The isotope plated on the Voyager Golden Record cover for dating purposes was uranium-238.
Each Voyager spacecraft carries a Golden Record whose aluminum cover includes an electroplated sample of ultra-pure uranium-238. The isotope was chosen because its extremely long half-life provides a durable clock for estimating when the record was made or encountered.
Uranium-238 has a half-life of about 4.468 billion years. A technically advanced finder could compare the remaining uranium with its decay products, using the ratio to estimate the record’s age. The cover also carries diagrams explaining how to play the record, decode its images, and locate the Solar System using pulsars.
The record itself is often confused with the cover: the 12-inch disk is copper, plated with nickel and gold, while the uranium-238 sample is on the aluminum cover. Plutonium-238 powered the spacecraft’s radioisotope generators, but it was not the isotope used for the cover’s dating device.