Cosmogenic neutron‐capture‐produced nuclides in stony meteorites

Abstract

The distribution of neutrons with energies below 15 MeV in spherical stony meteoroids is calculated using the ANISN neutron‐transport code. The source distributions and intensities of neutrons are calculated using cross sections for the production of tritium. The meteoroid's radius and chemical composition strongly influence the total neutron flux and the neutron energy spectrum, whereas the location within a meteoroid only affects the relative neutron intensities. Meteoroids must have radii of more than 50 g/cm2 before they have appreciable fluxes of neutrons near thermal energies. Meteoroids with high hydrogen or low iron contents can thermalize neutrons better than chondrites can. Rates for the production of 60Co, 59Ni, and 36Cl are calculated with these neutron fluxes and evaluated neutron‐capture cross sections and are reported for carbonaceous chondrites with high hydrogen contents, L‐chondrites, and aubrites. For most meteoroids with radii <300 g/cm2, the production rates of these neutron‐capture nuclides increase monotonically with depth. The highest calculated 60Co production rate in an ordinary chondrite is 375 atoms/min/g‐Co at the center of a meteoroid with a 250 g/cm2 radius. The production rates calculated for spallogenic 60Co and 59Ni are greater than the neutron‐capture rates for radii less than ∼50–75 g/cm2. Only for very large meteorpids and chlorine‐rich samples is the neutron‐capture production of 36Cl important. The results of these calculations are compared with those of previous calculations and with measured activities in many meteorites.

Monte Carlo simulation of GCR neutron capture production of cosmogenic nuclides in stony meteorites and lunar surface

“…Both previously mentioned models of Eberhardt et al (1963) and Spergel et al (1986) consider only one class of chondrites, the L chondrites, assuming the effects of different bulk composition on neutron fluxes and capture production to be negligible. As we have demonstrated, for some nuclides these effects can make measurable differences in production rates.…”

“…No such system is available for thermal neutron capture production of CN. Thermal neutron fluxes and neutron capture effects have previously been studied in stony meteorites and in the lunar surface using semi-empirical models (Eberhardt et al 1963;Spergel et al 1986;Lingenfelter et al 1972;Fanenbruck et al 1994).…”

Cosmogenic radionuclides in the recently fallen Kobe (CK4) meteorite.

“…For example, activity of typical neutron-capture product, 60 Co has been used as an indicator of their preatmospheric size (e.g., Eberhardt et al, 1963;Spergel et al, 1986), and those of spallation products, 22 Na, 54 Mn and 26 Al etc. reflect irradiation conditions such as shielding effect (e.g., Bhandari et al, 1993;Michel et al, 1995), exposure age (e.g., Heimann et al, 1974;Herpers and Englert, 1983) and flux of cosmic-rays (e.g., Evans et al, 1982).…”

Radiocarbon Beyond this World

“…The 14 C can be produced only by thermal neutrons on 14 N, or to a much lesser extent by neutron capture on 13 C. As we know that all samples of these meteorites so far recovered were irradiated as small objects in space with only trace water content, very few cosmicray generated neutrons can have been produced. Spergel et al (1986) had shown that for objects of radius less than ~50 g/cm 2 (about 150 cm in a rock, or approximately 19 kg in mass) that the cosmic ray induced thermal neutron flux is extremely small and neutron products are not detectable. For these meteorites, ALH84001, Nakhla and EETA79001 were much smaller than this size (2.1, 10, and 7.9 kg recovered mass, respectively), and thus the thermal neutron flux would be even, hence we can eliminate this production of 14 C in the organic components of these Martian meteorites.…”