Why are there so many different units to measure radiation?
When I was a kid, there were rems and reps and rads and I think at least one other. Now there’s becquerels and curies and sieverts and perhaps more I can’t think of. Why so many? Can’t we retire some of them? There seems to be more units for radiation than for pressure and that’s saying something.
Yeah, good luck with that. I’m sure you’ll be able to sort out pound, stone, and kilogram for everyone while you’re at it. 
There are different things being measured. You’ve got radioactivity, which is a measure of radioactive decays per second. This is measured in curies. Then you’ve got the absorbed dose, which is how much radioactive energy is absorbed. This is measured in rads. If you look at rads though it doesn’t necessarily give you a good idea of what the biological effects will be, so for that you’ve got the equivalent dose, which is measured in rems (Roentgen equivalent man). And, if you want to measure total exposure, you’ve got the roentgen. The differences are perhaps a bit subtle, but there are different things being measured.
And, just like pound and kilogram, you’ve got SI equivalents for each of these.
The SI equivalent of the curie is the becquerel.
The SI equivalent of the rad is the gray (Gy),
The SI equivalent of the rem is the sievert.
The SI equivalent of the roentgen is somewhat long and clunky coulomb/kilogram (C/kg).
Don’t forget the best one, the
Banana Equivalent Dose(BED).
That bastard Becquerel; fuckin’ with the Curies while they lived, and his ghost is still fuckin’ with 'em! (Well, my old radiation biophysics prof said that Becquerel persecuted the Curies horribly, though he never said how; it’s possible that he delayed Marie Curie’s doctorate to keep her working in his lab, but I’ve heard of nothing worse than that. Still, replacing the curie with the becquerel – damn you, SI!)
Meh. Without Becquerel nobody would never have heard of no Curies.
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radrisk.html#c1
Easy to understand science.