Cosmic rays interaction with meteorites

Aluminum-26 is an important nuclide for investigating the cosmic-ray exposure history of meteorites on their way to Earth from the asteroid belt. It can also be used to estimate the terrestrial age of a meteorite. In both of these applications, the 26 A1 is alive in the samples, having been produced by cosmic-ray interactions with elements heavier than aluminum, primarily silicon. Cosmic-ray-exposure dating will be discussed in Chapter 9. 

Tritium has also been observed in meteorites and material recovered from sateUites (see also Extraterrestrial materials). The tritium activity in meteorites can be reasonably well explained by the interaction of cosmic-ray particles and meteoritic material. The tritium contents of recovered sateUite materials have not in general agreed with predictions based on cosmic-ray exposure. Eor observations higher than those predicted (Discoverer XVII and sateUites), a theory of exposure to incident tritium flux in solar flares has been proposed. Eor observations lower than predicted (Sputnik 4), the suggested explanation is a diffusive loss of tritium during heating up on reentry. 

LB/10B ratios. That is, the B abundance in the meteoritic sample correlates with extra B. So although itis not possible for elements having but two isotopes to reveal which of the two is anomalous, it is sensible owing to the correlation with B abundance to think ofvaryingadmixtures ofa boron component thatis enriched in 11B. This has been interpreted as a component of boron produced by low-energy cosmic-ray interactions, perhaps even in the presolar cloud or even in the early solar system itself. (See B for more on this.)... 

Energetic particles of the galactic cosmic radiation (GCR) have a mean penetration depth in rock of about 50 cm, comparable to the typical size of a meteorite. GCR-induced effects therefore provide a means to study the history of meteorites as small objects in space or in the top few meters of their parent body. These effects include cosmic ray tracks, i.e., the radiation damage trails in a crystal lattice produced by heavy ions in the GCR (Fleischer et al. 1975), and thermoluminescence, i.e., the light emitted by a heated sample which had been irradiated by energetic particles (Benoit and Sears 1997). By far most important, however, are cosmogenic nuclides, produced by interactions of primary and secondary cosmic ray particles with target atoms. 

Interaction of Cosmic Rays with Meteorites, The Moon and The Earth s Surface... 

Figure 3, Sw vs. exposure age for various iron meteorites. Iron meteorites having the oldest exposure ages also have the lowest Sw values. The dashed line shows the maximum extent to which the 8w values of iron meteorites can be lowered by interaction with thermal neutrons produced during cosmic-ray exposure (23). The hatched area indicates the initial 8w of the solar system as determined from HfrW data for Allende CAIs. Tungsten isotope data for iron meteorites are from (5, 19, 20, 24) and exposure ages are from (25).

They confirmed the excesses in Cr/ Cr and Cr/ Cr for bulk carbonaceous chondrites, which may affect the use of the Mn-Cr chronometer in carbonaceous chondrites, reported by Shukolyukov and Lugmair (2004,2006). Bulk chondrites have small s Cr (1 e unit is 1 part in 10 ) excesses (up to 0.3) relative to the Earth, most likely reflecting the subchondritic Mn/Cr ratio of the Earth. The e Cr variations in chondrites probably correlate with Mn/Cr and yield an initial solar system Mn/ Mn value of 5.4( 2.4) x 10 , corresponding to an absolute age of4,566.4 ( 2.2) Ma. Interactions of cosmic rays result in excesses of s Cr and a Cr with 4 1 ratio in phases with high Fe/Cr. These are most dramatically verified in the iron meteorite Carbo. Hence, the Mn-Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic-ray exposure ages. 

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