Cosmogenic radioisotopes in the Almahata Sitta ureilite

Abstract

Asteroid 2008 TC₃ was predicted to fall in Sudan on October 7, 2008, and 2 months later, 15 meteorite fragments were recovered from the Nubian Desert. Most of these fragments were classified as polymict ureilites. In the largest ureilitic fragment #15, weighing 75 g, we have measured six γ emitting radionuclides (⁴⁶Sc, ⁵⁷Co, ⁵⁴Mn, ²²Na, ⁶⁰Co, and ²⁶Al) by nondestructive whole rock counting using a sensitive γ-ray spectrometer. The activities of ⁶⁰Co, produced mainly by neutron capture in cobalt, and ²⁶Al indicate that fragment #15 was located at a depth of 41 ± 14 cm inside the 1.5-2 m radius asteroid. The activity of other radionuclides is also consistent with this shielding depth within the asteroid. The ²²Na/²⁶Al activity ratio is higher than expected for the average cosmic ray flux, probably due to the unusually prolonged solar minimum before the fall.