Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

204 CHAPTER 6
The gray is universally applicable to all types of ionizing radiation dosimetry—
irradiation due to external fields of gamma rays, neutrons, or charged particles
as well as that due to internally deposited radionuclides.
rad (Radiation Absorbed Dose)
Before the introduction of the SI units, radiation dose was measured by a unit called
the rad.
One rad is defined as an absorbed radiation dose of 100 ergs
g .
1 rad = 100 ergs
. (6.3a)
g
Since 1 J = 10 7 ergs, and since 1 kg = 1000 g,
1Gy= 100 rads. (6.3b)
1 rad = 0.01 Gy = 1 centigray (cGy). (6.3c)
Although the gray is the newer unit and will eventually replace the rad, the rad and
its derivatives nevertheless continue to be useful units and are used in the official
radiation safety regulations in the United States.
It is important to understand that radiation absorbed dose concept, the gray and
the rad, is a macroscopic construct and is not intended for microdosimetry on the
cellular or subcellular levels. Radiation absorbed dose has been found to be correlated
with biomedical effects on the tissue, organ, and organism levels and thus
is appropriate for radiation safety measurements and for medical diagnostic and
therapeutic uses of radiation. The radiation absorbed dose concept implies that the
absorbed energy is uniformly distributed throughout the entire mass of the tissue
of interest. On the cellular and subcellular levels that are of interest to molecular
biologists, the biological effects are proportional to the number and types of intramolecular
bonds that are broken rather than to the concentration of absorbed
energy within the cell. On the tissue level, the number of such intramolecular breaks
in the tissue is proportional to the radiation absorbed dose. The distinction between
microdosimetry and radiation absorbed dose may be illustrated with the following
thought experiment.
W Example 6.1
Consider a single cell, with dimensions 10 μm × 10 μm × 10 μm and mass = 10 −12
kg, in a tissue of weight 0.1 g, in which a low LET particle, 1 keV/μm, transfers
1 keV to the cell as it passes through the cell. Calculate the radiation absorbed
dose to the tissue.