55191
He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its " half-life " ( 11 ½ minutes in this case ) .
 half-life
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-life