Natural Radiocarbon from Cosmic Radiation

C¹⁴ produced by the N¹⁴(n,p) reaction of cosmic-ray neutrons has been detected and identified. The C¹⁴ 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-M\"uller counters.The activity in biological methane was found to be 10.5 disintegrations per minute per gram C, or 0.95×{}10^-12 gram C¹⁴ 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.