Another radiation unit is kerma(acronym for kinetic energy released in
matter) which is defined as the sum of initial kinetic energies of all charged
particles liberated by uncharged ionizing radiation per unit mass of mate-
rial. For all practical purposes, kerma and rad are identical.
In SI units, the gray(Gy) is the unit of radiation absorbed dose and kerma
and is given by
1 Gy =100 rad (14.4)
=1 J/kg absorber (14.5)It can be shown that the energy absorbed per kilogram of air due to an
exposure of 1 R is
1R =86.9 × 10 −^4 J/kg in airTherefore,
1R =0.869 rad in airor,
1R =0.00869 Gy in air
The rad is not restricted by the type of radiation or absorber or by the
energy or intensity of the radiation. It should be understood that the rad is
independent of the weight of the material. This means that a radiation dose
of 1 rad (0.01 Gy) is always 1 rad (0.01 Gy) in 1, 2, or 10 g of the material.
However, the integral absorbed dose is given in units of gram-rad (g · rad
or g · Gy) and calculated by multiplying the rad (Gy) by the mass of mate-
rial. For example, if the radiation dose to a body of 45 g is 10 rad (0.1 Gy),
then the integral radiation dose to the material is 450 g · rad (or 4.5 g · Gy);
however, the radiation dose is still 10 rad (0.1 Gy).
The dose equivalent unit,rem, has been developed to account for the dif-
ferences in effectiveness of different types of radiation in causing biologi-
cal damage. In radiobiology, the rem is defined as
rem =rad ×RBE (14.6)where RBE is the relative biological effectiveness of the radiation. It is
defined as the ratio of the dose of a standard radiation to produce a par-
ticular biological response to the dose of the radiation in question to
produce the same biological response. Radiations of 250 KV x-rays are
normally chosen as the standard radiation because of their widespread
use. RBE varies with the linear energy transfer (LET) of the radiation,
radiation dose, dose rate, and the biological system in which RBE is
determined.
In radiation protection, RBE is replaced by the radiation weighting factor,
Wr, to account for differences in effectiveness of various radiations in
causing biological damage. The rem is then defined as
Radiation Units 209