CONTRIBUTION OF NEUTRON-CAPTURE REACTIONS TO OBSERVED TUNGSTEN ISOTOPIC-RATIOS

To study whether tungsten isotopic ratios observed in terrestrial samp les relative to W ratios in an iron meteorite could be made by neutron -capture reactions in the iron meteorite, the fluxes of thermal and ep ithermal neutrons in a large iron meteoroid, Toluca, were calculated u sing Monte Carlo particle production and transport codes. Peak neutron -capture rates on W isotopes were calculated to occur at depths of 30- 34 cm. The calculated changes in the W-182/W-183 due to neutrons are m uch less than those observed in terrestrial samples relative to iron m eteorites, indicating a non-cosmogenic origin of the observed W isotop ic ratios. (C) 1997 Elsevier Science B.V.