Predicted versus observed cosmic-ray-produced noble gases in lunar samples: improved Kr production ratios

New sets of cross sections for the production of krypton isotopes from targets of Rb, Sr, Y, and Zr were constructed primarily on the bases of experimental excitation functions for Kr production from Y. These cross sections were used to calculate galactic-cosmic-ray and solar-proton production rates for Kr isotopes in the moon. Spallation Kr data obtained from ilmenite separates of rocks 10017 and 10047 are reported. Production rates and isotopic ratios for cosmogenic Kr observed in ten well-documented lunar samples and in ilmenite separates and bulk samples from several lunar rocks with long but unknown irradiation histories were compared with predicted rates and ratios. The agreements were generally quite good. Erosion of rock surfaces affected rates or ratios for only near-surface samples, where solar-proton production is important. There were considerable spreads in predicted-to-observed production rates of 83Kr, due at least in part to uncertainties in chemical abundances. The 78Kr/83Kr ratios were predicted quite well for samples with a wide range of Zr/Sr abundance ratios. The calculated 80Kr/83Kr ratios were greater than the observed ratios when production by the 79Br(n,γ) reaction was included, but were slightly undercalculated if the Br reaction was omitted; these results suggest that Br(n,γ)-produced Kr is not retained well by lunar rocks. The productions of 81Kr and 82Kr were overcalculated by approximately 10% relative to 83Kr. Predicted-to-observed 84Kr/83 ratios scattered considerably, possibly because of uncertainties in corrections for trapped and fission components and in cross sections for 84Kr production. Most predicted 84Kr and 86Kr production rates were lower than observed. Shielding depths of several Apollo 11 rocks were determined from the measured 78Kr/83Kr ratios of ilmenite separates. 4 figures, 5 tables