Before revision: Plutonium-238, curium-244, strontium-90, and nowadays americium-241 are the most often cited candidate isotopes, but 43 more isotopes out of approximately 1300 were considered at the beginning in the 1950s.
After revision: Plutonium-238, curium-244, strontium-90, and most recently americium-241 are the most often cited candidate isotopes, but 43 more isotopes out of approximately 1,300 were considered at the beginning in the 1950s.