Measurement of Low Level Radiocarbon

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Abstract
Techniques are described for chemical purification and measurement of radiocarbon at natural levels (10−12 curie/g or less). The chemical cycle has the following steps: (1) conversion of the starting material to carbon dioxide, by combustion or hydrolysis, (2) precipitation as calcium carbonate, followed by evolution of the purified carbon dioxide, (3) reduction to elementary carbon with magnesium turnings, and (4) extraction with hydrochloric acid and water to remove magnesium and magnesium oxide. The over-all yield is about 85 percent, based on the carbon in the original sample. The counting is done in a screen-wall counter, twenty-four inches long, with an eight-inch effective counting length. The sample (about 8.5 g) is mounted uniformly in an ``infinitely thick'' layer, on a split cylinder which can be shifted from ``sample'' to ``background'' position by sliding from end to end of the counter. An argon-ethylene filling is used. The background of the counter (about 400 cpm, unshielded) is reduced to 4.5 cpm by the use of an eight-inch thick iron shield and eleven anti-coincidence counters, eighteen inches long. The calibration of the counter on an absolute basis is described. Its efficiency for a 20 mg/cm2 layer of carbon, over a 400 cm2 area, is 5.46±0.03 percent.