Obtaining a History of the Flux of Cosmic Rays using In Situ Cosmogenic Carbon-14 Trapped in Polar Ice
2019 July 22
Carbon-14 is produced in the atmosphere when neutrons from cosmic-ray air showers are captured by nitrogen nuclei. Atmospheric carbon-14 becomes trapped in air bubbles in polar ice as compacted snow (firn) transforms into ice. Carbon-14 is also produced in situ in ice grains by penetrating cosmic-ray neutrons and muons. Recent ice core measurements indicate that in the carbon monoxide (CO) phase, the carbon-14 is dominated by the in situ cosmogenic component at most ice coring sites. Thus, it should be possible to use ice-bound CO to reconstruct the historical flux of cosmic rays at Earth, without the transport and deposition uncertainties associated with beryllium-10 or the carbon cycle uncertainties affecting atmospheric carbon dioxide. To the extent that the CO is activated by low-energy secondary cosmic-ray particles, the flux history may also contain information about past changes in solar activity and solar irradiance. We describe recent measurements of CO in polar ice and the sensitivity of carbon-14 data to the historical flux of galactic cosmic rays.