, are the basic data obtained in a 14C laboratory. The conversion of this information into an age or geochemical parameters appears a simple matter at first. However, the path between counting and suitable 14C data reporting (table 1) causes headaches to many. Minor deflections in pathway, depending on personal interpretations, are possible and give end results that are not always useful for inter-laboratory comparisons. This discussion is an attempt to identify some of these problems and to recommend certain procedures by which reporting ambiguities can be avoided.

Infrared (IR) spectrophotometry and X-ray diffraction (XRD) were conducted on modern and fossil bone material from archaeological sites in the U S to determine post-mortem changes in bone apatite and to evaluate the effect of these changes on radiocarbon dating. IR absorption bands, XRD peak-broadening parameters, and XRD unit cell measurements indicated that during fossilization, bone apatite, a mineral similar to dahllite, was partially or completely recrystallized to francolite. Post-mortem changes involved then removal of some of the endogenous crystal carbonate both at surfaces and at internal OH-sites and introduction of exogenous carbonate into internal crystal PO4 sites. Increased fluorine content accompanied carbonate substitution.

Both the carbon isotopic composition and the amount of exogenous carbonate introduced into the apatite structure will affect the radiocarbon dating of bone apatite. Special sample pre-treatment may remove most of the substituted carbonate in some cases. Simulated experiments are suggested for a better understanding of the nature and mechanism of carbonate substitution in bone apatite for the removal of the exogenously substituted carbon and the improvement of radiocarbon dates.

An improved procedure for wet combustion of 14C NBS oxalic acid standard has been devised which gives consistent high carbon yields and average δ13C values of —19.1‰. The principal cause of fractionation in earlier attempts to prepare CO2 by the wet oxidation method was the inexact nature of the end-point. The new procedure employs a chocolate-brown end-point by adding 5ml more of the sulfuric acid-potassium permanganate solution after the initial reddish brown end-point is reached. The sulfuric acid-potassium permanganate solution is added to the NBS oxalic acid in a steady drop-wise flow, heat is applied to the generating apparatus, and a cycling technique is utilized to collect the CO2.

Fifteen samples of the NBS oxalic acid were processed. The per cent carbon yields range from 98.8 to 100.9% with an average of 99.7% and an average δ13C of —19.12‰. The results obtained by this procedure are much more consistent than previous results obtained in several laboratories by direct combustion.

The following list contains most of the measurements made during 1976, since our last list (R, 1976, v 18, p 151-160). Wood and charcoal samples were prepared as previously described (R, 1976, v 18, p 151). Peat samples were treated with cold N/10 NaOH and hot N/10 HCl, washed and dried.

Radiocarbon dating facilities were built at the Department of Nuclear Physics, Comenius University in 1967 (Usačev et al, 1973). Initially, sample pretreatment and combustion systems for a proportional counter filled with CO2 were installed (Chrapan, 1966). One group adopted methods based on the use of methane (Usačev et al, 1973), a second group continued radiocarbon dating using an Oeschger-type proportional counter filled with CO2 (Chrapan, 1968). Later a modified Oeschger-type proportional counter with 1L active volume and with a background of approximately 8.10–2 bq was built (Schmidt and Chrapan, 1970). The pressure used in this counter is 105 Pa. 0.95 NBS oxalic acid is used as a standard of the present biosphere and the year 1950 refers to the zero year. Calculated radiocarbon ages are based on a 5568 ± 30 year half-life as recommended by the 8th International Radiocarbon Dating Conference. Statistical errors are calculated as a combination of the 3σ standard deviations of the sample count and the background. Samples were treated by HCl, NaOH or other chemicals according to their initial conditions.

The following list includes determinations completed since publication of our last date list (R, 1976, v 18, p 43-49). Sample preparation and counting configuration remains the same as for DAL-I. Age determinations are based on at least two counter fillings (except as noted for very small samples) for a total of 3000 minutes and are calculated from at least 5000 minutes modern and background counts updated weekly by alternating modern and background samples in the counters over weekends. Age calculations are based on the Libby half-life of 5568 y and includes 1 σ statistical uncertainties of sample, modern, and background activity. Reference age is ad 1950 and is determined from 95 oxalic acid activity or age-corrected 1850 wood.

Since publication of Harwell I (R, 1974, v 16, p 178-191) the laboratory has been officially set up to operate a commercial radiocarbon dating service. To accommodate an increased number of samples the combustion and benzene synthesis rigs have been rebuilt as simultaneously-operated twin-line systems.

Most of the 14C measurements reported here were made between October 1975 and October 1976. Equipment, measurement, and treatment of samples are the same as reported previously (R, 1968, v 10, p 36-37; 1976, v 18, p 290).

Experimental procedures and calculation remain almost as previously described in R, 1977, v 19, p 326-331. The only difference is in the counting equipment. We now use a Beckman LS-230 counter where samples are counted for a total count of about 30,000.

The following dates are a partial list of archaeologic and geologic samples measured since spring of 1976. The method used is described by (Stipp et al, 1976). Errors reported are one standard deviation and include only the counting errors on the unknown sample, background, and modern standard. No corrections have been made on these dates. Sample descriptions and comments are based on information supplied by the submitters.

A radiocarbon laboratory at the US Geological Survey Western Regional Headquarters in Menlo Park, California was established in temporary rooms in March, 1976. In August, 1976 the laboratory was dismantled and moved into a new building that was designed specifically for the facility. This list contains results of operations in the temporary laboratory, which was located in the basement of a two-story building. The counter shield was 15cm lead and 5cm borated paraffin with an additional 5cm of lead above the counters. The anticoincidence ring consisted of copper tubes, 5cm in diameter, mounted around a copper tube with an inside diameter of 15cm. Installed in the anticoincidence ring were the four sample counters, whose characteristics are shown in Table 1. The counting electronics unit, which is designed to service five sample counters, follows the design of Gulliksen (1972) in most respects. The sample counters were constructed of copper and quartz; their design is described in detail by Robinson (1977). The CO2 counting gas was purified by recirculation over copper and silver at ca 450°C. Wood samples were typically pretreated by leaching for 24 hours alternately in 1N sodium hydroxide and hydrochloric acid solutions at 70°C.

The following list contains dates of samples measured since our previous list (R, 1975, v 17, p 149-155). As before, age calculations are based on the Libby half-life 5570 ± 30 yr and reported in years before 1950. The modern standard is 0.950 of the activity of NBS oxalic acid.