Depth and size dependence of cosmogenic nuclide production rates in stony meteoroids

Abstract

Depth profiles of the cosmogenic isotopes ³He, ²⁰e, ²¹Ne, ²²Ne, ¹⁰Be and ²⁶A1 have been measured by conventional and accelerator mass spectrometry in the chondrites Madhipura, Udaipur, and Bansur. Shielding depths of the samples and meteorite sizes were derived from cosmic ray track density data and from ²¹Ne exposure ages. In addition, ¹⁰Be and ²⁶A1 were measured in seven fragments of Dhajala. The measured data, together with the existing 53Mn profiles in these meteorites and with other well-investigated depth profiles of cosmogenic radionuclides and rare gas isotopes in ALHA 78084, Keyes, St. Severin, Jilin and Knyahinya, now provide an experimental data base describing the depth and size dependence of cosmogenic nuclides in ordinary chondrites for preatmospheric radii between 8.5 cm and about 100 cm. Production rates are found to change only slightly with depth in small meteorites (R ≤ 15 cm). For larger bodies (15 cm ≤ R ≤ 65 cm), the profiles show significant depth dependence, the cosmogenic production increases from the surface to the center by about 30%. The center production rates increase with 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 ≥ 70 cm, a significant decrease of center production rates is seen for ¹⁰Be, ²⁶A1,⁵³Mn and ²¹Ne, the individual depth profiles being essentially flat with shallow transition maxima. The observed depth profiles and the dependence of the center production rates on meteoroid size are well reproduced by model calculations based on Monte Carlo calculations of the intra- and internuclear cascade of galactic protons in meteoritic matter and on experimental and theoretical excitation functions of the underlying nuclear reactions. The model calculations provide a basis for identification of meteorites with anomalous levels of radioisotopes and give information about the irradiation history of meteorites and changes in the cosmic ray intensity with time and orbital space of the meteoroid. The results of the Dhajala chondrite are discussed in this context.