We have tested two versions of an 0.8-ml volume Teflon® vial, designed specifically for radiocarbon dating in the microvolume range, using a modern Quantulus 1220™ liquid scintillation (LS) spectrometer. We determined the counting performance of each vial type in conjunction with different designs of copper holder, viz., with and without the incorporation of a “Teflon® light coupler”. We also compared the losses of sample benzene during a typical 28-day counting period. Results show that neither vial design nor the type of vial holder used in the intercomparison had a significant influence on counting performance. We recorded an absolute 14C detection efficiency of 82% against a background count rate of 0.1 cpm, i.e., a “figure of merit (FM) value” = 67,240. This compares favorably with the operational parameters anticipated for microvolume 14C dating by modern LS spectrometry. However, variations in the design of the sealing method used between the vial types was reflected in an apparent approximate tenfold difference in the amount of benzene lost during routine counting. In the better case, the evaporation loss was equivalent to 0.032 mg of benzene per day.

We investigated the effect of radon (222Rn) contamination of samples and chemicals used for sample preparation and gas purification on the 14C age of samples. Radon count rate was monitored over several 222Rn half-lives in a proportional counter filled with methane. We analyzed ca. 240 samples of fossil and recent wood, charcoal, bone, dissolved inorganic carbon in water, travertine, peat, aquatic plants, organic soil and atmospheric CO2 for 222Rn contamination. Chemicals used for gas purification (MnO2, Mg(ClO4)2), ruthenium-coated pellets used for catalytic reduction of CO2 to CH4, and acids used for dissolution of calcareous samples (HCl and H3PO4) are also potential sources of Rn. Most geological samples contained a significant amount of Rn, as opposed to samples of wood, charcoal, aquatic plants and atmospheric CO2. We also studied Rn contamination of water samples during tritium activity measurements in a CH4-filled gas proportional counter. We found an increased count rate in the 3H channel, as well as above the 20 keV region in Rn-contaminated groundwater samples.

Low-level detector systems for 14C dating are frequently located in underground or basement laboratories to reduce the influence of cosmic radiation. However, if careful precautions are not taken, the presence of radioactive radon gas may severely disturb the analytical results. Measurements of the influence of radon on a proportional counter system, show that a radon level of 100 Bq m−3, a level not uncommon in basement rooms, is sufficient to produce an unacceptable uncertainty in the 14C results. Radon levels of up to 1500 Bq m−3 can be demonstrably reduced to about 30 Bq m−3, using a separate ventilation system that generates a slight overpressure in the laboratory.

The automatic β-counting system plays a significant role in obtaining high-level reproducibility and reliability in conventional radiocarbon dating. I review here the results achieved by using the “Fully Automatic Radiocarbon Dating System” developed by Omoto (1982). Since setting up the system in 1981, I was able not only to save operator time in β counting, but also to obtain accurate dates with only minimal uncertainties. Another positive result was the introduction of the automatic voltage correction program, which produced excellent results for counting sample materials over a long period.

We describe a two-phase study directed toward background reduction of a manual liquid scintillation counter and the interfacing of electronics for counting to a computer data acquisition system. Counter background reduction is achieved with afterpulse electronics, a high-performance cocktail, an auxiliary detector/guard and a special sample vial holder. The data acquisition system is comprised of an electronic signal processor and sorter for operating up to eight counters simultaneously and interfacing to a computer with software for data storage, acquisitions and age dating calculations. We discuss low-background counter modifications, electronic signal processing and computer software for 14C age dating.

We present here the principal ideas of a new, fully computerized data acquisition system with pulse-rise background reduction, developed in the Gliwice Radiocarbon Laboratory, and our first results. The new system uses a microprocessor-controlled pulse and coincidence analyzer for acquisition of data from 3 of 4 proportional counter sets. The analyzer acquires and stores information on the pulse's amplitudes and rise-times and their coincidence with guard counters and radio-frequency shield. This feature allows us to separate pulses using anticoincidence information and pulse-shape discrimination. The described method of background reduction led to a significant increase in the factor of merit on 2 of 3 counters tested.

Radiocarbon dating laboratories deal with many types of data and calculations, which include information on received and dated samples, age calculations and storage of results, printed reports to submitters and graphs plotted from sample measurements. I describe a computer program, designed to run on any PC-compatible computer with a hard disk, that can handle all the functions of a conventional liquid scintillation counting radiocarbon dating laboratory.

We report results of a three-year intercomparison experiment between the WHOI Radiocarbon Laboratory (now at University of California, Irvine) and the NSF-University of Arizona AMS Laboratory. The purpose of this study was to compare high-precision measurements of samples obtained routinely using gas proportional counting techniques with results obtained using AMS techniques. Three sets of annually banded, modern coral samples were used for the intercomparison. Each sample was acidified to CO2 at WHOI and split into two fractions. The larger fraction was converted to acetylene gas and counted at WHOI in quartz, gas-proportional beta counters. The smaller fractions were converted to graphite and analyzed for 14C using AMS techniques at the University of Arizona. Results of the three sample sets are presented. Except for a single outlier, the data from the two laboratories are in good agreement. Of the 13 samples in the third set of the intercomparison, for which a new high-intensity ion source was in operation at the Arizona AMS laboratory, agreement of results is excellent. This finding indicates that measurements made with precisions of < 3% are reproducible at both laboratories.

The 14C Quality Assurance Programme coordinated by the IAEA (Rozanski et al. 1992) prepared a set of five new intercomparison materials, including 40–50 ka old subfossil wood excavated from New Zealand peat bogs (IAEA C-4 standard). Statistical analysis of 79 14C measurements made on the wood indicated considerable variation in the results, with a marked skewness toward more modern values. The wide range of results and the possibility of inhomogeneity within the standard prompted the recovery and analysis of replacement material. The new subfossil wood sample is kauri (Agathis australis), at least 50 ka old, excavated from a swamp in Northland. It is in the form of a single plank, 6 m long, weighing 80 kg. It will be forwarded to the IAEA in Vienna for milling and distribution. Subsamples were obtained from both ends of the plank and analyzed by six laboratories. We present here the results of these analyses and compare them with the previous IAEA intercalibration results for the C-4 standard.

The Radiocarbon Tritium Laboratory at the Rudjer Bošković Institute participated in all three radiocarbon intercomparison studies. The first International Collaborative Study (ICS) showed that the age of organic samples measured at our laboratory is ca. 70 yr younger than the median ICS value. After introducing a slight modification in our chemical pretreatment procedure by acidifying samples after alkaline treatment, we obtained excellent agreement between our results and the median values reported by two subsequent intercomparisons (IAEA, TIRI) without any laboratory bias.