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

792 CHAPTER 14
PRINCIPLES OF RADIATION SAFETY
The principles of radiation protection for NIR are the same as those for ionizing
radiation—namely, time, distance, and shielding. Most of the recommended exposure
limits are 6-minute time-weighted averages, and some are based on frequency.
Thus, in the case of a 6-minute averaging period, a worker may be exposed at twice
the guideline level for a 3-minute period during any 6-minute interval. Generally, the
maximum exposure time during any t avg interval when the field intensity exceeds
the guideline limit is given by
tmax = tavg. × F
L
, (14.52)
where
t avg = frequency-dependent averaging time,
F = measured field intensity, either electric or magnetic, or power density,
and
L = limiting exposure.
Shielding against magnetic fields is easily accomplished through the use of ferromagnetic
materials, such as iron or alloys of high permeability (Table 14.22).
Attenuation of electric field intensity is accomplished through the use of solid metal
shields. The exact degree of attenuation is determined by the electrical properties
of the shielding material, the shield thickness, and the frequency of the radiation.
The relationship among these factors is given by
A = 3.34 × t × μ × σ × f , (14.53)
TABLE 14-22. Electric Conductivity and Magnetic Permeability of Selected Radio-Frequency
Shielding Materials
METAL RELATIVE CONDUCTIVITY RELATIVE PERMEABILITY
Silver 1.05 1
Copper, annealed 1.00 1
Copper, hard-drawn 0.97 1
Gold 0.70 1
Aluminum 0.61 1
Magnesium 0.38 1
Zinc 0.29 1
Brass 0.26 1
Cadmium 0.23 1
Nickel 0.20 1
Phosphor-bronze 0.18 1
Iron 0.17 1000
Tin 0.15 1
Steel, SAE 1045 0.10 1000
Beryllium 0.10 1
Lead 0.08 1
Hypernick 0.06 80,000
Monel 0.04 1
Mu metal 0.03 80,000
Permalloy 0.03 80,000
Steel, stainless 0.02 1000