What is the creeper's "activation" radius?

I've stared at creepers through a one-block glass wall, no problem. I later had a creeper blow next to the same wall when I was outside, some of the sand just inside the wall was blasted.

You should only need two blocks between you and the creeper. They only activate if they are within one block of you. I created a two layer wall in my base and would just stare at them all day long right in front of me, they never activated. You might get away with one block but just to be safe and since I know two works, I would stick with that.