Direct measurement of the production rate of ¹⁴C near Earth's surface

Environmental ¹⁴C is well known for its application in carbon dating organic material, as well as its use as a geochemical tracer in the oceans and atmosphere. More recently, ¹⁴C in the form of atmospheric ¹⁴CO has been used to estimate the concentration of hydroxyl radical, which is the primary oxidant of the atmosphere. This latter application is dependent upon accurate knowledge of the cosmogenic production rate, which has been calculated, but not directly measured. We have measured the atmospheric production rate of ¹⁴C near the earth's surface on a 300-meter research tower in Colorado, at the South Pole, at Scott Base, Antarctica, and near Mount Cook, New Zealand. The observed production rate near the surface was 50% of the production rate calculated by an earlier study. We also find that the ‘ground effect’, which causes an increase in the thermal neutron flux resulting in an enhancement of the production rate of ¹⁴C, is significantly smaller than calculated in earlier studies.