Neutron-capture 128Xe and 129Xe in the San Juan mass of the Campo del Cielo IAB iron meteorite: Evidence for a high fluence of thermalized neutrons.

PRAVDIVTSEVA O.; MARIA EUGENIA VARELA; MESHIK A.; CAMPBELL A.; TOPA D,

Meteoritics and Planetary Science

Wiley-Blackwell

Año: 2021

1945-5100

The petrographic study of the San Juan A2 polished section demonstratedtextural and compositional similarities with the Campo del Cielo IAB iron meteorite, with traceelement abundances in metal following the pattern of bulk Campo del Cielo.Xenon, neon and helium isotopic compositions have been measured in radial graphite rims,massive graphite inclusions, fine-grained graphite aggregates, cliftonite, and platy graphite. Twosilicate inclusions and two areas of metal were also analyzed. 3He/4He versus 4He/21Ne data forSan Juan metal plot next to the values reported for the El Taco fragment of Campo del Cielo,supporting San Juan being a part of the Campo del Cielo meteorite shower. Based on the Neisotopic composition of its components, and the observed correlation between 128Xe and 129Xe, theSan Juan A fragment of Campo del Cielo was well shielded from the primary galactic cosmic rayhigh-energy irradiation. Its size allowed the secondary neutrons to be fairly well thermalized,receiving an equivalent (normalized to the research reactor with highly thermalized neutronspectrum) fluence of thermal and epi-thermal neutrons of 6.6×1017 n/cm2. Considering 1.8×108years single stage and constant exposure geometry irradiation history for Campo del Cielo, andassuming the identical neutron flux spectra for the research reactor and Campo del Cielo, theaverage thermal equivalent neutron flux for San Juan is about 1.2×102 n/cm2s. Xe isotopiccomposition in the radial graphite rims and platy graphite show evidence of live 129I in San Juanand is consistent with a mixture of iodine-derived and tellurium-derived Xe.