Production of the cosmogenic isotopes 3H, 7Be, 10Be, and 36Cl in the Earth's atmosphere by solar and galactic cosmic rays

In a follow-up study to the earlier work of Webber and Higbie (2003) on Be-10 production in the Earth's atmosphere by cosmic rays, we have calculated the atmospheric production of the cosmogenic isotopes H-3, Be-7, Be-10, and Cl-36 using the FLUKA Monte Carlo code. This new calculation of atmospheric yields of these isotopes is based on 10(7) vertically incident protons at each of 24 logarithmically spaced energies from 10 MeV to 10 GeV, 10(2) times the number used in the earlier calculation, along with the latest cross sections. This permits a study of the production due to solar cosmic rays as well as galactic cosmic rays at lower energies where isotope production is a very sensitive function of energy. Solar cosmic ray spectra are reevaluated for all of the major events occurring since 1956. In terms of yearly production of Be-10, only the February 1956 solar event makes a major contribution. For Cl-36 these yearly SCR production contributions are 2-5 times larger depending on the solar cosmic ray energy spectra. We have determined the yearly production of Be-10, Cl-36, and other cosmogenic isotopes above 65 degrees geomagnetic latitude for the time period 1940-2006 covering six solar 11-year (a) cycles. The average peak-to-peak 11-a amplitude of this yearly production is 1.77. The effects of latitudinal mixing alter these direct polar production values considerably, giving an average peak-to-peak 11-a amplitude of 1.48 for the global average production.