Plutonium-244 Fission Tracks: Evidence in a Lunar Rock 3.95 Billion Years Old

Abstract
Tracks attributed to the spontaneous fission of plutonium-244 and of uranium-238 were detected in a large whitlockite crystal in the lunar breccia 14321 from the Fra Mauro formation. For a track-retention age of 3.95 ± 10 9 years the number of plutonium tracks relative to the number of uranium tracks is 0.51 ± 0.15, provided that the rock was not heavily neutron-irradiated 3.95 × 10 9 years ago.