Voyager has been going strong since 1977. Better than the energizer bunny!
Voyager uses a thermoelectric pile. Plutonium 238 oxide generates heat which a series-connected thermocouple device converts to electricity. Provides high power output over decades.
There are direct conversion radioactive battery which directly collect emitted beta (electron) or alpha (helium nuclei) particles to generate a high voltage. Typical small power output , but sustained for a very long duration.