DEPTH AND SIZE DEPENDENCE OF COSMOGENIC NUCLIDE PRODUCTION-RATES IN STONY METEOROIDS

Depth profiles of the cosmogenic isotopes He-3, Ne-20, Ne-21, Ne-22, B e-10, and Al-26 have been measured by conventional and accelerator mas s spectrometry in the chondrites Madhipura, Udaipur, and Bansur. Shiel ding depths of the samples and meteorite sizes were derived from cosmi c ray track density data and from Ne-21 exposure ages. In addition, Be -10 and Al-26 were measured in seven fragments of Dhajala. The measure d data, together with the existing Mn-53 profiles in these meteorites and with other well-investigated depth profiles of cosmogenic radionuc lides and rare gas isotopes in ALHA 78084, Keyes, St. Severin, Jilin, and Knyahinya, now provide an experimental data base describing the de pth and size dependence of cosmogenic nuclides in ordinary chondrites for preatmospheric radii between 8.5 cm and about 100 cm. Production r ates are found to change only slightly with depth in small meteorites (R less-than-or-equal-to 15 cm). For larger bodies (15 cm less-than-or -equal-to R less-than-or-equal-to 65 cm), the profiles show significan t depth dependence, the cosmogenic production increases from the surfa ce to the center by about 30%. The center production rates increase wi th meteoroid size and show a broad maximum for radii between 25 and 65 cm. The location of the maxima for different nuclides depends on the dominant energy of particles responsible for their production from the main target elements. For R greater-than-or-equal-to 70 cm, a signifi cant decrease of center production rates is seen for Be-10, Al-26, Mn- 53, and Ne-21, the individual depth profiles being essentially flat wi th shallow transition maxima. The observed depth profiles and the depe ndence of the center production rates on meteoroid size are well repro duced by model calculations based on Monte Carlo calculations of the i ntra- and internuclear cascade of galactic protons in meteoritic matte r and on experimental and theoretical excitation functions of the unde rlying nuclear reactions. The model calculations provide a basis for i dentification of meteorites with anomalous levels of radioisotopes and give information about the irradiation history of meteorites and chan ges in the cosmic ray intensity with time and orbital space of the met eoroid. The results of the Dhajala chondrite are discussed in this con text.