1-12. ABSORBED DOSE
a. The rad is a unit of absorbed dose and it describes energy absorbed per
gram of absorbing material.
1 rad = 100 ergs absorbed per gram of any substance
0.01 joules per kilogram
b. When water or soft tissue absorbs x or gamma radiation (100 KeV to 3 MeV),
the absorbed dose per roentgen is between 0.93 and 0.98 rad. Therefore, rads and
roentgens are approximately the same for x or gamma rays in those energies.
c. The rad is used for x and gamma rays and the particulate radiations, alpha
d. Recently, the 15th General Conference of Weights and Measures adopted
some new names and units. The Gray (symbol Gy) has been adopted as the special
unit of absorbed dose.
1 rad = 0.01 joule/kilogram
1 Gy = 100 rads
1-13. DOSE EQUIVALENT
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)
RBE = rads of Other Radiation (Same Effect) = 100 = 2