in what way are you using the radiation? if you are using it in an experiment testing a particular independent variable, then activity of the source is a variable that should be kept constant. If you use cobalt, the decay is so slow that the activity is relatively constant, same with radium. (I'm assuming you know what activity is). If you use bismuth, a half life of 20 minutes is too fast, so the activity will vary greatly during setting up the measurements, when taking reading e.Take Care. and if activity is a variable that should be kept constant then it beats logic to use bismuth.
