~~ ~~Abstract-We present a purely physical model for the calculation of depth-and size-dependent production rates of cosmogenic nuclides by galactic cosmic-ray (GCR) particles. Besides the spectra of primary and secondary particles and the excitation hnctions of the underlying nuclear reactions, the model is based on only one free parameter-the integral number of GCR particles in the meteoroid orbits. We derived this value from analysis of radionuclide data in Knyahinya. We also show that the mean GCR proton spectrum in the meteoroid orbits has been constant over about the last 10 Ma. For the major target elements in stony meteoroids, we present depth-and size-dependent production rates for loBe, 14C, 26A1, 36Cl, and 53Mn as well as for the rare gas isotopes 3He, ZoNe, 21Ne, 22Ne, 36Ar, and 38Ar. The new data differ from semiempirical estimates by up to a factor of 4 but agree within -20% with results obtained by earlier parametric or physical approaches. The depth and size dependence of the shielding parameter 22Ne/21Ne and the correlations 26AI vs. loge, 26AI vs. 53Mn, loBe/zlNe vs. 22Ne/21Ne, and 36Ar vs. 36Cl for deciphering preatmospheric sizes, shielding depths, terrestrial residence times, and exposure histories are also discussed.

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Abstract-We present a purely physical model for the calculation of depth-dependent production rates in 2n exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation hnctions 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 loge, 26A1, and 53Mn to measured data from the Apollo 15 drill core. The JO,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 JO,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 depthdependent production rates for IoBe, 14C, 26A1,36Cl and 53Mn, as well as for the rare gas isotopes 2O921,22Ne. In addition we present production rates for 36938Ar 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.

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