CA-41, AL-26 AND BE-10 IN LUNAR BASALT-74275 AND BE-10 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 a nd 11.2 dpm/kg but are nonetheless consistent with a small contributio n 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 nuc lides whose production is dominated by solar cosmic rays. Ca-41 activi ties decrease from about 22 dpm/kg at the surface to a minimum of simi lar 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 cos mic 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 d pm/kg. We model the production of Be-10, Al-26, and Ca-41 in lunar roc k 74275 by including published data that indicate a long exposure to g alactic cosmic rays at a depth of 140 g/cm(2) followed by one at the s urface lasting 2.8 Ma. Cosmogenic radionuclide production by galactic cosmic rays, and, in the case of Ca-41, by thermal neutrons is estimat ed from published measurements and semi-empirical calculations. Our mo del 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 particl es 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 e rosion rate of similar to 2 mm/Ma, a rigidity of 100 MV, and a 4 pi fl ux of protons with energies greater than 10 MeV of 89 cm(-2) s(-1). Fo r 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 ref lect uncertainties in the calculations of Ca-41 production or a secula r 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. Co pyright (C) 1998 Elsevier Science Ltd.