This radiocarbon laboratory was established by a grant from Brooklyn College to support the work of its archaeologists and geologists. The method of dating is that of benzene synthesis and liquid-scintillation counting developed by a number of investigators (Noakes et al, 1965; Polach and Stipp, 1967; Tamers, 1975). This list includes nearly all samples processed so far, most of which were done since September 1975.

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.

The following list consists of dates for archaeologic samples from countries other than the British Isles measured with a few exceptions over the period of mid-1970 to June 1974. The dates were obtained by liquid scintillation counting of benzene using a Model 3315 Packard Tricarb Liquid Scintillation Spectrometer. The laboratory procedures used were those outlined in the previous date list (R, 1976, v 18, p 16). As before, the dates, relative to ad 1950, are based on the Libby half-life for 14C of 5570 years, are corrected for isotopic fractionation (relative to the PDB standard) and are expressed in radiocarbon years uncorrected for natural 14C variations. NBS oxalic acid is used as the modern reference standard.

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.

The 14C dates listed in this report on archaeologic and geologic samples were obtained mainly from 1969 to 1974. Pretreatment of wood charcoal and carbonized fossil soil by boiling in acid, alkali, and acid, again, is standard procedure. The method employed is liquid scintillation counting of synthesized benzene using the basic techniques described by Arslanov and Gromova (1968) converting sample → Li2C2 → C2H2 → C6H6 with an overall yield to ca 90%. A vanadium-alumina catalyst is used at reduced temperature for the acetylene to benzene conversion. Anthracite coal was used for the dead carbon run.

Treatment of samples and counting equipment have remained essentially the same as described previously (R, 1974, v 16, p 404-306). Sample CO2 is used to fill a 1.2L steel proportional counter at 740mmHg.

This list comprises age measurements carried out from January 1976 to July 1976. Dated samples are all of archaeologic interest, from Italian territory. Pretreatment of samples, production of purest CO2 and counting techniques have been described elsewhere (Azzi, 1972; Azzi et al, 1973; Azzi et al, 1974). Ages of samples are calculated using the conventional half-life of 5570 ± 30 yr and refer to 1950. Errors are stated in terms of one standard deviation of counting statistics.

Radiocarbon dating of many types of samples has continued at the La Jolla Radiocarbon Laboratory since the last date list was published (Bien and Pandolfi, 1972). Since the 1969 relocation of the laboratory to its present location on Mt Soledad, at least 5 low-level 14C systems have generally been in operation. The laboratory has continued to employ acetylene gas proportional counting. Samples were usually measured for 2 to 3 days in each of 2 different detectors. The properties of the 5 counters used in the measurements of samples are as follows (background and net standard counts per minute, respectively, with acetylene of 900mmHg pressure are given in parentheses): 1) Bern counter: Oeschger-type counter-guard ring unit with 1.7L sensitive center volume (2.1, 21.5), 2) MBLE counter: Oeschger-type counter-guard ring unit with 1.6L center volume (1.3, 17.4), 3) 1L counter with separate Q-gas flow guard ring (4.2, 12.9), 4) 0.4L counter with separate guard ring (1.2, 4.2), and 0.1L counter with separate guard ring (0.7, 1.2).

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.

Most of the 14C measurements reported here were made between October 74 and March 76. The usual procedures and methods of sample preparation are those described in R, 1964, v 6, p 194-196. The electronic apparatus was constructed by J Galliot from results of studies on background of proportional counters (Delibrias & Rapaire, 1967) and was described briefly in R, 1966, v 8, p 286-291.

Radiocarbon measurements mainly on soil samples and soil organic matter fractions are being continued. Sample benzene preparation follows Scharpenseel & Pietig (1969; 1970a). Radioactivity is measured in single screw cap quartz vials using a Packard Tri-Carb 3075 as well as a Berthold Betaszint BF 5000.

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.

The Laboratory of Applied Nuclear Physics of the Brown Coal Research Institute in Most started the radiocarbon dating of archaeologic samples in 1972. Dates presented in this list were obtained from 1973 to 1974.

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 14C dates given below are continued from our previous list (R, 1974, v 16, p 331-357), and results obtained mainly during 1972-73 are described. A 2.7L stainless steel counter and a 3.3L copper counter are used as previously, yielding background counting rates of 3.9 and 5.6cpm, respectively, when filled with dead CO2 at ca 1.7atm. Dates have been calculated on the basis of the 14C half-life of 5568 ± 30 yr and 95% of NBS oxalic acid is modern standard. Errors (± 1σ) include standard deviations for sample counts, background and modern standard, that of half-life, and, also, effective standard deviations for reading of filling pressure and temperature. No correction has been made for any of the samples in this list.

The present list reports 14C measurements made in our laboratory from December 1972 to December 1973.

This date list includes most of the archaeologic and geologic samples dated in this laboratory since publication of our last date list (R, 1975, v 17, p 196-215) as well as many samples dated previously which lacked adequate sample information.

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.

The following list contains measurements made during the period 1972-5 which have not been previously published and which form part of a research project on marine shell dating (Gillespie, 1975). These sets of samples were measured to determine 1) “apparent age” of marine shells coll alive from Australian coastal waters before the advent of large-scale nuclear weapons testing, 2) the possibility of using post 1950 Australian marine shells as a modern reference, 3) “apparent age” of Australian marine shells in the past as shown by comparisons between stratigraphically equivalent charcoal and marine shell samples.