Plutonium-244 fission tracks: evidence in a lunar rock 3.95 billion years old.

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 x 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 X 10(9) years ago.