C 14 produced by the N u (n, p) reaction of cosmic-ray neutrons has been detected and identified. The C 14 concentration of biological methane was enriched by factors of up to 260-fold by thermal diffusion to raise the activity to convenient levels for counting. The pure methane was used as the filling gas in large Geiger-Mtiller counters.The activity in biological methane was found to be 10.5 disintegrations per minute per gram C, or 0.95 X10~1 2 gram C 14 per gram C. This is in reasonable agreement with the value predicted from the estimated values of the cosmic-ray neutron flux and the amount of carbon in exchange equilibrium with the atmosphere. No significant activity was detected in petroleum methane.A discussion of the thermal diffusion enrichment factor is given.
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