Sometime in the past few years, I've become aware of the vast number of units related to the measurement of radiation, and I've decided to do something about it, and de-ignorise myself about them.
Here are the units:
Whew. Let's start.
One Curie is roughly the activity of 1 gram of the radium isotope 226Ra
"Its continued use is discouraged", so let's ignore that.
A gray (Gy) is a unit of radiation dose absorbed by matter. To gauge biological effects the dose is multiplied by a 'quality factor' which is dependent on the type of ionising radiation. Such measurement of biological effect is called "dose equivalent" and is measured in sievert (Sv).
The sievert (symbol: Sv) is the International System of Units (SI) derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in grays.
So a gray is a measurement of the amount of absorbed radiation, but sievert is the biological effect of the absorbed radiation.
REM means Roentgen equivalent man.
The roentgen (R, also roentgen) is a unit of measurement for exposure to ionizing radiation (such as X-ray and gamma rays)
In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose (= 0.01 Gy).
So, it's starting to link. 1 roentgen into a human is 1 gray, which will be 1 sievert if the type of radiation is electron and photon radiation (e.g. gamma).