a. The absorbed dose does not provide us with an indication of biological damage since it is concerned with energy absorption only. It was desirable to establish an index of damage produced by different kinds of radiation. The relative biological effectiveness (RBE) factor was introduced.

b. Using damage produced by x-rays (200 KeV) as the standard, other forms of radiation were compared and RBE values assigned.

c. For example, if our 200 KeV x-rays produce reddening of the skin with a dose of 200 rads, but another type of radiation produces the same effect with 100 rads, we can compare dose and calculate the RBE.

rads of Standard (Skin Reddening) 200
RBE = rads of Other Radiation (Same Effect) = 100 = 2

So we would assign the other radiation an RBE value of two. This value is used primarily in the study of radiobiology.

d. The RBE demonstrated the need for a unit of dose that would take into account biological damage. Thus, a unit of dose equivalent was established and called the roentgen equivalent man (rem).

rem = rad x RBE (Radiobiology)


rem = rad x Q x N

where Q is the quality factor and N is any other modifying factor. This is used in radiation protection.

e. Recently, a new name and unit of dose equivalent has been introduced. The Sievert (symbol Sv) is the new unit of dose equivalent.

1 rem = 0.01 joule/kilogram = 0.01 Sv

Therefore, 1 Sv = 100 rem.

X-ray Schools | X-ray and Radiation Safety
For Informational Purposes Only - Based On US Army Radiation Safety Training