If you look at the amount remaining, you get a logarithmic curve. It slowly tapers off towards zero, but never quite gets there. (Which never made sense to me, because there are a finite number of atoms, and eventually the last one has to either go or not.)
The half-life remains the same the entire time, so in that sense, the decay rate (percent mass/time) is the same. But if you consider how much mass is being converted at a time...
In the example above, half a pound went during the first hour. A quarter of a pound went during the second hour. And so on.
So in that sense, the decay rate (total mass/time) goes down. The fact that you have half the sample size means that half the number of atoms are going.
Make sense?
As for the time-release coating... You'd have to get the radioisotope broken down to tiny pieces. And then you'd have to find a coating that can temporarily block the radiation. And then you'd have to get that coating around. And then put it all into solution. And still have that be small enough to fit through the needle. Doable, I think, but not easy. Time-consuming, at the least. Unless you have nanites or something to do the work. (Hey, at LexLabs, maybe they do...)
And yes, the more radioactive something is, the shorter the half-life. Isn't that what I said?
