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

RADIATION DOSIMETRY 213
Figure 6-5. Fractional number of photons transmitted through an air wall of thickness equal to the
maximum range of the secondary electrons. (From Protection Against Betatron–Synchrotron Radiations
up to 100 Million Electron Volts. Washington, DC: US Government Printing Office; 1955. NBS
Handbook 55.)
very thick walls are necessary. However, when the walls are sufficiently thick, on the
basis of the range of the primary electrons, they attenuate the photons (gamma
radiation) to a significant degree, as shown in Figure 6-5. Under these conditions,
it is not possible to attain electronic equilibrium since the radiation intensity within
the wall is not constant and the primary electrons, consequently, are not produced
uniformly throughout the entire volume of wall from which they may reach the
cavity.
Exposure–Dose Relationship
The air-wall chamber, as the name implies, measures the energy absorption in air.
In most instances, we are interested in the energy absorbed in tissue. Since energy
absorption is approximately proportional to the electronic density of the absorber in
the energy region where exposure units are valid, it can be shown that the tissue dose
is not necessarily equal to the air dose for any given radiation field. For example, if
we consider muscle tissue to have a specific gravity of 1 and to have an elementary
composition of 5.98 ×10 22 hydrogen atoms per gram, 2.75 ×10 22 oxygen atoms per
gram, 0.172 ×10 22 nitrogen atoms per gram, and 6.02 ×10 21 carbon atoms per gram,
then the electronic density is 3.28 ×10 23 electrons per gram. For air, whose density is
1.293 ×10 −3 g/cm 3 , the electronic density is 3.01 ×10 23 electrons/gram. The energy