41Ca, 26Al and 10Be in lunar basalt 74275 and 10Be in the double drive tube 74002/74001

We report depth profiles of the cosmogenic radionuclides Be-10, Al-26, and Ca-41 in the titanium-rich lunar basalt 74275. The Be-10 profile is flat: Be-10 activities are confined to a narrow range between 9.6 and 11.2 dpm/kg but are nonetheless consistent with a small contribution of about 1-2 dpm/kg from solar cosmic rays. The Al-26 profile shows the steep decrease with increasing depth that is characteristic of nuclides whose production is dominated by solar cosmic rays. Ca-41 activities decrease from about 22 dpm/kg at the surface to a minimum of similar to 9 dpm/kg at a depth of 4.7 g/cm(2) and then increase to similar to 11 dpm/kg at a depth of 15.8 g/cm(2). The sharp decrease near the surface identifies for the first time production of Ca-41 by solar cosmic rays. We also report Be-10 measurements for six samples from lunar core 74002/1. The Be-10 activities range from approximately 8 to 14 dpm/kg. We model the production of Be-10, Al-26, and Ca-41 in lunar rock 74275 by including published data that indicate a long exposure to galactic cosmic rays at a depth of 140 g/cm(2) followed by one at the surface lasting 2.8 Ma. Cosmogenic radionuclide production by galactic cosmic rays, and, in the case of Ca-41, by thermal neutrons is estimated from published measurements and semi-empirical calculations. Our model includes a new calculation of production rates due to solar cosmic rays and incorporates recently published cross section measurements. Although many parameterizations of the flux of solar energetic particles give acceptable fits to the experimental data for 74275, we prefer a best fit obtained for Be-10 and Al-26 alone, which incorporates an erosion rate of similar to 2 mm/Ma, a rigidity of 100 MV, and a 4 pi flux of protons with energies greater than 10 MeV of 89 cm(-2) s(-1). For Ca-41 alone, the corresponding values are 2 mm/Ma, 80 MV, and 198 cm(-2) s(-1). The differences between the two sets of parameters may reflect uncertainties in the calculations of Ca-41 production or a secular change in the solar cosmic ray flux. Calculations for a slab and for a hemispherical knob with a radius of 23 cm yield similar results. Copyright (C) 1998 Elsevier Science Ltd.