c. Organizations responsible for defining safe radiation limits provide
recommendations that cover all types of ionizing radiation, whether applied internally or
externally. Our major concern is exposure to x-radiation from an external source, the x-
ray tube. Among the organizations responsible for defining safe radiation limits are:
National Committee on Radiation Protection and Measurement.
Nuclear Regulatory Agency.
Environmental Protection Agency.
Energy Research and Development Administration.
d. The safe limits known as Radiation Protection Guides (RPGs) as
recommended by some of the organizations previously mentioned are divided into two
categories: those governing radiation workers and those governing all others (meaning
the general population). A further breakdown specifies the limits for whole-body and
partial-body radiation exposure.
4-28. UNITS OF RADIATION DOSAGE
Fundamentally, the harmful consequences of ionizing radiation to a living
organism are due to the energy absorbed by the cells and tissues that form the
organisms. This absorbed energy (or dose) produces chemical decomposition of the
molecules present in the living cells. The mechanism of the decomposition appears to
be related to ionization and excitation interactions between the radiation and atoms
within the tissue. The amount of ionization or number of ion pairs produced by ionizing
radiation in the cells or tissues provides some measure of the amount of physiological
damage that might be expected from a given quantity or dose. The ideal basis for
radiation-dose measurement, therefore, would be the number of ion pairs produced
within the medium of interest. However, for certain practical reasons, the medium used
in establishing a unit of measurement was air.
a. Roentgen. A roentgen (R) is a unit of exposure. One roentgen is the
exposure of x- or gamma radiation such that the associated corpuscular emission per
kilogram of air produces in air, ions carrying 2.58 x 10-4 coulomb of electrical charge of
either sign (IR = 2.58 x 104 coulomb/Kg = 7.74102 electrostatic units per gram of air.).
b. Absorbed Dose (rad). Absorbed dose is the energy imparted to matter in a
suitably small element of volume by ionizing radiation, divided by the mass of that
element of volume. The rad is the unit of absorbed dose; one rad equals 0.01 joule per
kilogram, which equals 100 ergs per gram.