Direct measurement of the radiocarbon production at altitude

Details Direct measurement of the radiocarbon production at altitude Direct measurement of the radiocarbon production at altitude

Jun 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007nimpb.259..558b&link_type=abstract

Nuclear Instruments and Methods in Physics Research Section B, Volume 259, Issue 1, p. 558-564.

Physics

Scientific paper

The use of radiocarbon as an environmental tracer is widespread and yet the actual production rate in the atmosphere has not been measured in any great detail. The research we present here set out to make direct measurements of the production rate at the cruising altitude of commercial aircraft where the production rate is much higher than at ground level. Modelled estimates (e.g. [R.E. Lingenfelter, R. Ramaty, Astrophysical and geophysical variations in 14C production, in: I.U. Olsson (Ed.), Radiocarbon Variations and Absolute Chronology, Wiley, 1970; K. O’Brien, Secular variations in the production of cosmogenic isotopes in the earth’s atmosphere, J. Geophys. Res. 84 (A2) (1979) 423; J. Masarik, R.C. Reedy, Terrestrial cosmogenic nuclide production calculated from numerical simulations, Earth Planet. Sci. Lett. 136 (1995) 381]) put the estimated total production rate at between 1.8 2.4 14C atoms cm-2 s-1. A previous experimental study [J.E. Mak, C.A.M. Brenninkmeijer, J.R. Southon, Direct measurements of the production rate of 14C near earth’s surface, Geophys. Res. Lett. 26 (22) (1999) 3381 3384] measuring the production rate at the ground had found the production to be lower than expected from the models by a factor of almost two. In this experiment, 90 measurements were made on 14CO and 14CO2 generated in compressed air cylinders carried in a Lufthansa aircraft over a period of two years. A further 12 blank measurements were also made on identical cylinders stored at ground level. The correlation of radiocarbon production with flight time is good but the results were different from those expected in two respects. Firstly, the 14CO/14CO2 ratio was lower than expected (five rather than 20) which may be the result of the high pressure in the gas bottles, although there is no observed correlation with gas pressure over the range of samples we took. Even more significantly, the production rate is considerably higher than expected from the models, even taking into account the calculated enhancement-effect of the aircraft.

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