Cosmogenic radionuclides in four fragments of the portales valley meteorite shower: influence of different element abundances and shielding

Abstract

Portales Valley is an unusual meteorite with H6 chondritic matrix, interspersed by heterogeneous distribution of metallic veins. Four fragments with different abundances of veins, as determined by their bulk densities (3.49-4.86 gcm^-3 ) were analysed for measurement of the cosmogenic radionuclides ⁵⁷Co, ⁵⁴Mn, ²²Na, ⁶⁰Co, ⁴⁴Ti and ²⁶Al using a gamma ray spectrometer. Chondritic and metallic proportions were estimated for each fragment. One sample was essentially free of metal veins. The shielding depth of each fragment (14 to 81 gcm^-2) was estimated using cosmic ray heavy nuclei tracks which varied by more than two orders of magnitude in the four fragments, ranging between < 0.6 × 104 to 1.7 × 106 cm^-2. The activity ratio ²²Na/²⁶Al was found to be similar (1.53 ± 0.03) in the four fragments, independent of shielding depth and consistent with the expected activity at the time of fall during the beginning phase of the solar cycle 23.