FIG. 1. Over-all view of counting system with front of iron shield removed to show counters 
Radiocarbon Dating System 
Dating range has been extended to 45,000 years and accuracy of “‘living-carbon” 
assay increased to 0.3% by carefully instrumented system centered on a 7.7-liter 
proportional counter filled with up to 3 atmospheres of pure CO: 
By G. J. FERGUSSON 
Dominion Physical Laboratory 
Lower Hutt, New Zealand 
AN IMPROVED METHOD for radiocarbon 
dating has been developed to supplant 
the solid-sample method (1) previously 
used here (2, 3). The system (Fig. 1) 
centers around a well-shielded 7.7-liter 
proportional counter filled with up to 
3 atmospheres of especially purified 
CO». The output pulse heights are 
analyzed electronically. In addition, 
pulses are sorted according to whether 
or not they are in coincidence with a 
190 
surrounding ring of G-M_ counters. 
This identifies most of cosmic-ray back- 
ground. 
For “living” carbon the counting 
rate is 37.5 cpm above a background of 
9.8 cpm for a counter-filling pressure of 
1 atm and 103 cpm above 14 cpm back- 
ground for a filling pressure of 3 atmos- 
pheres. For most problems a filling 
pressure of 1 atmosphere appears to 
offer the best compromise between 
accuracy and man-hours of work per 
sample. When increased accuracy is 
desired a 3-atmosphere filling is used. 
The maximum age that can be meas- 
ured is arbitrarily defined here as the 
age for which the net sample count is 
equal to four times its statistical stand- 
ard deviation. This definition has also 
been used by several other workers (4). 
For a l-day count on sample and on 
background the maximum age is 35,000 
yr for 1 atmosphere and 42,000 yr for 
3 atmospheres filling pressure. For a 
2-day count on sample and on back- 
ground with 3-atmosphere filling, maxi- 
mum age is 45,000 yr. 
Apparatus in General 
Carbon in the sample to be dated is 
converted to COs, which is then purified 
according to a procedure developed by 
