The production of cosmogenic nuclides by galactic cosmic‐ray particles for 2π exposure geometries

Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for ¹⁰Be, ²⁶Al, and ⁵³Mn to measured data from the Apollo 15 drill core. The J_0,GCR value of 4.54 cm^?2 s^?1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J_0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl and ⁵³Mn, as well as for the rare gas isotopes ^20,21,22Ne. In addition we present production rates for ^36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the ²²Ne/²¹Ne ratio as a shielding parameter is also discussed.