Samples were counted in 20ml low potassium glass vials with special brass caps and indium seals. Most samples achieved the optimum counting volume of 15ml, but those which did not were diluted using AnalaR benzene to the required counting volume. The scintillant used was butyl PBD in the proportions 15.66g/l, and the modern standard NIST oxalic acid II. Two oxalic acid standards were run along with the Stonehenge samples, one having been produced by combustion as described above and one by wet oxidation. The combusted sample was used to determine the dates. The background was determined by counting AnalaR benzene and applying a synthesisation factor (determined by in house experiments) to account for 14C added during benzene synthesis. Samples and standards were cycled through the counters at 100 minute intervals. All samples were counted for between 4300 and 4500 minutes which gave about 300,000 counts per sample or a counting precision of approximately 1.8 per mil. (eg ± 15 years). The counting errors on the samples were combined with the errors in determining the activity of the modern standard and the background. This combined error was multiplied by an error multiplier which reflects the reproducibility of dates on replicate samples within our laboratory. Counting was carried out in a purpose built basement laboratory that has a 1.8m concrete ceiling to act as a passive shield. The laboratory temperature and humidity are carefully controlled so that environmental variability does not affect counting efficiency or stability. The counter used for all Stonehenge results is designated by us as "Q1" and is an LKB Quantulus 1220 liquid scintillation counter which was set up and specially modified as described by McCormac (1992).
Results from the Y and Z Holes