RADIATION UNITS

rad - unit of "absorbed dose" - energy absorbed per unit mass of tissue

rem - unit of "equivalent dose"

Equivalent dose = absorbed dose x radiation weighting factor (w_R)

w_R takes into account that ionizing radiation other than x-rays and gamma-rays can be more damaging to the body for the same amount of absorbed dose.

X-rays and gamma-rays are assigned a radiation weighting factor ( w_R ) of one.

Other radiations can be assigned weighting factors up to 20, though these in reality are only approximate assignments. Depending on the biological effect at which you are observing a particular type of radiation (other than x-rays and gamma-rays) may have an enhanced biological damage that varies over a wide range, 5 to 50 for example. Using a single number for the radiation weighting factor is a dramatic simplification that has been accepted into use for radiation safety purposes.

(These other types of ionizing radiation come from radioactive materials, particle accelerators, fission, and fusion, so we don't have to worry about them here. For your information, though, electrons or beta particles from radioactive decay also have a weighting factor of one, while alpha particles are assigned a weighting factor of 20.)

For x-rays (and for gamma-rays): 1 rad = 1 rem, always

New Units (SI units):

gray (Gy) - unit of "absorbed dose"

sievert (Sv) - unit of "equivalent dose"

1 Gy = 100 rad 1 Sv = 100 rem

For x-rays: 1 Gy = 1 Sv