When a radioactive sample is purchased what is of particular interest is its activity . Activity refers to the number of nuclear disintegrations per second that occur in a sample. Rad and Gray The 'rad' is used to describe the quantity or dose of radiation absorbed by some material. The rad does not account for the kind of damage done, only for how much radiation goes in.
Neutrons are more dangerous then radiation of the same energy and intensity.
To take into account this fact the REM was derived. To find the dose in REMS the dosage in rads is multiplied by a conversion factor that reflects the effectiveness of the kind of radiation causing the damage. 1 Doses in REMS are additive whereas doses in rads are not. Radioactivity can ionize molecules. The ionized molecule will then chemically react with adjoining molecules to form new compounds.