An international review of liquid scintillation low-level counting procedures and instrumentation made it possible to assess in detail those elements which lead to high-precision liquid scintillation radiocarbon dating with a Figure of Merit of 32,000. Current research is documented and future possibilities are alluded to.

Results are presented of a study of counter performance and vial characteristics for three liquid scintillation counters used at the SMU Radiocarbon Laboratory: the Intertechnique LS20, Packard Tri-Carb 460C, and LKB Wallac Rack Beta 1217. Modifications to photomultiplier tube high voltage, pre-amplifier gain, energy window settings, counting vial design, and sample holder design have resulted in reduced background, higher counting efficiency, and greater long-term stability for the Intertechnique and Packard counters. Square quartz counting vials are used in the Intertechnique and Packard counters with excellent results. Use of Teflon vials in the LKB counter requires careful cleaning procedures and long counting times.

The measurements reported here are a continuation of tropospheric radiocarbon measurements in carbon dioxide carried out since 1961 at our China Lake, California collection facility. The data show a continued decrease in radiocarbon activity from ca 330‰ in 1977 to 215‰ in 1983 in agreement with similar analyses in Europe for the same time interval.

Bone would seem to be an ideal material for 14C dating because this calcified tissue contains 20 weight per cent protein. Fossil bone, however, can lose most of its original organic matter and frequently contains contaminants having different 14C ages. Numerous 14C dates on bone have been available to archaeologists and geologists but many age determinations have been inaccurate despite over 30 years of research in the field following the first 14C age determinations on bone (Arnold & Libby, 1951). This situation remained unchanged until simple pretreatments were abandoned and more bone-specific fractions were isolated. The ideal solution is to use accelerator mass spectrometer 14C dating, which facilitates the use of milligram-sized amounts of highly purified compounds—an approach impossible to pursue using conventional 14C decay-counting methods.

Four different bone collagen preparation procedures were compared and were found useful as a means of assessing the nature of contaminants present in a sample. Weathered bone however appeared to contain contaminants that could not be eliminated by any of the procedures studied.

The vertical profile of radiocarbon at (30° N, 170° E) measured in 1980 was compared with the GEOSECS data measured in 1973. 14C was extracted from 200L of sea water, converted to C2H2, and analyzed with a gas proportional counter. Our profile and that of GEOSECS were in good agreement below 700m depth without systematic deviation of Δ14C values between both measurements. On the other hand, a Δ14C increase was observed above 700m depth, reflecting the transient addition, in 6.6 years, of bomb 14C to the intermediate layer from the atmosphere.

Samples processed since the last list was published (R, 1978, v 20, no. 3, p 192–199) are reported here. The dates were obtained by liquid scintillation counting of benzene, using laboratory procedures outlined in previous articles (R, 1976, v 18, no. 2, p 151–160; R, 1977, v 19, no. 3, p 383–388).

The following list consists of dates for archaeologic samples mostly measured from June 1984 to June 1985. The dates were obtained by liquid scintillation counting of benzene using the laboratory procedures outlined in previous lists (see, eg, BM-VIII, R, 1976, v 18, p 16). Dates are expressed as suggested by Stuiver and Polach (1977), ie, in 14C years relative to AD 1950, based on the Libby half-life for 14C of 5570 yr, and corrected for isotopic fractionation (δ13C values are given relative to PDB). No corrections have been made for natural 14C variations. The modern reference standards are the NBS oxalic acids (SRM 4990 and RM 49). Errors quoted are the counting error for the sample, combined with an estimate of the errors contributed by the modern and background samples. This estimate includes both counting and non-counting errors, and is computed from differences in the overall count rates observed among the individual backgrounds and moderns. The overall error is given as ± 1 standard deviation (± 1σ). Descriptions, comments, and references to publications are based on information supplied by submitters.

The results presented in this list include some recently measured samples (1984) but mostly ones from our earlier years of operation which had not been previously published in RADIOCARBON. It is the first of a number of special lists prepared over the last year so that the backlog of unpublished dates of this laboratory will be cleared. The samples are all archaeologic from the United Kingdom most of which have originated from “rescue”-type excavations.

This list contains results obtained between 1981 and 1985. Since the first description of the laboratory (Carmi, Noter & Schlesinger, 1971) the following changes were made. Two proportional counters are now used: 1) 0.5L volume, 0.865 ± 0.023 cpm background, 12.830 ± .134 cpm NBS oxalic acid standard (old); 2) 0.25L volume, 0.484 ± 0.023 cpm background, 6.185 ± .123 cpm NBS oxalic acid standard (old). The passive shield has been increased by 2cm of mercury next to the counters. For anticoincidence, a modular, hand-made gas counter is used. The laboratory was transferred to the ground floor of a 7-storey building. Data acquisition and processing are done with a sealer/buffer built at the institute and an IBM PC computer. Samples are filled into the counters and counted for ca 1000min at least twice. The sample preparation method and counter filling pressure have not been changed.

The following radiocarbon date list contains dates of samples from Plitvice Lakes measured since our previous list dealing with tufa from this region (Srdoč et al, 1982). Tufa measurements from the Knin area in S Croatia and some localities in Bosnia are also listed. Age calculations are based on the Libby half-life (5570 ± 30) yr and reported in years before 1950. Reported ages are based on the initial activity of 85% except for lake sediments where calculations of initial activity have been performed. The modern standard is 0.95 of the NBS oxalic acid activity. Sample pretreatment and counting technique are essentially the same as described in R, 1971, v 13, p 135–140, supplemented by new techniques for groundwater processing (R, 1979, v 21, p 131–137). Statistical processing of data has been computerized (Obelić & Planinić, 1977; Obelić, 1980). The errors quoted correspond to 1 σ variation of sample net counting rate and do not include the uncertainty in 14C half-life.

The following radiocarbon date list contains dates of samples measured since our previous list (R, 1984, v 26, no. 3, p 449–460). As before, age calculations are based on the Libby half-life (5570 ± 30) yr and reported in years before 1950. The modern standard is 0.95 of the NBS oxalic acid activity. Sample pretreatment, combustion and counting technique are essentially the same as described in R, 1971, v 13, no. 1, p 135–140, supplemented by new techniques for groundwater processing (R, 1979, v 21, no. 1, p 131–137).

We strongly welcome the investigation by Geyh and Hennig (1986) and agree with their conclusion that the boundaries of the interglacial periods cannot be determined exactly using the methods applied so far in the Heggen cave. However, since we have been engaged for several years in the application of these methods (except 14C), and since the paper gives a very different interpretation of the ESR results from that presented by one of us (Grün, 1985), we feel the need to make some comments on this paper.

Preliminary observations of changes of the background of three proportional counters during May and June, 1986, are presented. Some isotopes contributing to the rise of background were identified on the basis of their half-life determinations.