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 ¹⁰Be production in the Earth's atmosphere by cosmic rays, we have calculated the atmospheric production of the cosmogenic isotopes ³H, ⁷Be, ¹⁰Be, and ³⁶Cl using the FLUKA Monte Carlo code. This new calculation of atmospheric yields of these isotopes is based on 10⁷ vertically incident protons at each of 24 logarithmically spaced energies from 10 MeV to 10 GeV, 10² 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 ¹⁰Be, only the February 1956 solar event makes a major contribution. For ³⁶Cl 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 ¹⁰Be, ³⁶Cl, and other cosmogenic isotopes above 65° 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.