47969
potassium-40 and rubidium-87 have very long half-lives and thus occur naturally , although it has no naturally occurring radioisotopes .
 half-lives
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 half-lives