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The dates detailed below are based on measurements made from September 1962 to August 1964. Work was often seriously interrupted due to difficulties with electronic equipment and also, from the late summer of 1963, high levels of tritium in the local water supplies used in the synthesis of acetylene (from nuclear weapons tests) made it difficult to obtain accurate measurements with acetylene as a filling gas for the proportional counter. The gas preparation equipment was therefore modified for the preparation of high purity CO2, and from Sample BM-165 onwards, the proportional counter was operated with CO2 as the filling gas at a pressure of 210 cm Hg at 22°C. instead of 140 cm Hg pressure of acetylene at 22°C. as reported previously (British Museum I). Background and net modern count rates under these conditions are 3.59 c.p.m. and 8.35 c.p.m., respectively. (In practice these values are taken as the rolling mean of the past 20 weeks’ measurements and are very constant.) The calculations of age are based on the half-life of 5570 yr and error terms are widened to include contributions of ± 80 yr for possible isotopic fractionation effects and ± 100 yr for de Vries-effects. Safeguards against inaccuracies are as described previously (British Museum I).
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