Argon 37/argon 39 activity ratios in meteorites and the spatial constancy of the cosmic radiation

Abstract

Argon 37 and argon 39 have been measured in a bulk sample and a metal-rich fraction of the Lost City stone meteorite. In the bulk sample the activity of both isotopes is in fair agreement with the activities reported previously for this meteorite. The activities calculated for the pure metal phase (37Ar = 15.1 ± 2.5 dpm/kg, 39Ar = 20.0 ± 1.5 dpm/kg), however, are different. In particular, the activity ratio of 0.75 ± 0.10 is considerably higher than the previously published values. Consequently the radial heliocentric gradient of the galactic cosmic radiation is found to be much smaller. The data available for the activity ratios in the metal of different meteorites are discussed. Because the wide scatter in the ratios observed in meteorites that fell at comparable times during the solar cycle is due to different decay rates of 37Ar, it is argued that at least during solar maximum the ratios depend essentially on the perihelions of the meteoroid orbits and on whether the meteorites fall on earth when they are on their inward-bound trip or when they are going out. Except in special cases like Lost City, different aphelions will have a noticeable influence only for meteorites falling near solar minimum, provided that the spatial gradient of the cosmic-ray intensity is small or zero at solar minimum.