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

NONIONIZING RADIATION SAFETY 773
The effectiveness of an absorber for radiation of any given wavelength is measured
by the penetration depth in the absorber. At an absorber thickness t = 1/α, the power
density is reduced by absorption to
E 2
E 1
= e −2 = 0.135
of its original value. Since the power density is reduced to 13.5%, this means that
86.5% of the energy is absorbed in a thickness equal to 1/α. This thickness is called
the penetration depth, δ, and its exact value depends on the frequency of the radiation
and on the electrical and magnetic properties (μ, σ , and ɛ) of the absorber.
In the case of the 2450-MHz radiation of Example 14.24, the penetration depth in
muscle tissue is
δ = 1
α
δ =
1
= 1.67 cm.
0.6 cm−1 (14.39)
The rate of heat generation in any absorber, such as biological tissue, is inversely
proportional to the square of the penetration depth. Thus, a tissue with a relatively
small penetration depth because of high water content, such as muscle, will heat
much faster under microwave radiation than will a tissue such as fat, whose penetration
depth is relatively large because of its very low water content. At 2450 MHz, for
example, the penetration depths in muscle and fat are 1.7 and 11.2 cm, respectively.
The rate of heating in the muscle, therefore, from a given power density, will be
about (11.2/1.7) 2 = 43.4 times greater than in the fat. Table 14-17 lists the dielectric
parameters and penetration depth for muscle tissue for several frequencies and
Table 14-18 lists these parameters for several other frequencies for muscle and for
fat. Note that the penetration depth decreases with increasing frequency, and that
at a frequency of 100,000 MHz (100 GHz), only a very thin layer of tissue, 0.03 cm,
absorbs 86.5% of the incident energy.
TABLE 14-17. Penetration Depth and Dielectric Parameters for Muscle Tissue vs. Frequency
RELATIVE PENETRATION
FREQUENCY DIELECTRIC CONDUCTIVITY, DEPTH,
(MHz) CONSTANT ɛ T σ (S/m) δ (cm)
0.1 1850 0.56 213
1.0 411 0.59 70
10 131 0.68 13.2
100 79 0.81 7.7
1000 60 1.33 3.4
10000 42 13.3 0.27
100000 8 60 0.03
Reproduced with permission from Adair ER, Peterson RC. Biological effects of radiofrequency/microwave radiation. IEEE
Trans Microw Theory Tech. 2002; 50(3):953–962.